Accepted Manuscript Title: Thermodynamic evaluation of vesicles shed by erythrocytes at elevated temperatures Author: V. Vodyanoy PII: DOI: Reference:
S0927-7765(15)00386-0 http://dx.doi.org/doi:10.1016/j.colsurfb.2015.06.013 COLSUB 7143
To appear in:
Colloids and Surfaces B: Biointerfaces
Received date: Revised date: Accepted date:
21-4-2015 28-5-2015 5-6-2015
Please cite this article as: V. Vodyanoy, Thermodynamic evaluation of vesicles shed by erythrocytes at elevated temperatures, Colloids and Surfaces B: Biointerfaces (2015), http://dx.doi.org/10.1016/j.colsurfb.2015.06.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights
2
Number of vesicles shed by erythrocytes at increases elevated temperatures.
3
Eighty percent of erythrocyte released vesicles are smaller than 0.4 μm.
4
Freshly drawn mammalian blood contains more than 1 million vesicles per one microliter.
5
Erythrocyte vesicles can serve as diagnostic tool of physical performance.
6
Vesicle release is driven by entropy with enthalpy-entropy compensation.
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Thermodynamic evaluation of vesicles shed by erythrocytes at elevated temperatures.
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V. Vodyanoy1
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Abstract
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Erythrocytes undergo structural transformation and shed small vesicles at elevated temperatures. To
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Department Anatomy, Physiology and Pharmacology, College of Veterinary Medicine, Auburn, AL 36849; School of Kinesiology, Auburn University, Auburn, AL 36849, The Edward Via College of Osteopathic Medicine, Auburn, AL 36849
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Vitaly Vodyanoy, telephone: 334-844-5405, fax: 334-844-5388, e-mail: [email protected].
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1
characterize mechanisms of the phenomenon, the Arrhenius and Eyring equations were used for analysis
11
of the literature, experimental data on vesiculation of human and rat erythrocytes after the temperature
12
was elevated by physical exercise or by exposure to external heat. The thermodynamic analysis of the
13
data showed that erythrocyte transformation, vesicle release, and other associated processes are driven by
14
entropy with enthalpy-entropy compensation. It is suggested that the physical state of the hydrated cell
15
membrane is responsible for the compensation. The increase of vesicle number related to elevated
16
temperatures may be indicative of the heat stress level and serve as diagnostic of erythrocyte stability and
17
human performance.
18
Keywords
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Erythrocytes, echinocytes, vesicles, entropy, microscopy, heat stress
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1. Introduction
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In his work entitled “A heated microscope stage and its use in the investigation of the blood” Schultze [1]
22
used a compound microscope and a specially constructed wet chamber to observe and document changes
23
of blood cells at well controlled elevated temperatures (37-65 oC) under magnification up to 800 X.
24
Human subjects, dogs, cats, and rabbits were studied. He analyzed a small droplet of live blood collected 2 Page 2 of 30
from humans via fingerpick or venous blood on the glass microscope slide and carefully covered with a
2
coverslip. At the temperature of 37 oC, most of the red blood cells are oval and flexible biconcave disks of
3
about 8 microns in diameter. At elevated temperatures approaching 52 oC, he observed large
4
morphological changes in red blood cells. The red blood cells change from the biconcave discs to the
5
crenated discs, crenated spheres, and then into spheres with very small spicules. He noted that many red
6
blood cells appear to shed multiple vesicles. More than 100 years later, the major morphological changes
7
in red blood cells at elevated temperatures described by Schultze were confirmed by electron microscopy
8
[2].
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1
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The vesiculation induced by elevated temperatures up to 49 oC in normal human erythrocytes was studied
11
with the use of a Nomarski differential interference-contrast (DIC) microscope at the original
12
magnification of 1000 X (Wagner et al., 1986). This technique allowed for the observation of real life cell
13
images. It was observed that the erythrocyte cell membrane went through spontaneous vesiculation in
14
vivo during and following exposure to heat. The cell turns into a crenated shape or becomes echinocyte,
15
with many small projections (spicules). After some time, the projections extend and pinch off to create
16
small free-floating vesicles. Based on the presented biochemical data, it was suggested that a localized
17
disruption of the normal membrane skeletal protein and lipid interaction were the essential steps in
18
vesiculation.
19
It is accepted that intracellular erythrocyte oxidation of hemoglobin to methemoglobin leads to ATP
20
depletion with follow on dephosphorylation of membrane proteins and lipids [3]. The change of K+ and
21
Cl- membrane permeability causes the loss of KCL and osmotically driven water, which in turn leads to
22
cell contraction, externalization of phosphatidylserine (the processes specific to eryptosis) [4] and
23
erythrocyte-echinocyte transitions [5]. It has been demonstrated that during vesiculation of human
24
erythrocytes in vitro, acetylcholinesterase redistributes on the cell surface and becomes enriched in the
25
released vesicles [6, 7]. Additionally, it was shown that acetylcholinesterase (AcCHE) release correlates
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with vesiculation of red blood cells [8]. It is important to note that the process of erythrocyte hemolysis
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leads to cell death, but it is very different from the programmed cell death – eryptosis. The main
2
difference lies in the fact that during hemolysis the osmotically driven water moves inside the cell,
3
causing it to swell and break it apart, while in eryptosis, water moves outside, causes cell shrinkage and
4
vesiculation. The hemolysis is an adverse cellular phenomenon, but eryptosis allows defective
5
erythrocytes to escape hemolysis and prolongs cell life [9]. When erythrocytes are subjected to the
6
hypotonic solution, as, in osmotic fragility test [10], the cells experience morphological changes that are
7
similar to those during a regular eryptosis. Under the influence of osmotic pressure, water moves from
8
inside out, and forces the cells to experience remarkable changes: the first is the appearance of small bud-
9
like protrusions on an occasional erythrocyte; the next recognizable change, many of the erythrocytes
10
show single or multiple buds; then many erythrocytes are converted to spheroid cells, and finally, buds
11
are pinched off creating many free vesicles of one micron and smaller in sizes [10].
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An essential information about the mechanism of red blood cell vesiculation was obtained by the
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thermodynamic analysis of experiments with erythrocyte hemolysis of man, rabbits, guinea-pigs, rats,
15
sheep, cats and pigs [11]. The author came to the conclusion that the mechanism of the induced hemolysis
16
is identical for all the cell irrespective of species and depends directly on the physical state of water in the
17
cell and cell membrane.
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In order to elucidate the mechanism of erythrocyte transformation and vesiculation thermodynamic
20
analysis was applied to the experiential results on the kinetics of vesicle loss by erythrocytes from human
21
and rat blood following temperature increase generated by physical exercise or by exposure to external
22
heat [12, 13]. Additionally, other literature results obtained for the erythrocyte hemolysis [14, 15], the
23
formation of methemoglobin , the K+ and ACHE release [16], the ion permeability [17], the osmotic
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fragility [10], the discocyte-echinocyte transition [18], and the heat survival of the Chinese hamster ovary
25
cultured cells [19] have been analyzed. This work may shed some light on mechanisms of heat stress.
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1 2
The thermodynamic analysis of transformations in live cells is an important tool for the quantitative study
3
of the energy transductions that occur during these changes. It allows to define the nature and function of
4
the chemical processes underlying these transformations [20]. The purpose of this section is to present
5
kinetic and thermodynamic equations that can be used for calculation of activation energy, free energy,
6
enthalpy, and entropy changes in cells at different temperatures. In the next section, these thermodynamic
7
variables will be calculated from the literature experimental data.
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During a normal lifespan, an erythrocyte reduces its volume and surface area in part by a process of
9
vesiculation – losing free vesicles. In steady-state conditions, the number of vesicles is constant because
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2. Thermodynamic calculations of erythrocyte vesiculation
of equilibrium between vesicles generated by erythrocytes and vesicles destructed by Kupffer cell
11
mechanisms [21, 22].
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First-order kinetics can describe the apparent rate of vesicle destruction at constant temperature
13
dN kN dt
14
where N is the concentration of vesicles being destructed, k represents the first-order rate constant, and t,
15
the time of the thermal destruction. The solution of the equation Eq (1) can be expressed as
16
N / N 0 e kt
17
where N 0 is the initial concentration of vesicles and N / N 0 represents the ratio of the concentration of
18
destructed vesicles to the initial vesicle concentration. This fraction defines the ratio of intact vesicles.
19
Taking the logarithm of both sides of Eq (2), one obtains
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log( N / N 0 ) kt / 2.303
(1)
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(2)
(3)
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1
A plot of the left-hand side of Eq (3) versus time (t) yields an estimate of k from the slope. The Arrhenius
2
equation [23] can be used to express the temperature dependence of the first-order activation kinetics:
3
k Ae
4
where R is the universal gas constant, Ea represents the apparent activation energy, and A -- the pre-
5
exponential Arrhenius factor. Taking the logarithm of Eq (4) yields:
6
log k
7
If the logarithm of k in Eq (5) is plotted against the reciprocal of temperature, 1/T, then the slope of this
8
graph yields the activation energy (Ea), the thermal activation level of transitions from the intact to the
9
destroyed vesicle. The rate constant of the vesicle destruction process depends on the thermodynamic
Ea RT
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Ea 1 log A 2.303R T
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(4)
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activation parameters of this transition state and can be described by the Eyring equation [24]:
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k
12
where k is the rate constant, ΔG is the standard Gibbs free energy of activation, h is Planck’s constant, ΔS
13
is the standard entropy of activation, ΔH is the standard enthalpy of activation, kb is the Boltzmann
14
constant, R is the gas constant, and T is the absolute temperature in Kelvin. Taking the logarithm of both
15
sides of Eq (6), one obtains:
16
log ( ) log(
17
If the plot of the left-hand side of Eq (7) versus 1/T is linear, one can compute the value of ΔH from the
18
slope, ΔS from the ordinate intercept and the Gibbs free energy of activation (ΔG) by the relation:
19
ΔG=ΔH-TΔS
H
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k T
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S
kbT RTG k bT R RT e = e e h h
d
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kb S H 1 ) h 2.303R 2.303R T
(6)
(7)
(8)
6 Page 6 of 30
1
From Eq 4 we can define the energy of activation as:
2
Ea R
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The substitution of k in Eq (9) with the k equivalent relation in Eq (6) and differentiation of this new
4
expression with respect to 1/T shows that the Arrhenius (Eq 4) and the Eyring (Eq 6) expressions can be
5
related as:
6
Ea=ΔH+RT
7
A(
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Taking the logarithm of both sides of Eq11, one obtains:
9
log A log(
ln k 1 T
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(9)
(10)
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and S
(11)
(12)
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ekT S ) h 2.303R
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ek bT R )e h
suggesting a linear relation between the logarithm of the Arrhenius frequency factor and entropy of
11
activation obtained from the Eyring equation for the transitional state.
12
The first-order kinetic model (Eq 3) was used to determine the best fits of the data with the apparent rate
13
constant k. The Arrhenius and Eyring equations were then applied to the data in order to determine Ea, A,
14
ΔG, ΔH, and ΔS of the temperature-dependent viability transitions.
15
For two temperature points T1 and T2, it follows from Eq 5 that
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(13)
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(14) 7 Page 7 of 30
Substituting logA in Eq 13 from Eq 12, it follows that entropy (15)
2
3
Enthalpy H is found from Eq 6 as (16)
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Using S from Eq 15 and H from Eq 16, the Gibbs free energy of activation is calculated from
6
Eq 8.
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3. Results
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3.1 Thermodynamics of erythrocyte vesiculation in the human case study A human erythrocyte loses 15 μm3 of volume during its lifespan (120 days) [25]. The volume of a single
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vesicle (v) is estimated as v=πdv3/6= π(0.403)3=3.43×10-2 μm3. If all lost erythrocyte volume (L) was
11
converted to vesicles, the number of produced vesicles (n) during the erythrocyte lifespan n=L/v=19.47
12
μm3/4.49×10-2 μm3=437.
13
If surface area of a single vesicle, s=πds2 =π (0.377)2=0.446 μm2, the total area (S) of 437 vesicles would
14
be equal to 437×0.446 μm2=194 μm2 that is much higher than the total area of erythrocyte (136 μm2).
15
During a lifespan, erythrocyte reduces its surface area by 20% [22], what amounts to the area loss
16
Ls=0.2×136 μm2=27.2 μm2. Comparing the estimated total area S=194 μm2 to the experimental loss Ls=
17
27.2 μm2, one can conclude that only 27.2/194≈14% of lost area are converted to vesicles. A similar
18
estimate made for rat erythrocytes shows that only 8% of lost erythrocyte material is lost by shedding
19
vesicles.
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1
When the human core temperature rises during exercise from 37.1 to 38.1, 37.1 to 39.2, and 37.1 to 40.0
2
o
3
8.17×10-3, 2.22×10-2, and 4.46×10-2 min-1, respectively (Fig.1 a).
4
The percent of echinocytes in blood is also increased with elevated temperature (Fig. 1b), so there is a
5
linear relationship between vesicle concentration and percent of erythrocyte-echinocyte transitions (Fig.
6
1c). Fig. 1d and 1e show the frequency and cumulative frequency distributions of vesicle concentration
7
and diameters at 37.1 oC, respectively. A comparison of the cumulative frequency distribution of vesicle
8
concentration at 37.1 oC (before exercise) and those at temperatures of 38.1, 39.2 and 40.0 oC (after
9
exercise) is shown in Figure 1f. When the temperature increases, the distribution moves toward higher
10
vesicle concentrations. Vesicle concentration and diameter are shown in comparison with the literature
11
data in Table 1.
12
The graph of the logarithm of the apparent rate constants as a function of 1000/T (Eq 5) represents the
13
Arrhenius plot shown in Fig. 2 a. The slope of the plot yields the activation energy (Ea), the thermal
14
activation level of vesiculation and logA from the ordinate intersection. The same rate constants were
15
used to generate the Eyring plots shown in Fig. 2 b. The linear plot of the left-hand side of Eq 7 vs 1/T
16
brings the value of H from the slope, S from the ordinate intercept and the Gibbs free energy of
17
activation (G) by Eq 8. Activation energy (Ea) and S can be calculated in a second way from Eqs 10
18
and 12. The calculated values of Ea, H, S, and G in the process of human erythrocyte vesiculation are
19
shown in Table 2.
20
Table 1. Comparison of properties of fresh, intact erythrocytes from rat and human blood. Property Human Rat Life span 120 days [26] 61 days [27] Diameter 7 μm [28] 6.9 μm [29] Volume 90 μm3 [28] 64.9 μm3 [29] 2 2 Surface area 136 μm [28] 103 μm [29] Concentration 5×106 μL-1 [30] 8×106 μL-1 [31] 13 11 Total number 2.5×10 (75 kg) [30] 1.3×10 (250 g) [32] Blood volume 5 L (75 kg men) [30] 16 mL (250 g rat) [32]
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C, the change of relative vesicle concentration per minute (apparent rate constant, k) is found to be
9 Page 9 of 30
Table 2. Thermodynamic analysis of erythrocyte hemolysis and vesiculation Activation Enthalpy Entropy Process T o C energy, H, S, Ea, KJ.mol-1 J.mol-1 KJ.mol-1 K-1 773.5
2,342
36.743.2
776.4
772.8
2,364
19-37
63.5
66.4
19-37
63.5
19-37
129.6
37-57
102.0
48-49 38-45 45-50 38-45 15-35 41-43
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-19.5
Free energy, ∆G, KJ.mol1
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776.1
Entropy contribution TS, KJ.mol-1 728.3
cr
37-39
45.2
736.3
36.6
-24.6
91.0
-20.9
-26.4
87.3
127.0
29.8
37.2
89.5
99.0
-5.7
-7.6
106.6
2400 131.7 342 290 206.1
2401 129.1 339.1 286 71.9
7,167 71.5 346 536 -10.9
2,305 22.5 234.5 174 -12.5
96.0 106.6 104.6 112 84.4
1266
1264
3,736
1,178
86
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60.9
d
The increase of number of vesicles in blood as a result of the human core temperature increase during exercise [12]. The increase of vesicles in rat blood as a result of environmental heating at 45 oC for 25 minutes [13]. K+ release from erythrocytes in vitro vesiculation [16]. AcCHE release during vesiculation in vitro [16]. Formation of methemoglobin in erythrocytes in vitro [16] Basal ion permeability in erythrocytes in vitro [17]. Erythrocyte osmotic fragility [10]. Erythrocyte hemolysis [14]. Erythrocyte hemolysis [14]. Erythrocyte hemolysis [15]. Discocyte-echinocyte transition [18]. Heat survival of CHO cells in cultures [19].
te
1 2 3
The experimental data on cell decomposition in references [12-19] were used for calculation of thermodynamic variables.
5 6
The concentration of free vesicles increased after exposure to heat stress from (1.47)×106 to (3.87)×106
7
vesicles/μL (Fig. 1g). The values are significantly different at 0.001 level. The ear rat temperature
8
increased during a heat exposure from 36.8 to 40.4 oC. Fig. 1 h and i show the frequency and cumulative
9
frequency distributions of vesicle concentration and diameter. The figures indicate a significant shift
3.2 Thermodynamics of erythrocyte vesiculation in rats
10
toward higher vesicle concentrations and diameter after a heat stress. The mean vesicle diameter at 36.7
11
o
C is 0.44 μm, and at 40.3 oC is 0.56 μm.
10 Page 10 of 30
Rat erythrocytes lose 30% of volume during its lifespan [31]. Taking a rat erythrocyte volume from Table
2
1, the lost volume, L v= 0.3×64.9 μm3 =19,47 μm3. The volume of a single vesicle (v) is estimated as
3
v=πdv3/6= π(0.441)3=4.49×10-2 μm3. If all lost erythrocyte volume was converted to vesicles, the number
4
of produced vesicles during the erythrocyte lifespan would be equal n =L/v=19.47 μm3/4.49×10-2
5
μm3=434.
6
If the surface area of a single vesicle, s=πds2 =π (0.438)2=0.602 μm2, the total area (S) of 434 vesicles
7
would be equal to 434×0.602 μm2=261 μm2 that is much higher than the whole area of erythrocyte (103
8
μm2). During a lifespan, erythrocytes reduce their surface area by 20% [31], which amounts to the area
9
loss Ls=0.2×103 μm2=20.6 μm2. Comparing the estimated total area S=261 μm2 to the experimental loss
10
Ls= 20.6 μm2, one can conclude that only 20.6/261≈8% of lost area is converted to vesicles. 8% of the lost
11
erythrocyte volume V= 8%× L v=0.8×19.47 μm3=1.54 μm3. Then, number of vesicles produced by a
12
single erythrocyte during its lifespan, n=V/v=1.54 μm3 /4.49×10-2 μm3 =34.22. Taking into account total
13
number of erythrocytes is 1.3×1011 and lifespan is 61 days (Table 1), it follows that vesicle destruction
14
rate at basal temperature of 36.7 oC, k1=1.3×1011×34.22:61:24:60≈5.0×107 min-1.
15
When the rat is exposed to heat, the rat core temperature increases from t1= 36.7 to t2 = 40.3 oC, vesicle
16
concentration rises from 1.4×106 to 3.8×106 vesicles/μL. Taking into account the duration of hyperthermia
17
(25 min) and rat blood volume (16 mL) (Table 1), the vesicle destruction rate (k2) at the transition from t1
18
to t2 can be estimated as follows
19
k2=(3.8×106-1.4×106) vesicles/μL×1.6×104μL/25 min=1.54×109 min-1.
20
Using values of k1 and k2 at absolute temperatures T1 and T2 and Eq 14, one can estimate the apparent
21
activation energy Ea=776.4 kJmol-1.
22
A value of logA calculated by Eq 13 and inserted in Eq 15 allows calculation of entropy change (S) at
23
temperature T=(T1+T2)/2 =(309.7+313.3)/2≈311.5oK (38.5 oC). S is found to be equal to 2364 Jmol-1K-1.
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Inserting the values of Ea and RT in Eq 16, one can calculate enthalpy change H=772.8 kJmol-1.
2
Finally, substituting H and S in Eq 8 with found above values, it follows that free Gibbs energy (G)
3
is equal to 36.6 kJmol-1. The calculated values of Ea, H, S, and G for the process of rat erythrocyte
4
vesiculation are shown in Table 2.
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1
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The release of potassium ions and acetylcholinesterase (AcChE) from the human erythrocytes, and
7
formation of methemoglobin that accompanied erythrocyte transformation and vesiculation were shown
8
to be sensitive to elevated temperatures [16]. From the experimental temperature dependences of these
9
three processes, the values of variables Ea, H, S, and G were calculated with formulae given in the
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3.3 Thermodynamics of erythrocyte transformation
Section 2 and showed in Table 2. Similarly, were calculated the same thermodynamic variables from
11
experiments with the temperature activation of the ion permeability during the thermal necrosis of human
12
erythrocytes [17] (Table 2). Studies on the destruction of human and canine red blood cells reviled the
13
strong temperature dependency of the erythrocyte osmotic fragility [10]. This dependence was used for
14
calculation of Ea, H, S, and G, which were also shown in Table 2. The rates of human red blood cell
15
hemolysis were measured as a function of temperature [14, 15]. The variables of Ea, H, S, and G for
16
the erythrocyte hemolysis were calculated by using the experimental rates of cell decomposition (Table
17
2). The shape of human red blood cells as a function of temperature [18] was used for calculation of
18
variables Ea, H, S, and G after discocyte-echinocyte transition of erythrocytes (Table 2). Finally, the
19
same thermodynamic variables were obtained from the experimental results on the Chinese hamster ovary
20
cultured cells [19] (Table 2).
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21 22
When an enthalpy change (H ) for vesiculation of human and rat erythrocytes calculated in this work
23
together with H related to other phenomena related to the thermal cell decomposition given in Table 2
3.4 Thermodynamic correlates of erythrocyte vesiculation
12 Page 12 of 30
1
were plotted as a function of entropy change (S), the data were fitted with a single line that corresponds
2
to the equation H=Go+ToS, where Go=97.9 kJxmole-1 and To=315.5 oK (Fig. 3).
4
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3
6
4 Discussion
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Table 3. Vesicle concentration (C ) and diameter (d) in rat and human blood at different conditions. Subject Condition Method t, oC C, μL-1 d, μm Reference Human Live Light microscopy 37.1 1.5×106 0.365 [12] Rat Live Light microscopy 36.7 1.4×106 0.436 [13] Human Storage, 5 days Flow cytometry 4 3,370
S0927-7765(15)00386-0 http://dx.doi.org/doi:10.1016/j.colsurfb.2015.06.013 COLSUB 7143
To appear in:
Colloids and Surfaces B: Biointerfaces
Received date: Revised date: Accepted date:
21-4-2015 28-5-2015 5-6-2015
Please cite this article as: V. Vodyanoy, Thermodynamic evaluation of vesicles shed by erythrocytes at elevated temperatures, Colloids and Surfaces B: Biointerfaces (2015), http://dx.doi.org/10.1016/j.colsurfb.2015.06.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights
2
Number of vesicles shed by erythrocytes at increases elevated temperatures.
3
Eighty percent of erythrocyte released vesicles are smaller than 0.4 μm.
4
Freshly drawn mammalian blood contains more than 1 million vesicles per one microliter.
5
Erythrocyte vesicles can serve as diagnostic tool of physical performance.
6
Vesicle release is driven by entropy with enthalpy-entropy compensation.
cr
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1
Ac ce p
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1 Page 1 of 30
2
Thermodynamic evaluation of vesicles shed by erythrocytes at elevated temperatures.
3
V. Vodyanoy1
4 5 6
1
7
1
8
Abstract
9
Erythrocytes undergo structural transformation and shed small vesicles at elevated temperatures. To
ip t
Department Anatomy, Physiology and Pharmacology, College of Veterinary Medicine, Auburn, AL 36849; School of Kinesiology, Auburn University, Auburn, AL 36849, The Edward Via College of Osteopathic Medicine, Auburn, AL 36849
cr
Vitaly Vodyanoy, telephone: 334-844-5405, fax: 334-844-5388, e-mail: [email protected].
us
1
characterize mechanisms of the phenomenon, the Arrhenius and Eyring equations were used for analysis
11
of the literature, experimental data on vesiculation of human and rat erythrocytes after the temperature
12
was elevated by physical exercise or by exposure to external heat. The thermodynamic analysis of the
13
data showed that erythrocyte transformation, vesicle release, and other associated processes are driven by
14
entropy with enthalpy-entropy compensation. It is suggested that the physical state of the hydrated cell
15
membrane is responsible for the compensation. The increase of vesicle number related to elevated
16
temperatures may be indicative of the heat stress level and serve as diagnostic of erythrocyte stability and
17
human performance.
18
Keywords
19
Erythrocytes, echinocytes, vesicles, entropy, microscopy, heat stress
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d
te
Ac ce p
20
an
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1. Introduction
21
In his work entitled “A heated microscope stage and its use in the investigation of the blood” Schultze [1]
22
used a compound microscope and a specially constructed wet chamber to observe and document changes
23
of blood cells at well controlled elevated temperatures (37-65 oC) under magnification up to 800 X.
24
Human subjects, dogs, cats, and rabbits were studied. He analyzed a small droplet of live blood collected 2 Page 2 of 30
from humans via fingerpick or venous blood on the glass microscope slide and carefully covered with a
2
coverslip. At the temperature of 37 oC, most of the red blood cells are oval and flexible biconcave disks of
3
about 8 microns in diameter. At elevated temperatures approaching 52 oC, he observed large
4
morphological changes in red blood cells. The red blood cells change from the biconcave discs to the
5
crenated discs, crenated spheres, and then into spheres with very small spicules. He noted that many red
6
blood cells appear to shed multiple vesicles. More than 100 years later, the major morphological changes
7
in red blood cells at elevated temperatures described by Schultze were confirmed by electron microscopy
8
[2].
us
cr
ip t
1
9
The vesiculation induced by elevated temperatures up to 49 oC in normal human erythrocytes was studied
11
with the use of a Nomarski differential interference-contrast (DIC) microscope at the original
12
magnification of 1000 X (Wagner et al., 1986). This technique allowed for the observation of real life cell
13
images. It was observed that the erythrocyte cell membrane went through spontaneous vesiculation in
14
vivo during and following exposure to heat. The cell turns into a crenated shape or becomes echinocyte,
15
with many small projections (spicules). After some time, the projections extend and pinch off to create
16
small free-floating vesicles. Based on the presented biochemical data, it was suggested that a localized
17
disruption of the normal membrane skeletal protein and lipid interaction were the essential steps in
18
vesiculation.
19
It is accepted that intracellular erythrocyte oxidation of hemoglobin to methemoglobin leads to ATP
20
depletion with follow on dephosphorylation of membrane proteins and lipids [3]. The change of K+ and
21
Cl- membrane permeability causes the loss of KCL and osmotically driven water, which in turn leads to
22
cell contraction, externalization of phosphatidylserine (the processes specific to eryptosis) [4] and
23
erythrocyte-echinocyte transitions [5]. It has been demonstrated that during vesiculation of human
24
erythrocytes in vitro, acetylcholinesterase redistributes on the cell surface and becomes enriched in the
25
released vesicles [6, 7]. Additionally, it was shown that acetylcholinesterase (AcCHE) release correlates
26
with vesiculation of red blood cells [8]. It is important to note that the process of erythrocyte hemolysis
Ac ce p
te
d
M
an
10
3 Page 3 of 30
leads to cell death, but it is very different from the programmed cell death – eryptosis. The main
2
difference lies in the fact that during hemolysis the osmotically driven water moves inside the cell,
3
causing it to swell and break it apart, while in eryptosis, water moves outside, causes cell shrinkage and
4
vesiculation. The hemolysis is an adverse cellular phenomenon, but eryptosis allows defective
5
erythrocytes to escape hemolysis and prolongs cell life [9]. When erythrocytes are subjected to the
6
hypotonic solution, as, in osmotic fragility test [10], the cells experience morphological changes that are
7
similar to those during a regular eryptosis. Under the influence of osmotic pressure, water moves from
8
inside out, and forces the cells to experience remarkable changes: the first is the appearance of small bud-
9
like protrusions on an occasional erythrocyte; the next recognizable change, many of the erythrocytes
10
show single or multiple buds; then many erythrocytes are converted to spheroid cells, and finally, buds
11
are pinched off creating many free vesicles of one micron and smaller in sizes [10].
an
us
cr
ip t
1
M
12
An essential information about the mechanism of red blood cell vesiculation was obtained by the
14
thermodynamic analysis of experiments with erythrocyte hemolysis of man, rabbits, guinea-pigs, rats,
15
sheep, cats and pigs [11]. The author came to the conclusion that the mechanism of the induced hemolysis
16
is identical for all the cell irrespective of species and depends directly on the physical state of water in the
17
cell and cell membrane.
te
Ac ce p
18
d
13
19
In order to elucidate the mechanism of erythrocyte transformation and vesiculation thermodynamic
20
analysis was applied to the experiential results on the kinetics of vesicle loss by erythrocytes from human
21
and rat blood following temperature increase generated by physical exercise or by exposure to external
22
heat [12, 13]. Additionally, other literature results obtained for the erythrocyte hemolysis [14, 15], the
23
formation of methemoglobin , the K+ and ACHE release [16], the ion permeability [17], the osmotic
24
fragility [10], the discocyte-echinocyte transition [18], and the heat survival of the Chinese hamster ovary
25
cultured cells [19] have been analyzed. This work may shed some light on mechanisms of heat stress.
4 Page 4 of 30
1 2
The thermodynamic analysis of transformations in live cells is an important tool for the quantitative study
3
of the energy transductions that occur during these changes. It allows to define the nature and function of
4
the chemical processes underlying these transformations [20]. The purpose of this section is to present
5
kinetic and thermodynamic equations that can be used for calculation of activation energy, free energy,
6
enthalpy, and entropy changes in cells at different temperatures. In the next section, these thermodynamic
7
variables will be calculated from the literature experimental data.
8
During a normal lifespan, an erythrocyte reduces its volume and surface area in part by a process of
9
vesiculation – losing free vesicles. In steady-state conditions, the number of vesicles is constant because
an
us
cr
ip t
2. Thermodynamic calculations of erythrocyte vesiculation
of equilibrium between vesicles generated by erythrocytes and vesicles destructed by Kupffer cell
11
mechanisms [21, 22].
12
First-order kinetics can describe the apparent rate of vesicle destruction at constant temperature
13
dN kN dt
14
where N is the concentration of vesicles being destructed, k represents the first-order rate constant, and t,
15
the time of the thermal destruction. The solution of the equation Eq (1) can be expressed as
16
N / N 0 e kt
17
where N 0 is the initial concentration of vesicles and N / N 0 represents the ratio of the concentration of
18
destructed vesicles to the initial vesicle concentration. This fraction defines the ratio of intact vesicles.
19
Taking the logarithm of both sides of Eq (2), one obtains
20
log( N / N 0 ) kt / 2.303
(1)
Ac ce p
te
d
M
10
(2)
(3)
5 Page 5 of 30
1
A plot of the left-hand side of Eq (3) versus time (t) yields an estimate of k from the slope. The Arrhenius
2
equation [23] can be used to express the temperature dependence of the first-order activation kinetics:
3
k Ae
4
where R is the universal gas constant, Ea represents the apparent activation energy, and A -- the pre-
5
exponential Arrhenius factor. Taking the logarithm of Eq (4) yields:
6
log k
7
If the logarithm of k in Eq (5) is plotted against the reciprocal of temperature, 1/T, then the slope of this
8
graph yields the activation energy (Ea), the thermal activation level of transitions from the intact to the
9
destroyed vesicle. The rate constant of the vesicle destruction process depends on the thermodynamic
Ea RT
cr
Ea 1 log A 2.303R T
ip t
(4)
us
(5)
M
an
activation parameters of this transition state and can be described by the Eyring equation [24]:
11
k
12
where k is the rate constant, ΔG is the standard Gibbs free energy of activation, h is Planck’s constant, ΔS
13
is the standard entropy of activation, ΔH is the standard enthalpy of activation, kb is the Boltzmann
14
constant, R is the gas constant, and T is the absolute temperature in Kelvin. Taking the logarithm of both
15
sides of Eq (6), one obtains:
16
log ( ) log(
17
If the plot of the left-hand side of Eq (7) versus 1/T is linear, one can compute the value of ΔH from the
18
slope, ΔS from the ordinate intercept and the Gibbs free energy of activation (ΔG) by the relation:
19
ΔG=ΔH-TΔS
H
Ac ce p
k T
te
S
kbT RTG k bT R RT e = e e h h
d
10
kb S H 1 ) h 2.303R 2.303R T
(6)
(7)
(8)
6 Page 6 of 30
1
From Eq 4 we can define the energy of activation as:
2
Ea R
3
The substitution of k in Eq (9) with the k equivalent relation in Eq (6) and differentiation of this new
4
expression with respect to 1/T shows that the Arrhenius (Eq 4) and the Eyring (Eq 6) expressions can be
5
related as:
6
Ea=ΔH+RT
7
A(
8
Taking the logarithm of both sides of Eq11, one obtains:
9
log A log(
ln k 1 T
us
cr
ip t
(9)
(10)
an
and S
(11)
(12)
te
ekT S ) h 2.303R
d
M
ek bT R )e h
suggesting a linear relation between the logarithm of the Arrhenius frequency factor and entropy of
11
activation obtained from the Eyring equation for the transitional state.
12
The first-order kinetic model (Eq 3) was used to determine the best fits of the data with the apparent rate
13
constant k. The Arrhenius and Eyring equations were then applied to the data in order to determine Ea, A,
14
ΔG, ΔH, and ΔS of the temperature-dependent viability transitions.
15
For two temperature points T1 and T2, it follows from Eq 5 that
Ac ce p
10
16
(13)
17
(14) 7 Page 7 of 30
Substituting logA in Eq 13 from Eq 12, it follows that entropy (15)
2
3
Enthalpy H is found from Eq 6 as (16)
cr
4
ip t
1
Using S from Eq 15 and H from Eq 16, the Gibbs free energy of activation is calculated from
6
Eq 8.
an
us
5
3. Results
8
3.1 Thermodynamics of erythrocyte vesiculation in the human case study A human erythrocyte loses 15 μm3 of volume during its lifespan (120 days) [25]. The volume of a single
d
9
M
7
vesicle (v) is estimated as v=πdv3/6= π(0.403)3=3.43×10-2 μm3. If all lost erythrocyte volume (L) was
11
converted to vesicles, the number of produced vesicles (n) during the erythrocyte lifespan n=L/v=19.47
12
μm3/4.49×10-2 μm3=437.
13
If surface area of a single vesicle, s=πds2 =π (0.377)2=0.446 μm2, the total area (S) of 437 vesicles would
14
be equal to 437×0.446 μm2=194 μm2 that is much higher than the total area of erythrocyte (136 μm2).
15
During a lifespan, erythrocyte reduces its surface area by 20% [22], what amounts to the area loss
16
Ls=0.2×136 μm2=27.2 μm2. Comparing the estimated total area S=194 μm2 to the experimental loss Ls=
17
27.2 μm2, one can conclude that only 27.2/194≈14% of lost area are converted to vesicles. A similar
18
estimate made for rat erythrocytes shows that only 8% of lost erythrocyte material is lost by shedding
19
vesicles.
Ac ce p
te
10
8 Page 8 of 30
1
When the human core temperature rises during exercise from 37.1 to 38.1, 37.1 to 39.2, and 37.1 to 40.0
2
o
3
8.17×10-3, 2.22×10-2, and 4.46×10-2 min-1, respectively (Fig.1 a).
4
The percent of echinocytes in blood is also increased with elevated temperature (Fig. 1b), so there is a
5
linear relationship between vesicle concentration and percent of erythrocyte-echinocyte transitions (Fig.
6
1c). Fig. 1d and 1e show the frequency and cumulative frequency distributions of vesicle concentration
7
and diameters at 37.1 oC, respectively. A comparison of the cumulative frequency distribution of vesicle
8
concentration at 37.1 oC (before exercise) and those at temperatures of 38.1, 39.2 and 40.0 oC (after
9
exercise) is shown in Figure 1f. When the temperature increases, the distribution moves toward higher
10
vesicle concentrations. Vesicle concentration and diameter are shown in comparison with the literature
11
data in Table 1.
12
The graph of the logarithm of the apparent rate constants as a function of 1000/T (Eq 5) represents the
13
Arrhenius plot shown in Fig. 2 a. The slope of the plot yields the activation energy (Ea), the thermal
14
activation level of vesiculation and logA from the ordinate intersection. The same rate constants were
15
used to generate the Eyring plots shown in Fig. 2 b. The linear plot of the left-hand side of Eq 7 vs 1/T
16
brings the value of H from the slope, S from the ordinate intercept and the Gibbs free energy of
17
activation (G) by Eq 8. Activation energy (Ea) and S can be calculated in a second way from Eqs 10
18
and 12. The calculated values of Ea, H, S, and G in the process of human erythrocyte vesiculation are
19
shown in Table 2.
20
Table 1. Comparison of properties of fresh, intact erythrocytes from rat and human blood. Property Human Rat Life span 120 days [26] 61 days [27] Diameter 7 μm [28] 6.9 μm [29] Volume 90 μm3 [28] 64.9 μm3 [29] 2 2 Surface area 136 μm [28] 103 μm [29] Concentration 5×106 μL-1 [30] 8×106 μL-1 [31] 13 11 Total number 2.5×10 (75 kg) [30] 1.3×10 (250 g) [32] Blood volume 5 L (75 kg men) [30] 16 mL (250 g rat) [32]
Ac ce p
te
d
M
an
us
cr
ip t
C, the change of relative vesicle concentration per minute (apparent rate constant, k) is found to be
9 Page 9 of 30
Table 2. Thermodynamic analysis of erythrocyte hemolysis and vesiculation Activation Enthalpy Entropy Process T o C energy, H, S, Ea, KJ.mol-1 J.mol-1 KJ.mol-1 K-1 773.5
2,342
36.743.2
776.4
772.8
2,364
19-37
63.5
66.4
19-37
63.5
19-37
129.6
37-57
102.0
48-49 38-45 45-50 38-45 15-35 41-43
Ac ce p
4
us
an
-19.5
Free energy, ∆G, KJ.mol1
ip t
776.1
Entropy contribution TS, KJ.mol-1 728.3
cr
37-39
45.2
736.3
36.6
-24.6
91.0
-20.9
-26.4
87.3
127.0
29.8
37.2
89.5
99.0
-5.7
-7.6
106.6
2400 131.7 342 290 206.1
2401 129.1 339.1 286 71.9
7,167 71.5 346 536 -10.9
2,305 22.5 234.5 174 -12.5
96.0 106.6 104.6 112 84.4
1266
1264
3,736
1,178
86
M
60.9
d
The increase of number of vesicles in blood as a result of the human core temperature increase during exercise [12]. The increase of vesicles in rat blood as a result of environmental heating at 45 oC for 25 minutes [13]. K+ release from erythrocytes in vitro vesiculation [16]. AcCHE release during vesiculation in vitro [16]. Formation of methemoglobin in erythrocytes in vitro [16] Basal ion permeability in erythrocytes in vitro [17]. Erythrocyte osmotic fragility [10]. Erythrocyte hemolysis [14]. Erythrocyte hemolysis [14]. Erythrocyte hemolysis [15]. Discocyte-echinocyte transition [18]. Heat survival of CHO cells in cultures [19].
te
1 2 3
The experimental data on cell decomposition in references [12-19] were used for calculation of thermodynamic variables.
5 6
The concentration of free vesicles increased after exposure to heat stress from (1.47)×106 to (3.87)×106
7
vesicles/μL (Fig. 1g). The values are significantly different at 0.001 level. The ear rat temperature
8
increased during a heat exposure from 36.8 to 40.4 oC. Fig. 1 h and i show the frequency and cumulative
9
frequency distributions of vesicle concentration and diameter. The figures indicate a significant shift
3.2 Thermodynamics of erythrocyte vesiculation in rats
10
toward higher vesicle concentrations and diameter after a heat stress. The mean vesicle diameter at 36.7
11
o
C is 0.44 μm, and at 40.3 oC is 0.56 μm.
10 Page 10 of 30
Rat erythrocytes lose 30% of volume during its lifespan [31]. Taking a rat erythrocyte volume from Table
2
1, the lost volume, L v= 0.3×64.9 μm3 =19,47 μm3. The volume of a single vesicle (v) is estimated as
3
v=πdv3/6= π(0.441)3=4.49×10-2 μm3. If all lost erythrocyte volume was converted to vesicles, the number
4
of produced vesicles during the erythrocyte lifespan would be equal n =L/v=19.47 μm3/4.49×10-2
5
μm3=434.
6
If the surface area of a single vesicle, s=πds2 =π (0.438)2=0.602 μm2, the total area (S) of 434 vesicles
7
would be equal to 434×0.602 μm2=261 μm2 that is much higher than the whole area of erythrocyte (103
8
μm2). During a lifespan, erythrocytes reduce their surface area by 20% [31], which amounts to the area
9
loss Ls=0.2×103 μm2=20.6 μm2. Comparing the estimated total area S=261 μm2 to the experimental loss
10
Ls= 20.6 μm2, one can conclude that only 20.6/261≈8% of lost area is converted to vesicles. 8% of the lost
11
erythrocyte volume V= 8%× L v=0.8×19.47 μm3=1.54 μm3. Then, number of vesicles produced by a
12
single erythrocyte during its lifespan, n=V/v=1.54 μm3 /4.49×10-2 μm3 =34.22. Taking into account total
13
number of erythrocytes is 1.3×1011 and lifespan is 61 days (Table 1), it follows that vesicle destruction
14
rate at basal temperature of 36.7 oC, k1=1.3×1011×34.22:61:24:60≈5.0×107 min-1.
15
When the rat is exposed to heat, the rat core temperature increases from t1= 36.7 to t2 = 40.3 oC, vesicle
16
concentration rises from 1.4×106 to 3.8×106 vesicles/μL. Taking into account the duration of hyperthermia
17
(25 min) and rat blood volume (16 mL) (Table 1), the vesicle destruction rate (k2) at the transition from t1
18
to t2 can be estimated as follows
19
k2=(3.8×106-1.4×106) vesicles/μL×1.6×104μL/25 min=1.54×109 min-1.
20
Using values of k1 and k2 at absolute temperatures T1 and T2 and Eq 14, one can estimate the apparent
21
activation energy Ea=776.4 kJmol-1.
22
A value of logA calculated by Eq 13 and inserted in Eq 15 allows calculation of entropy change (S) at
23
temperature T=(T1+T2)/2 =(309.7+313.3)/2≈311.5oK (38.5 oC). S is found to be equal to 2364 Jmol-1K-1.
Ac ce p
te
d
M
an
us
cr
ip t
1
11 Page 11 of 30
Inserting the values of Ea and RT in Eq 16, one can calculate enthalpy change H=772.8 kJmol-1.
2
Finally, substituting H and S in Eq 8 with found above values, it follows that free Gibbs energy (G)
3
is equal to 36.6 kJmol-1. The calculated values of Ea, H, S, and G for the process of rat erythrocyte
4
vesiculation are shown in Table 2.
ip t
1
5 6
The release of potassium ions and acetylcholinesterase (AcChE) from the human erythrocytes, and
7
formation of methemoglobin that accompanied erythrocyte transformation and vesiculation were shown
8
to be sensitive to elevated temperatures [16]. From the experimental temperature dependences of these
9
three processes, the values of variables Ea, H, S, and G were calculated with formulae given in the
an
us
cr
3.3 Thermodynamics of erythrocyte transformation
Section 2 and showed in Table 2. Similarly, were calculated the same thermodynamic variables from
11
experiments with the temperature activation of the ion permeability during the thermal necrosis of human
12
erythrocytes [17] (Table 2). Studies on the destruction of human and canine red blood cells reviled the
13
strong temperature dependency of the erythrocyte osmotic fragility [10]. This dependence was used for
14
calculation of Ea, H, S, and G, which were also shown in Table 2. The rates of human red blood cell
15
hemolysis were measured as a function of temperature [14, 15]. The variables of Ea, H, S, and G for
16
the erythrocyte hemolysis were calculated by using the experimental rates of cell decomposition (Table
17
2). The shape of human red blood cells as a function of temperature [18] was used for calculation of
18
variables Ea, H, S, and G after discocyte-echinocyte transition of erythrocytes (Table 2). Finally, the
19
same thermodynamic variables were obtained from the experimental results on the Chinese hamster ovary
20
cultured cells [19] (Table 2).
Ac ce p
te
d
M
10
21 22
When an enthalpy change (H ) for vesiculation of human and rat erythrocytes calculated in this work
23
together with H related to other phenomena related to the thermal cell decomposition given in Table 2
3.4 Thermodynamic correlates of erythrocyte vesiculation
12 Page 12 of 30
1
were plotted as a function of entropy change (S), the data were fitted with a single line that corresponds
2
to the equation H=Go+ToS, where Go=97.9 kJxmole-1 and To=315.5 oK (Fig. 3).
4
ip t
3
6
4 Discussion
an
M
7
us
cr
5
Table 3. Vesicle concentration (C ) and diameter (d) in rat and human blood at different conditions. Subject Condition Method t, oC C, μL-1 d, μm Reference Human Live Light microscopy 37.1 1.5×106 0.365 [12] Rat Live Light microscopy 36.7 1.4×106 0.436 [13] Human Storage, 5 days Flow cytometry 4 3,370
