Liver Glutamine Metabolism DIETER HÄUSSINGER From the Medizinische Universitätsklinik,

ABSTRACT. A fundamental conceptional change in the field of hepatic glutamine metabolism is derived from an understanding of the unique regulatory properties of hepatic glutaminase, the occurrence of glutamine cycling, and the discovery of marked hepatocyte heterogeneities in nitrogen metabolism, with metabolic interactions between differently localized subacinar hepatocyte populations. This change provided new insight into the role of the liver in maintaining ammonia and bicarbonate homeostasis under physiologic and pathologic conditions. Glutamine synthetase is present only in a specialized cell population at the hepatic venous outflow of the liver acinus; these cells act as scavengers for ammonia and probably also for

Freiburg, Federal Republic of Germany

various signal molecules ("perivenous scavenger cell hypothesis"). The function of mitochondrial glutaminase is that of a pH- and hormone-modulated ammonia amplification system that controls carbamoylphosphate synthesis and urea cycle flux in periportal hepatocytes. Not only is hepatic glutamine metabolism essential for maintenance of bicarbonate and ammonia homeostasis, but glutamine itself can act in the liver as a signal modulating hepatic metabolism. This article summarizes some major aspects of hepatic glutamine metabolism, based on previous reviews.( Journal of Parenteral and Enteral Nutrition

:56S-625, 1990) 14

The functional units of the liver are the so-called acini, erogeneously distributed among subacinar cell populawhich extend from the terminal portal venule along the tions. On the other hand, other amino acids, such as sinusoids to the terminal hepatic venule. Periportal he- proline, alanine, and glutamine, are taken up predomipatocytes (near the sinusoidal inflow) can be distin- nantly by periportal hepatocytes,7,9,15 ie, into a compartguished from perivenous hepatocytes located more down- ment with high gluconeogenic and ureagenic capacity.’,’ The reciprocal distribution of urea cycle enzymes and stream (near the sinusoidal outflow). Periportal and perivenous hepatocytes differ in their enzyme equipment glutamine synthetase has been shown for rat, mouse, and and their metabolic functions (&dquo;functional hepatocyte human liver and seems to be unique to mammalian heterogeneity,&dquo; &dquo;metabolic zonation&dquo;).1-6 This hepato- liver.18 The complementary distribution of carbamoylcyte heterogeneity is remarkably developed with respect phosphate synthetase and glutamine synthetase in the rat liver acinus is controlled at a pretranslational to nitrogen metabolism.1-6 In the intact liver acinus, urea 18 synthesis occurs and glutaminase is present in periportal level.l3, 14, The ontogenesis of this enzymic zonation is whereas is found hepatocytes, glutamine synthetase only developmentally regulated and seems to be related to the in perivenous hepatocytes. This difference was evidenced development of hepatic acinar architecture, rather than in the structurally and metabolically intact perfused rat being a result of perinatal adaptation.&dquo; Zonal dynamics liver by comparison of metabolic fluxes during antegrade are reviewed elsewhere.1,4,5 and retrograde perfusion’ in experiments with zonal liver SCAVENGER FUNCTION OF GLUTAMINE SYNTHETASE damage;8~9 by immunohistochemistry of carbamoylphosphate synthetase,10 of argininosuccinate synthetase,11 of In the intact liver acinus, the two major ammoniaarginase,11 and of glutamine synthetase 12 ; and, more resystems, urea and glutamine synthesis, are detoxicating cently, by means of mRNA in situ hybridization.13,14 As switched behind each other. Accordingly, shown by immunohistochemistry, the borderline be- anatomically the portal blood first encounters periportal hepatocytes tween the periportal urea-synthesizing and the pericapable of urea synthesis before reaching glutaminevenous glutamine-synthesizing compartment is very synthesizing cells just at the venous outflow of the acinar strict;lo,12 glutamine synthetase is found exclusively in a bed.~ In functional terms, this organization represents small hepatocyte population (about 7% of all hepatocytes the sequence of a periportal low-affinity, but high-capacof an acinus) surrounding the terminal hepatic venule. and a perivenous high-affinity These cells are virtually free of urea cycle enzymes. As ity system (ureagenesis) for ammonia detoxification (glutamine synthesystem shown in label-incorporation studiesl5-1; and by autosis) 3.7.8 The difference in ammonia affinity between

radiography (Fig. 1, Stoll, Buscher, Haussinger, unpublished data), vascular glutamate, aspartate, and a-oxoglutarate are taken up almost exclusively by this small perivenous cell population, but not by the much larger periportal cell population capable of urea synthesis. Thus, plasma membrane transport systems are also het-

Reprint requests: Prof Dr Dieter Haussinger, Medizinische Universitätsklinik, Hugstetterstrasse 55, D-7800 Freiburg, Federal Republic of Germany.

urea synthesis and perivenous glutamine syn1-2 mM2° for NH4~ thesis results from the higher Km of carbamoylphosphate synthetase, the rate-controlling enzyme of the urea cycle 2’ as compared with that for isolated glutamine synthetase (Km 0.3 mM).22 A similar difference in Ko_5 values of urea and glutamine synthesis from NH4CI is also observed in isolated perfused rat liver15,23 and is even more pronounced in human liver (Ko.5(NH4~) for urea and glutamine synthesis are 3.6 mM

periportal

=

=

56S Downloaded from pen.sagepub.com at UNIV NEBRASKA LIBRARIES on February 3, 2015

57S and 0.11 mM, respectively). 21 Perivenous glutamine synthetase acts as a high-affinity scavenger for the ammonia that has escaped periportal detoxification bv urea synthesis.3.7.8 In perfused rat liver, about one-third of a physiologic portal ammonia load reaches perivenous hepatocy-tes and is converted into glutamine, although the first passes the large urea-synthesizing comportal blood partment.’7 Similar observations have been made in vivo.25 When ammonia is produced during amino acid breakdown in periportal hepatocytes, it is even released from these cells into the sinusoidal space, despite the enzyme activity in this compartment.’ however, is delivered via the blood stream to perivenous hepatocytes and is used for glutamine synthesis.’ These findings not only underline the comparatively low affinity of periportal urea synthesis for ammonia and the importance of ammonia scavenging by perivenous hepatocytes for efficient hepatic ammonia detoxification, but they also demonstrate metabolic in-

high

urea

cycle

This ammonia,

teractions between different subacinar cell populations.’ The important scavenger role of the perivenous glutamine synthesis for maintenance of physiologically low ammonia concentrations in hepatic venous blood becomes evident after the inhibition of glutamine synthe-

by methionine sulfoximine’ or the destruction of perivenous cells by CCl4 treatment’; hyperammonemia ensues, due to an almost complete failure to synthesize glutamine, although periportal urea synthesis is not affected. Recent data suggest that perivenous hepatocytes also play an important role in the inactivation of signal molecules, such as extracellular nucleotides and eicosanoids, thereby extending their well-documented scavenger role for ammonia to a variety of other compounds. This finding prompted the &dquo;perivenous scavenger cell hypothesis,&dquo;26 which assumes that one function of perivenous hepatocytes is to eliminate a variety of potentially toxic and harmful compounds just before the sinusoidal blood enters the systemic circulation. In the discussion that follows, the term &dquo;perivenous scavenger cell&dquo; is synonymous with perivenous glutamine synthetase-containing hepatocyte. One major regulatory advantage arises from this structural and functional organization: flux through the urea cycle in the periportal compartment can be varied without threat of hyperammonemia, because glutamine synthesis in perivenous scavenger cells acts as an effective &dquo;back-up system&dquo; for ammonia detoxification, guaranteeing efficient ammonia detoxification even when urea cycle flux is decreased. Such an uncoupling of urea cycle flux from the vital need for maintaining systemic ammonia homeostasis is an important prerequisite for efficient and sensitive feedback control of urea synthesis by acid-base status. Independent of the portal ammonia load, bicarbonate-consuming urea synthesis in the periportal compartment is adjusted to the requirements of acid-base homeostasis, whereas more downstream peritase

scavenger cells maintain ammonia homeostasis. This arrangement provides the basis for a rapid switching of hepatic ammonia detoxification from urea to net glutamine synthesis in acidosis. Further details on the role of hepatic urea synthesis in systemic acid-base homeo-

venous

stasis are given elsewhere* ’&dquo; On the other hand, flux through the urea cycle will determine the amount of ammonia reaching the perivenous hepatocytes and will, accordingly, exert an indirect control on downstream glutamine synthesis. The capacity of’ perivenous cells to synthesize glutamine from NH-t -.- is remarkable. The highest rates of glutamine synthesis in perfused rat liver are about 0.6 Jlmol/min/g liver,16 which corresponds to 8-10 ~mol/ min/g perivenous cells. The carbon skeleton required for glutamine synthesis is, in part, provided by an almost exclusive uptake of vascular n-oxoglutarate,16 other citric acid cycle dicarboxylates (unpublished result) and glutamate15.1h.:n into these perivenous cells. AMPLIFIER FUNCTION OF GLUTAMINASE

Whereas glutamine synthetase is localized perivenously, glutaminase is found in periportal hepatocytes’ and has a joint mitochondrial localization together with carbamoylphosphate synthetase. Phosphate-dependent glutaminase is the major enzyme involved in hepatic glutamine degradation and is immunologically and kinetically different from glutaminases in other tissues.:i,;J2,;n The liver enzyme is not inhibited by its product glutamate but requires ammonia, its other product, as essential activator.2:U.B In the absence of ammonia, liver glutaminase is virtually inactive. Half-maximal ammonia activation of glutaminase occurs at the physiologic portal ammonia concentration of 0.2-0.3 mmol/liter.:J;;-:36 Thus, fluctuations of the portal ammonia concentration in the physiologic range are paralleled by activity changes of hepatic glutaminase (so-called &dquo;interorgan feed-forward&dquo; between intestinal ammonia production and hepatic glutamine breakdown).:¡;’ This remarkable feature allows glutaminase to act as an amplification system for ammonia inside the mitochondria (Fig. 2). Because urea synthesis is normally controlled by flux through carba’3’~

F!G. 1. Hepatocyte heterogeneity in glutamate uptake as demonstrated by autoradiography following [’H)glutamate injection in isolated perfused rat liver. Radioactivity is accumulated in a small cell population surrounding hepatic venules. whereas uptake of glutamate is not detectable in periportal hepatocytes (from Stoll B. Buscher HP, Haussinger D, manuscript in preparation). This autoradiographic distribution is in line with recent studies on radiolabel-incorporation kinetics, demonstrating an almost exclusive uptake of vascular glutamate

by perivenous glutamine 5ythetase-containing

Downloaded from pen.sagepub.com at UNIV NEBRASKA LIBRARIES on February 3, 2015

scavenger

cells.&dquo;

58S INTERCELLULAR GLUTAMINE CYCLING

periportal glutaminase and periveglutamine synthetase are simultaneously active, resulting in periportal breakdown and perivenous resynthesis of glutamine (so-called &dquo;intercellular glutamine cycle&dquo;).’ Glutamine cycling is under complex metabolic, hormonal, and cell volume control, involving flux changes through both periportal glutaminase and perivenous glutamine synthetase.1-4, 7,35,36,42 Accordingly, net glutamine balance across the liver is the result of opposing metabolic glutamine fluxes and can, depending on the experimental conditions, be positive, negative, or even zero. With respect to hepatic nitrogen metabolism, FIG. 2. Intercellular glutamine cycling and ureagenesis. Periportal glutaminase is activated by ammonia and acts as a pH- and hormone- intercellular glutamine cycling provides an effective means for adjusting ammonia flux into either urea or modulated ammonia amplifier inside the mitochondria. The activity of this amplifier determines flux through the urea cycle (&dquo;low-affinity glutamine according to the needs of the acid-base situafor ammonia Glutamine in In the intact liver,

nous

detoxification&dquo;).

system

cells

synthetase

perive-

for the ammonia escaping periportal urea synthesis (&dquo;high-affinity system for ammonia detoxication&dquo;). This anatomical sequence of low- and high-affinity detoxication systems uncouples urea synthesis from the primary need to maintain nontoxic ammonia levels and provides the basis for acid-base control of urea synthesis without threat of hyperammonemia. A complete conversion of a portal ammonia load into urea occurs in a well-balanced acid-base situation. Under these conditions, periportal glutamine consumption (ammonia amplifying) and perivenous glutamine synthesis (ammonia scavenging) match each other: there is no net glutamine turnover by the liver, but portal ammonia is converted efficiently into urea despite the low ammonia affinity of carbamoylphosphate synthetase.’

nous

acts as scavenger

moylphosphate synthetase,21

which depends largely on the actual ammonia concentration inside the mitochondria, amplification of mitochondrial ammonia via glutaminase flux control becomes an important determinant of urea cycle flux. This is especially relevant in view of the physiologically low ammonia concentrations, which are about one order of magnitude below the Km (ammonia) of carbamoylphosphate synthetase. Here, the low ammonia affinity of carbamoylphosphate synthetase is counteracted by amplification of the operational ammonia concentration in the mitochondria.’ In line with this, a 50% increase of urea synthesis is observed in perfused rat liver, when glutamine (0.6 mmol/liter) is added to the influent perfusate already containing a physiologic ammonia concentration. Several further features add to the functional link between hepatic glutaminase and carbamoylphosphate synthetase: (1) both enzymes are activated by N-acetylglutamate, 31 so this compound, as well as ammonia, may play a role in coordinating the activity of both enzymes; (2) there is some evidence for a channeling of glutaminase-derived ammonia into car-

bamoylphosphate synthetase4°; (3) in contrast to ammonia being delivered via the portal vein, glutaminederived ammonia is utilized for carbamoylphosphate synthesis without control by mitochondrial carbonic anhydrase V~1; and (4) as anticipated from a control of urea cycle flux by glutaminase activity, factors known to affect urea cycle flux are, indeed, associated with parallel activity changes of the &dquo;mitochondrial ammonia amplifier&dquo; glutaminase. Apart from the portal ammonia concentration, these factors include the effects of glucagon, aadrenergic agonists, vasopressin, acidosis/alkalosis,

feeding

of

a

high-protein diet.

changes. 1-4.32-1:1

and liver cell volume

tion. In

a

I

well-balanced acid-base situation, intercellular

glutamine cycling allows the maintenance of a high urea flux, despite the low affinity of carbamoylphosphate synthetase for ammonia’ and the presence of physiologically low ammonia concentrations. This is achieved by peri-

portal glutamine consumption during the mitochondrial ammonia-amplifying process, which increases urea cycle flux, whereas periportally consumed glutamine is simultaneously resynthesized by perivenous scavenger cells from ammonia that has escaped the periportal compartment (Fig. 2). In perfused rat liver and with physiologic concentrations of ammonia and glutamine, up to 30% of the urea produced is attributable to periportal glutamine breakdown.’ At normal extracellular pH, flux through glutaminase roughly matches that through glutamine synthetase.3° Thus, there is no significant net glutamine turnover, but by means of glutamine cycling the complete conversion of a physiologically low portal ammonia load into urea is achieved. In metabolic acidosis, however, flux through glutaminase decreases, resulting in a decrease of urea synthesis. This, and a simultaneous increase of perivenous glutamine synthesis, switches hepatic ammonia detoxication from urea to net glutamine production. 30,36 Because urea synthesis is a major pathway for irreversible bicarbonate elimination, 21 this mechanism

represents a pH homeostatic response by the liver in an interorgan team effort with the kidney27-30 (Fig. 3). Thus, glutaminase acts as a pH- and hormone-controlled am-

monia-amplifying system inside the mitochondria. GLUTAMINE AND LIVER CELL VOLUME REGULATION: GLUTAMINE-INDUCED CELL SWELLING AS A SIGNAL FOR METABOLISM

The concentrative

uptake

of amino acids into hepatoto osmotic cell

cytes in the postprandial phase leads

swelling,42-44 thereby activating volume-regulatory

K+

efflux mechanisms.45,46 When glutamine is added to isolated perfused rat liver, there is a rapid increase in liver weight by about 5% within 2 min. Thereafter, further cell swelling is counteracted by a volume-regulatory Km efflux, although glutamine accumulation inside the cell continues for 10-15 min (Fig. 4) until a steady-state intra/extracellular glutamine concentration gradient is built Up.12 Half-maximal cell swelling due to glutamine

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59S

FIG. 3. Hepatic NH4+ metabolism and HC03- homeostasis NH4+ and HC03- generation are ultimately linked in a 1:1 stoichiometry during protein catabolism, as is the irreversible elimination of both compounds via hepatic urea synthesis. Flux through the urea cycle is sensitively controlled by extracellular pH, [HC03 and [C02]; these mechanisms adjust bicarbonate-consuming urea synthesis to the requirements of acid-base homeostasis.’ When urea synthesis decreases relative to the rate of protein catabolism in acidosis, bicarbonate is spared, and NH4+ is excreted as such into urine (&dquo;renal ammoniage-+ nesis&dquo;), with glutamine serving as a nontoxic transport form of NH4+ from liver to kidney. When NH4+ is excreted into urine, there is no net production or consumption of a-oxoglutarate (a-OG) in the organism. Numbers in circles refer to major points of flux control by the acidbase status. In metabolic acidosis, flux through the urea cycle (reaction 1) and hepatic glutaminase (reaction 2) is decreased, whereas flux through hepatic glutamine synthetase (reaction 3) and renal glutaminase (reaction 4) is increased. This interorgan team effort between liver and kidney results in NH4+ disposal without concomitant HC03removal from the organism.64

is observed at the physiologic glutamine concentration of about 0.8 mM46a. Cell swelling and/or cell volumeregulatory K+ movements, in turn, were recently recognized as important determinants of the hepatocyte’s FIG. 4. Glutamine-induced cell swelling and volume-regulatory K+ metabolic state.42>45 Osmotic cell swelling decreases lacLivers were perfused in an open tate, pyruvate, and glucose output from the liver,45 acti- efflux in isolated perfused rat liver. with Krebs-Henseleit buffer. Concenpass&dquo;) system (&dquo;single perfusion vates glycogen synthesis,44 stimulates glutathione release trative uptake of glutamine (3 mmol/liter) leads to a rapid increase of and urea synthesis from amino acids and inhibits prote- liver weight (cell swelling due to free water influx into the cells) and olysis (Haussinger et al, unpublished data). Cell swelling elicits a subsequent volume-regulatory K+ release that parallels glutamine accumulation inside the cell thereby discontinuing cell swelling, also switches hepatic glutamine balance from net production to net uptake because of a stimulation of flux despite further intracellular glutamine accumulation.;2 through periportal glutaminase and an inhibition of per- port systems build up glutamine concentration gradients: ivenous glutamine synthesis42 (Fig. 5). It is likely that at a physiologic extracellular glutamine concentration of this activation of glutaminase is attributable to prothe cytosolic and mitochondrial concentrations 0.6 longed mitochondrial swelling, which is known to stim- are mM, about 7 and 20 mM, respectively, in rat liver in vivo ulate glutaminase.4’ Thus, glutamine interferes in a comunder and in uitro.4g~49 Therefore, glutaminase plex way with hepatic metabolism. On the one hand, glu- physiologic conditions at concentrationsoperates of glutamine tamine modifies liver metabolism via cell volume changes near its Km (glutamine) of about 28 mM.32 Control of the and/or volume-regulatory responses. On the other hand, mitochondrial steady-state glutamine concentration by glutaminase activation (&dquo;ammonia amplifying&dquo;) by the activity of glutamine transporters, in turn, regulates amino acid-induced cell swelling seems well designed to flux and, accordingly, mitochondrial amglutaminase augment urea synthesis,42 to stimulate glycogen synthe- monia One example is transport control by sis,44 and/or to inhibit glycogen breakdown&dquo; and prote- pH: at amplifying. a constant extracellular glutamine concentration olysis (unpublished result) when the liver receives an of 0.6 mM, an increase of the extracellular pH from 7.3 increased amino acid load. to 7.7 increases the mitochondrial glutamine concentration from 15 to 50 mM.49 This is paralleled by a 3- to 4GLUTAMINE TRANSPORT fold increase of flux through glutaminase.:n:36 It is interAnother important site controlling glutaminase flux esting to note that the activity of glutamine transport (ie, the activity of the ammonia amplifier) is glutamine increases under conditions known to stimulate urea syntransport across the plasma and the mitochondrial mem- thesis, such as dexamethasone treatment&dquo;4 and feeding branes.2,4,48-50 Plasma membrane transport of glutamine of a high-protein diet.&dquo;&dquo; Intercellular glutamine cycling occurs via the Na+-dependent system N , 51-53 whereas the implies opposite net glutamine movements across the glutamine transport system across the mitochondrial plasma membrane of periportal and perivenous hepatomembrane has not yet been characterized. These trans- cytes, respectively. There is some Evidence that periporDownloaded from pen.sagepub.com at UNIV NEBRASKA LIBRARIES on February 3, 2015

60S

FiG. 5. Effect of cell swelling on hepatic glutamine metabolism in isolated perfused rat liver. Livers were perfused with a medium containing [L-14C]glutamine (0.6 mmol/liter) and NH4C1 (0.5 mmol/liter). Cell swelling was induced by lowering the extracellular osmolarity from 305 mOsm/liter to 225 mOsm/liter (by decreasing the NaCI concentration of influent perfusate by 40 mmol/liter). Hypotonicity switches net glutamine release to net uptake by the liver and stimulates 14C02 production from [L-14C]glutamineY

tal

glutamine uptake

N,48,51,52

via Na+-dependent system newly synthesized glutamine cells is Na+ independent and

occurs

whereas release of

from perivenous scavenger involves facilitated diffusion.52,56 Both transport systems are inhibited by histidine.48,51,52 GLUTAMINE AND AMMONIA METABOLISM IN CHRONIC LIVER DISEASE

Although the structural-functional organization of nitrogen-metabolizing pathways in the liver acinus was established in studies mainly in the rat, there is now good evidence that the situation is similar in human liver.24,57-60 The development of hyperammonemia in chronic liver disease has been ascribed to portosystemic shunting and a loss of urea cycle enzyme activity. In line with previous reports on ureagenesis,61,62 the capacity of human liver slices to synthesize urea from NH4CI was found to be decreased by about 80% in cirrhosis, 24,63 whereas glutaminase flux (&dquo;mitochondrial ammonia-amplifying system&dquo;) was increased 4- to 5-fold .2’ Flux through glutaminase, as determined in human liver slices from patients with various degrees of liver dysfunction, is inversely correlated with the capacity to synthesize urea (Fig. 6A ). This finding suggests that activation of the &dquo;mitochondrial ammonia-amplifying system&dquo; glutaminase represents a compensatory mechanism maintaining life-compatible urea cycle flux, although the maximal ureagenic capacity is markedly reduced in liver disease.24 On the other hand. the capacity to synthesize glutamine from

~H4CI is impaired in parallel to urea synthesis,24,63 ie, by about 80% in liver cirrhosis. This fact indicates

FIG. 6. Relationship between urea synthesis and flux through glutaminase in human liver slices (A) and in vivo plasma bicarbonate concentration (B). The diagnosis of liver disease [normal liver (0), fatty liver {1), cirrhosis (M)] was evaluated histologically in patients free of renal and pulmonary disease and not medicated with diuretics or antacids. The liver disease was at a well-compensated state (no ascites, no history of gastrointestinal bleeding or manifest encephalopathy). Biochemical assessment of liver disease and patient selection are described elsewhere.24,63 Urea and glutamine synthesis and glutaminase flux were determined in slices prepared from liver biopsy specimens obtained during diagnostic laparoscopy. Urea synthesis was measured in a 3-hr incubation of liver slices with NH,CL (10 mmol/ liter), ornithine, lactate, and pyruvate), thus reflecting maximal urea synthesis capacity. On the other hand, glutaminase flux was measured as glutamine consumption in incubations containing glutamine (0.6 mmol/liter), methionine sulfoximine, and NH,CL (10 mmol/liter), ie, under nonsaturating substrate concentrations. Metabolic flux rates were linear with the incubation time and the amount of liver tissue

incubated. 24,11

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61S that pathogenesis of hyperammonemia in chronic liver disease involves a defect of the perivenous high-affinity system for ammonia detoxication and, accordingly, a perivenous scavenger cell defect. 24,63 Whether there is also a disturbance of the scavenger function for compounds other than ammonia, such as biologically highly potent signal molecules, is at present unclear. Many acute liver diseases, such as poisoning with paracetamol or viral hepatitis, are associated with the development of perivenous liver necrosis, and hyperammonemia may develop under these conditions because of a selective destruction of perivenous scavenger cells without impairment of periportal urea synthesis. This was shown in an experimentally induced perivenous necrosis following CCl4 injury.’ Because urea synthesis is a major pathway for irreversible disposal of bicarbonate,1,2,27-30 impairment of urea synthesis in liver disease may diminish hepatic bicarbonate disposal and give rise to metabolic alkalosis24,63 (Fig. 3). In line with this, in vivo plasma bicarbonate concentration increases with the progressive loss of urea cycle capacity in vitro63 (Fig. 6B ). It should be emphasized that no other causes for the development of hyperbicarbonatemia (such as renal disease, diuretic or antacid treatment, vomiting, or hyperaldosteronism) were detectable in the patients studied 61 (Fig. 6). Because alkalosis, in turn, is a potent stimulus of the ammonia amplifier glutaminase, a feedback circuit between urea synthesis, bicarbonate accumulation, and ammonia amplifying via glutaminase can be hypothesized: a decrease of urea cycle capacity leads to hyperbicarbonatemia and alkalosis that, in turn, activates glutaminase. Glutaminase activation augments urea synthesis and restores a normal urea cycle flux (despite diminished capacity to synthesize urea). Thus, a more alkaline steady state is maintained in the cirrhotic patient, which keeps glutaminase at an activated level, thereby permitting lifecompatible urea cycle flux.

during ureagenesis 8.

8:822-830, 1988

181:709-716, 1989 17.

18.

Häussinger D, Stoll B, Stehle T, et al: Hepatocyte heterogeneity in glutamate metabolism and bidirectional transport in isolated perfused rat liver. Eur J Biochem 185:181-187, 1989 Smith DD, Campbell JW: Distribution of glutamine synthetase and carbamoylphosphate synthetase I in vertebrate liver. Proc

Natl Acad Sci USA 85:160-164, 1988 19. Lamers WH, Gaasbeek-Jansen JW, te Kortschot A, et al: The development of enzymic zonation in liver parenchyma is related to the development of the acinar architecture. Differentiation 35:228-

235, 1987 20.

Lusty C: Carbamoylphosphate synthetase I of rat-liver mitochondria : Purification, properties and polypeptide molecular weight.

21.

Meijer AJ, Lof C, Ramos I,

22. 23.

25.

1990 2.

3. 4. 5.

6.

et al: Hepatic nitrogen metabolism and acid-base homeostasis. IN pH Homeostasis, Haussinger D, (ed). Academic Press, London, 1988, pp 337-377 Häussinger D, Sies H (eds): Glutamine Metabolism in Mammalian Tissues. Springer Verlag, Heidelberg, 1984 Meijer AJ, Lamers WH, Chamaleau RAFM: Nitrogen metabolism and ornithine cycle function. Physiol Rev 1989 in press Jungermann K, Katz N: Functional specialization of different hepatocyte populations. Physiol Rev 69:708-764, 1989 Traber PG, Chianale J, Gumucio JJ: Physiologic significance and regulation of hepatocellular heterogeneity. Gastroenterology 95:30-

26.

27.

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29. 30.

D: Hepatocyte heterogeneity in glutamine and ammonia metabolism and the role of an intercellular glutamine cycle

Häussinger

Eur J Biochem 85:373-383, 1978 et al: Control of ureagenesis. Eur J Biochem 148:189-196, 1985 Deuel TF, Louie M, Lerner A: Glutamine synthetase from rat liver. J Biol Chem 253:6111-6118, 1978 Häussinger D, Weiss L, Sies H: Activation of pyruvate dehydrogenase during metabolism of ammonium ions in hemoglobin-free perfused rat liver. Eur J Biochem 52:421-431, 1975 Kaiser S, Gerok W: Häussinger D: Ammonia and glutamine metabolism in human liver slices: New aspects of the pathogenesis of hyperammonemia in chronic liver disease. Eur J Clin Invest 18:535-542, 1988 Cooper AJL, Nieves E, Coleman AE, et al: Short-term metabolic fate of [ 1 N] 3 ammonia in rat livr in vivo. J Biol Chem 262:1073-

1080, 1987

Häussinger D, Meijer AJ, Gerok W,

43, 1988 7.

liver. Eur J Biochem 133:269-

10. Gaasbeek-Janzen JW, Lamers WH, Moorman AFM. et al: Immunohistochemical localization of carbamoylphosphate synthetase (ammonia) in adult rat liver. J Histochem Cytochem 32:557-564. 1984 11. Saheki T, Yagi Y, Sase M, et al: Immunohistochemical localization of argininosuccinate synthetase in the liver of control and citrullinemic patients. Biomed Res 4:235-238, 1983 12. Gebhardt R, Mecke D: Heterogeneous distribution of glutamine synthetase among rat liver parenchymal cells in situ and in primary culture. Embo J 2:567-570, 1983 13. Moorman AFM, De Boer AJ, Geerts WJC, et al: Complementary distribution of carbamoylphosphate synthetase (ammonia) and glutamine synthetase in rat liver acinus is regulated at a pretranslational level. J Histochem Cytochem 36:751-755, 1988 14. Gebhardt R, Ebert A, Bauer G: Heterogeneous expression of glutamine synthetase mRNA rat liver parenchyma by in situ hybridization and Northern blot analysis of RNA from periportal and perivenous hepatocytes. FEBS Lett 241:89-93, 1988 15. Häussinger D, Gerok W: Hepatocyte heterogeneity in glutamate uptake in isolated perfused rat liver. Eur J Biochem 136:421-425, 1983 16. Stoll B, Häussinger D: Functional hepatocyte heterogeneity: Vascular oxoglutarate is almost exclusively taken up by perivenous glutamine-synthetase-containing hepatocytes. Eur J Biochem

24.

Häussinger D: Nitrogen metabolism in liver: Structural-functional organization and physiological relevance. Biochem J 267:289-290,

rat

Interact 48:191, 1984 9. Gebhardt R. Burger HJ, Schreiber KL. et al: Alterations of hepatic enzyme levels and of acinar distribution of glutamine synthetase in response to experimental liver injury in the rat. Hepatology

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perfused

Häussinger D. Gerok W: Hepatocyte heterogeneity in ammonia metabolism: Impairment of glutamine synthetase in 4 CCl induced liver cell necrosis with no effect on urea synthesis. Chem Biol

ACKNOWLEDGMENTS

Our own studies reported herein were supported by Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 154 &dquo;Klinische und Experimentelle Hepatologie,&dquo; and the Heisenberg Program.

in

274. 1983

Häussinger D, Stehle

T: Hepatocyte heterogeneity in response to eicosanoids: The perivenous scavenger cell hypothesis. Eur J Biochem 175:395-403, 1988 Haussinger D (ed): pH Homeostasis. Academic Press. London, San Diego, 1988 Atkinson DE, Camien M: The role of urea synthesis in the removal of metabolic bicarbonate and the regulation of blood pH. Curr Top Cell Regul 21:261-302, 1982 Atkinson DE, Bourke E: The role of ureagenesis in pH homeostasis. Trends Biochem Sci 9:297-300, 1984 Häussinger D. Gerok W, Sies H: Hepatic role in pH regulation: Role of the intercellular glutamine cycle. Trends Biochem Sci

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Liver glutamine metabolism.

A fundamental conceptional change in the field of hepatic glutamine metabolism is derived from an understanding of the unique regulatory properties of...
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