Int Arch Occup Environ Hlth 36,67-73 (1975) © by Springer-Verlag 1975

Effect of S-Methylfenitrothion on the Activity of Cholinesterase and on the Excretion of Its Metabolites in Rats ANNA HLADKA and VACLAV KRAMPL Institute of Industrial Hygiene and Occupational Diseases, Bratislava Received July 15, 1975 / Accepted August 5, 1975 Summary esterase

The effect of S-methylfenitrothion on the activity of cholin(ChE) of erythrocytes and plasma was studied as well as the time

of its reactivation in rats after peroral administration of 2/3 to 1/100 of LD 50 The degradation of S-methylfenitrothion in the organism of rats was evaluated according to the urinary excretion of its metabolite p-nitro-m-cresol The S-methyl isomer was found to be a more potent inhibitor of cholinesterase than fenitrothion A remarkable decrease of activity was found even after the administration of 1/100 of LD 5 0 The reactivation of Ch E after the administration of 2/3 LD 5 0 took 6 weeks. The excretion was faster and the excreted amounts of p-nitro-m-cresol were significantly higher than they were after fenitrothion administration. Key words: S-methylfenitrothion

Urinary excretion of p-nitro-m-cresol Cholinesterase inhibition and reactivation.

INTRODUCTION

Fenitrothion (F) of technical grade, which represents the active component of a series of commercial insecticides, usually contains S-methyl isomer of fenitrothion (SMF), chemically 0, S-dimethyl-O-( 3-methyl-4-nitrophenyl) phosphorothiolate SMF appears to be a compound possessing a high biological activity either in insects or in mammals In contrast to F, however, SMF is able to inhibite cholinesterases (ChE) directly without prior enzymatic activation After peroral administration of SMF the LD 5 0 in rats was found to be 315 mg/kg (Rosival et al , 1974), while the LD 5 0 for F was 740 mg/kg (Gaines, 1969) Kovacicov & Btora (1973) compared the values of I50 for SMF and F They found that in vitro the anticholinesterase activity of SMF for Ch E from horse serum, human serum, and fly heads was on an order of about 2 5 higher than that of F. 67

In this study the rapidity of SMF degradation was measured on the basis of the urinary excretion of p-nitro-m-cresol (PNMC) in rats Simultaneously, changes of cholinesterase activity in plasma and red cells were measured after a single administration of various doses of SMF.

MATERIALS AND METHODS Chromatographically pure SMF was emulsified in water containing Tween 80 and administered by means of oral tube to female Wistar rats (1 8 0-2 00 g) The solution was prepared immediately before administration Each control and experimental group consisted of 5 animals. 1

Estimation of PNMC in Urine

The animals were kept in all-glass

metabolic cages The urine was collected after administration of SMF in various intervals up to 72 hrs PNMC in urine was measured spectrophotometrically after its isolation with TLC on silica gel as previously described (Hladk&, 1969). 2

Estimation of ChE Activity

The blood was withdrawn under light

ether anesthesia from the abdominal aorta with heparinized syringes The activity of Ch E in plasma and erythrocytes was measured by the pH-metric method of Pickering & Martin (1970).

RESULTS The data on the urinary excretion of PNMC within 72 hrs after single administration of SMF are presented in Table 1 As shown, SMF appears to be relatively rapidly metabolized in the organism The greatest part of PNMC was excreted during 12 hrs and no more PNMC was found in the urine after 72 hrs A total of 66-76% of the theoretical amount of PNMC was excreted within 72 hrs following administration of SMF The excreted amount of PNMC was directly related to the administered dose of SMF. No statistically significant differences of average values (each value represents the mean amount of PNMC excreted in 72 hrs) were found in percent of excreted PNMC between various doses of SMF administered However, elimination of metabolite tends to occur at a faster rate with low dosage level. The activity of ChE for control groups was determined on days 0, 7, 14, 28 and 42 The average value of plasma Ch E activity in 25 control rats was 0 77 ± O 12 A pH/hr (mean ±S E ), that in red cell being 0 62 ± O 07 A pH/2 hr (mean ± S E ). The time course of inhibition and reactivation of ChE after

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Fig 1 Changes of ChE activity after administration of 200 mg SMF/kg plasma; * = ChE in erythrocytes

o = ChE in

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Fig 2 Changes of ChE activity after administration of 50 mg SMF/kg plasma; = ChE in erythrocytes

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ChE in

the administration of SMF (approximately 2/3 to 1/100 of LD 50) is depicted in Figs 1-4 It may be concluded that the velocity of return of Ch E activity to normal level is inversely related to the administered dose of SMF The degree of plasma Ch E inhibition, at low doses of SMF, is greater than it is for acetylcholinesterase Even a dose of about 1/100 LD 50 decreased significantly the plasma and red cell activity of ChE.

70

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Fig 3 Changes of ChE activity after administration of 12 5 mg SMF/kg and 12 5 mg F/kg = SMF (o = ChE in plasma; = ChE in erythrocytes). = F (o = ChE in plasma; · = Ch E in erythrocytes)

0 Fig 4 Changes of activity of ChE after administration of 3 13 mg SMF/kg Ch E in plasma;

o =

= Ch E in erythrocytes

DISCUSSION

Similarly to F (Hladk&, 1966), its S-methyl isomer gives a formation of PNMC by a biotransformation in the organism In the data presented in this study the excretion of PNMC after SMF shows a more rapid increase and the excreted amounts of PNMC are higher than those after the administration of F

71

(Hladka & Nosal, 1967) The more rapid degradation of SMF in the organism is in good agreement with several orders higher constant of hydrolysis of this compound (Kova6icov & Btora, 1973) It may be supposed that after the administration of SMF, as in a case of F, a part of the administered compounds is excreted as demethylated metabolites (Hollingsworth et al , 1967) The amounts of PNMC are proportional to the dose of SMF administered to rats as found in the case of F (Hladka & Nosal, 1967). If the changes of ChE activity after the administration of SMF to rats are compared with those found after the administration of F (Fig 3), it may be stated that after the administration of SMF the period of reactivation is specially prolonged The faster rate of Ch E reactivation following administration of F observed in this paper agrees with data reported by Miyamoto et al (1963) The reactivation of plasma Ch E after high doses of SMF shows a biphasic pattern Rosival et al (1974) showed impairment of liver cells resulting from the action of SMF Doenicke (1967) reported that pseudocholinesterases are presumably synthesized in the liver The biphasic pattern of pseudocholinesterases reactivation observed in our experiments may be related to the dates of these investigators. In conclusion it may be stated that SMF is a part of technical F which is more rapidly degraded in the organism, but shows a higher anticholinesterase activity than fenitrothion. The results show the importance of the evaluation of the toxic effect of compounds concomitant to pesticides, the presence of which may influence the toxicity of commercial product. Acknowledgements

We wish to thank Mrs

E

tolcov& for her technical

cooperation in this work.

REFERENCES Doenicke, A : Klinische Bedeutung der Pseudocholinesterase cholinesterasen, H W Goedde, A Doenicke, K Berlin-Heidelberg-New York: Springer 1967 Gaines, T B : Acute toxicity of pesticides 515-534 (1969)

In: Pseudo-

Altland, eds , pp 150-157.

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Hladk&, A , Noshl, M : The determination of the exposition to metathion (fenitrothion) on the basis of excreting its metabolite p-nitro-mcresol through urine in rats 209-214

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Kovacicov&, J , Btora, V , Truchlik, S : Hydrolysis rate and in vitro anticholinesterase activity of fenitrothion and S-methyl fenitrothion. Pesticid Sci 4, 759-763 (1973) Miyamoto, J , Sat 6, J , Kadota, T , Fujinami, A : Studies on the mode of action of organophosphorus compounds 669-676

Part II

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Pickering, R G , Martin, J G : Modification of the Michel Ap H method for the estimation of plasma, erythrocyte and brain cholinesterase activities of various species of laboratory animals 195 (1970)

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Rosival, L , Vargov&, M , Hladkh, A , Szokolayov&, J , B&tora, V , Kovaic6ovA, (in Slovak)

J : On the toxicology of S-methyl isomer of fenitrothion Proc Toxicological Section Czechoslovak Chem Assoc ,

pp.108-112, Liblice, microbiologica, No

Dipl Ing

1973 14

In: Acta hygienica, epidemiologica et

(1974)

Anna Hladk&

Institute of Industrial Hygiene and Occupational Diseases Duklianska 20 Bratislava, Czechoslovakia

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