METHOMYL EXPLANATION Methomyl was evaluated for acceptable daily intake by the Joint Meeting in 1978 and in 1986 (Annex 1, FAO/WHO, 1979a, 1986d). A temporary ADI was estimated in 1986 with a request for additional in vitro studies in human blood to demonstrate the time-course of plasma and red blood cell cholinesterase inhibition and reactivation. Data on the kinetics of human and rat acetylcholinesterase inhibition and spontaneous reactivation have been received and are summarized in this monograph addendum. EVALUATION FOR ACCEPTABLE DAILY INTAKE BIOLOGICAL DATA Special study on cholinesterase activity kinetics Acetylcholinesterase from human erythrocytes or from hemolyzed whole blood from male rats was incubated with technical grade methomyl (purity >98%) at 30°C for a fixed time period (apparently 10 minutes) and the residual activity was assayed. Three concentrations of methomyl were assayed with human enzyme and two with rat enzyme (at least duplicate assays with two concentrations with each enzyme). Control assays were run to determine the activity of the uninhibited enzymes. Enzyme activity was determined at 30°C with 18 ml saline solution (0.08 M MgCl2 and 0.2 M NaCl), 1 ml enzyme solution and 1 ml acetylcholine bromide solution (0.055 M) adjusted immediately to pH 7.6 with 0.01 M NaOH. During the reaction the pH was kept constant at 7.6 (± 0.01 pH units) by addition of the sodium hydroxide solution. The logarithm of the ratio of the activity of inhibited enzyme over the activity of uninhibited enzyme was plotted against time and the slope determined. The rate constant (k1) and the I50 value (the molar concentration of inhibitor resulting in a 50% inhibition of the enzymatic activity at a fixed time of incubation, in this study (10 minutes) were calculated. Spontaneous reaction of enzymatic activity was also determined for both enzymes. Enzyme solution was incubated for 10-15 minutes at 30°C with enough methomyl to give 74-81% inhibition of human enzyme or 32-47% inhibition of rat enzyme and then passed through a Sephadex G-25 column to separate the enzyme and the methomyl. Fractions 5 and 6 containing the enzyme were combined and incubated. Periodically, aliquots were removed for cholinesterase activity measurement. From the slope of the least squares line of log % enzyme inhibited versus time, the regeneration constant (k3) was determined and the time for one-half of the enzyme activity to be regenerated (t1/2) was calculated. The human enzyme was about six times more sensitive to methomyl inhibition than the rat enzyme. I50 for human enzyme was 2.65 x 10-6M while that for rat enzyme was 1.56 x 10-5M. Rat enzyme regenerated about 1.4 times faster than the human enzyme; t1/2 = 26.6 min for rat vs. 38.0 minutes for humans (Carakostas, 1987). Actual Cholinesterase Kinetics Inhibition K1 (M-1 min-1) I50 Human 2.96 x 104 2.65 x 10-6 Rat 4.73 x 103 1.56 x 10-5 Regeneration K3 (min-1) t1/2 (min-1) Human 1.85 x 10-2 38.0 Rat 2.62 x 10-2 26.6 COMMENTS Rate constants of inhibition of human and rat AChE by methomyl and its spontaneous reactivation have been reported. The human enzyme is about five times more sensitive to methomyl inhibition than the rat, but the rates of spontaneous reactivation do not differ in the two species. TOXICOLOGICAL EVALUATION Level causing no toxicological effect Mouse: 50 ppm in the diet, equal to 8.7 mg/kg bw/day Rat: 50 ppm in the diet, equivalent to 2.5 mg/kg bw/day Dog: 100 ppm in the diet, equal to 3.1 mg/kg bw/day. Estimate of acceptable daily intake for humans 0-0.03 mg/kg bw. Studies which will provide information valuable in the continued evaluation of the compound Observations in humans. REFERENCE Carakostas, M.C. (1987) Inhibition and regeneration kinetics for human and rat acetyl-cholinesterase exposed to methomyl in vitro. Unpublished Report No. HLR-379-88 from Haskell Laboratory. Submitted to WHO by E.I. du Pont de Nemours and Co., Inc., Washington, Delaware, USA.
See Also: Toxicological Abbreviations Methomyl (EHC 178, 1996) Methomyl (HSG 97, 1995) Methomyl (ICSC) Methomyl (WHO Pesticide Residues Series 5) Methomyl (Pesticide residues in food: 1976 evaluations) Methomyl (Pesticide residues in food: 1977 evaluations) Methomyl (Pesticide residues in food: 1978 evaluations) Methomyl (Pesticide residues in food: 1986 evaluations Part II Toxicology) Methomyl (JMPR Evaluations 2001 Part II Toxicological)