Malathion carboxylesterase titer and its relationship to malathion toxicity

The titer of malathion carboxylesterase in liver and sera of rats, three strains of mice, and five groups of pretreated mice was correlated with the malathion LD50 values measured in these groups. The equation of a regression line relating enzyme titer to toxicity was used to predict the median lethal dosage of purified malathion to humans. The mean value, 3655 mg/kg, was compared with dosages ingested in actual cases of human malathion poisonings. A discrepancy was noted between the LD50 value predicted in this study and the life-threatening doses of commercial malathion reported in the clinical literature. The unexpectedly high toxicity of commercial malathion may be related to its content of esterase-inactivating impurities.