Inhibition of human erythrocyte and plasma cholinesterases by methamidophos.

Methamidophos (O, S-dimethyl phosphoramidothioate) inhibition of human erythrocyte and plasma cholinesterase was studied in vitro. The pseudo-biomolecular rate constant of inhibition was 3.14 +/- 0.22 X 10(3) M-1 min-1 for erythrocyte cholinesterase and 1.85 +/- 0.01 X 10(3) M-1 min-1 for plasma cholinesterase. Thus methamidophos, although quite toxic to mammals, is a slow, rather non-selective inhibitor of human cholinesterases, as was also observed previously in rats.