Degradation and activation of parathion analogs by microsomal enzymes

A kinetic analysis of hepatic microsomal activation of parathion and chlorpyrifos in control and phenobarbital‐treated rats

“…While a single Kmapp was calculated in the dearylation experiments, it is recognized that the dearylation assay is much less sensitive; it is possible that a low Km,,, activity exists for dearylation also. The different Km,,,s observed here for desulfuration and dearylation are consistent with the reports of others, indicating that different P450s vary in their ratios of the reactions (11)(12)(13). Similar to what was reported in our previous, more limited kinetic study (8), the Vmax/Km,pp ratios (the clearance terms or efficiencies) showed that there was a greater capacity to activate the more toxic parathion and a greater capacity to detoxify the less toxic chlorpyrifos.…”

“…The K, , , , s of 0.23 and 71.3 pM reported here surround the 10.2 pM reported by others for parathion (11); likewise, the 20.0 pM reported for chlorpyrifos (6) is centered between our values of 1.64 and 50.4 pM. The very low substrate concentrations that were studied here allowed the satu- ration curve to be dissected into two activities and, therefore, the calculation of a Km,,, in the sub-to low-micromolar range.…”

Metabolism of Parathion by Two Species of Rhizobium1

“…The more toxic oxidative activation analog of parathion, paraoxon, which is nearly always formed when parathion is administered to a mammal or an insect, (Hazleton 1955, Casida 1956, Krueger et al 1960, Nakatsugawa and Dahm 1965, Nakatsugawa and Dahm 1967a, Nakatsugawa and Dahm 1967b, Neal 1967, Nakatsugawa et al 1968, Nakatsugawa et al 1969 was not detected in this investigation. Ahmed and Casida (1958) found "little if any oxidation" of parathion, Dow ET-57, dimefox, and schradan by pure cultures of the green alga Chlorella pyrenoidosa (Emerson's strain).…”

Metabolism of parathion by two species of Rhizobium

“…Dry soils which retarded microorganism activity also were slow to degrade parathion. The more toxic oxidative activation analog of parathion, paraoxon, which is nearly always formed when parathion is administered to a mammal or an insect, (Hazleton 1955, Casida 1956, Krueger et al 1960, Nakatsugawa and Dahm 1965, Nakatsugawa and Dahm 1967a, Nakatsugawa and Dahm 1967b, Neal 1967, Nakatsugawa et al 1968, Nakatsugawa et al 1969) was not detected in this investigation. Ahmed and Casida (1958) found "little if any oxidation" of parathion, Dow ET-57, dimefox, and schradan by pure cultures of the green alga Chlorella pyrenoidosa (Emerson's strain).…”