IN-VITRO INHIBITION OF MIDGUT MICROSOMAL P450S FROM SPODOPTERA-ERIDANIA CATERPILLARS BY DEMETHYLATION INHIBITOR FUNGICIDES AND PLANT-GROWTH REGULATORS

The demethylation inhibitor fungicides diniconazole, propiconazole, and triadimefon, and the plant growth regulators uniconazole and pachlobutrazole inhibit microsomal cytochrome P450s from midguts of Spodoptera eridania sixth-instar caterpillars. The I-50 values for O-demethylation of methoxyresorufin in microsomes from caterpillars fed a diet containing 0.2% pentamethylbenzene (PMB) for 3 days ranged from 0.3 to 130 mu M, For N-demethylation of p-chloro-N-methylaniline in microsomes from caterpillars fed a control diet, i.e., without inducer, the I(50)s were from 33 to 857 mu M. There were no consistent differences in inhibitory efficacy between fungicides and plant growth regulators. The induced isozyme(s) catalyzing the O-demethylation was more sensitive than the constitutive isozyme(s) catalyzing N-demethylation to inhibition by the compounds. Inhibition of both activities was mixed, indicating the presence of more than one isozyme catalyzing each reaction. Both substrates and all inhibitors used formed complexes with oxidized microsomes and caused type 2 spectral perturbations. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting with an antibody to a Drosophila melanogaster P450 isozyme of microsomes from control and induced S. eridania also indicate the presence of several P450 isozymes; at least three, which may be P450s, were induced. Two of the control isozymes and three of the isozymes in microsomes from PMB-fed caterpillars cross-reacted strongly with the antibody. (C) 1994 Academic Press, Inc.