Covalent binding of metabolites of 4-ipomeanol to rabbit pulmonary and hepatic microsomal proteins and to the enzymes of the pulmonary cytochrome P-450-dependent monooxygenase system.

The covalent binding of metabolites of 4-ipomeanol, a potent lung toxin, to proteins in rabbit pulmonary and hepatic microsomal preparations and in purified monooxygenase systems was investigated. The rate of binding was 12-fold greater in pulmonary preparations than in hepatic preparations. Covalent binding in pulmonary microsomal fractions was inhibited 39 to 49% by antibodies to rabbit pulmonary cytochrome P-450II or P-450I and 90% by antibodies to cytochrome P-450 reductase. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and scintillation autoradiography of pulmonary microsomal proteins revealed the presence of heavily labeled bands throughout the molecular weight range (Mr) examined. Two of these bands corresponded in mobility to pulmonary cytochrome P-450I (Mr 52,000) and P-450II (Mr 58,000). In addition, there was a great deal of binding associated with very high molecular weight proteins, probably in the form of cross-linked aggregates which were unable to penetrate the gel matrix. In the absence of cofactor, no binding was observed. Binding was decreased by the addition of the following: antireductase greater than glutathione = NADH (without NADPH) greater than anti-II greater than anti-I. The electrophoretic patterns of the proteins from incubation of [3H]-4-ipomeanol with purified pulmonary P-450-dependent monooxygenase enzymes were also examined. In the complete system, the majority of the binding was associated with high molecular weight species located at the origin and with low molecular weight species that migrated with the tracking dye. In the absence of cofactor, some binding to proteins that corresponded with cytochrome P-450 and P-450 reductase was observed. Protease digestion of incubation mixtures resulted in the migration of all bound material at the dye front.