Microsomal cytochrome P-450 degradation by in vitro lipid peroxidation

In this study, the influence of in vivo lipid peroxidation (LPO) on cytochrome P-450 (P-450) degradation was investigated using rat liver microsomes. To identify the nature of P-450 degradation, three different perturbant LPO-initiation systems were employed: NADPH/ADP-Fe, cumene hydroperoxide (CHP), and 2,2'-azobis (2-amidino- propane) hydrochloride (AAPH). The results show that each of these systems readily induced P-450 degradation during in vitro LPO and that the progression and extent of the degradation increased with incubation time. However, attempts to elicit P-450 degradation by the use of hydrogen peroxide, superoxide, or hexanal failed to induce damage. Interestingly, the addition of several well-known radical scavengers and radical scavenging enzymes, including superoxide dismutase and catalase, into the incubation media provided little protection against P-450 degradation or malondialdehyde (MDA) formation. It was found, however, that sulfhydryl compounds, including GSH and substrates of P-450-dependent monooxygenases, provided varying degrees of protection. Based on the specificity of protective action, it was concluded that the structural stability of P-450 to defend against LPO requires reduced thiols and/or substrate binding. This suggests that P-450 degradation by LPO is closely related to the oxidation of certain essential thiol groups located at the substrate binding site of the P-450 molecule during LPO reaction.