Identification of cross-linked cytochrome P-450 in rat liver microsomes by enzyme-immunoassay

Microsomal monooxygenase as a multienzyme system: the role of P450-P450 interactions

INTRODUCTION

There is increasing evidence of physical interactions (association) among cytochromes P450 in the membranes of the endoplasmic reticulum. Functional consequences of these interactions are often underestimated.

AREAS COVERED

This article provides a comprehensive overview of available experimental material regarding P450-P450 interactions. Special emphasis is given to the interactions between different P450 species and to the functional consequences of homo- and heterooligomerization.

EXPERT OPINION

Recent advances provide conclusive evidence for a substantial degree of P450 oligomerization in membranes. Interactions between different P450 species resulting in the formation of mixed oligomers with altered activity and substrate specificity have been demonstrated clearly. There are important indications that oligomerization impedes electron flow to a fraction of the P450 population, which renders some P450 species nonfunctional. Functional consequences of P450-P450 interactions make the integrated properties of the microsomal monooxygenase remarkably different from a simple summation of the properties of the individual P450 species. This complexity compromises the predictive power of the current in vitro models of drug metabolism and warrants an urgent need for development of new model systems that consider the interactions of multiple P450 species.

Competitive interactions between cytochromes P450 2A6 and 2E1 for NADPH-cytochrome P450 oxidoreductase in the microsomal membranes produced by a baculovirus expression system

The present study investigated the interactions between cytochrome P450 (P450) enzymes and the NADPH:cytochrome oxidoreductase (OR) in the microsomal membrane. Microsomes containing human cytochrome P450 2A6 (h2A6) coexpressed with human OR (hOR) via a baculovirus expression system displayed coumarin hydroxylase activity with apparent Km and Vmax values of 0.41 microM and 4.05 nmol/min/nmol P450, respectively. Incorporation of purified rat liver cytochrome b5 (b5) into the microsomes increased the Vmax 2.5-fold, but did not affect the Km. The N-nitrosodimethylamine (NDMA) demethylase activity of human cytochrome P450 2E1 (h2E1) coexpressed similarly was characterized previously. Coumarin was shown not to be a substrate nor an inhibitor of h2E1, and NDMA was not a substrate nor an inhibitor of h2A6. In microsomes containing h2A6, h2E1, and hOR (M-h2A6-h2E1-hOR) obtained from a triple expression system, the two P450 enzymes were shown to compete with each other for interaction with hOR. In incubations with M-h2A6-h2E1-hOR, the presence of a h2A6 substrate (coumarin) decreased NDMA demethylase activity by a maximum of 47%, and the presence of a h2E1 substrate (NDMA) decreased coumarin hydroxylase activity by a maximum of 19%. This substrate-induced competition between h2A6 and h2E1 was decreased by the addition of purified b5. In the absence of a substrate, the NADPH-dependent H2O2 formation was high in both M-h2A6-h2E1-hOR and M-h2E1-hOR, but low in M-h2A6-hOR. The addition of NDMA had little effect on the H2O2 formation in M-h2A6-h2E1-hOR and M-h2E1-hOR. The addition of coumarin, however, slightly decreased H2O2 formation in M-h2A6-h2E1-hOR, but drastically increased H2O2 formation in M-h2A6-hOR. These results suggest that the presence of a h2A6 substrate decreased the electron flow to h2E1 in M-h2A6-h2E1-hOR. The activities of coumarin hydroxylase and NDMA demethylase of M-h2A6-h2E1-hOR were decreased and increased, respectively, by an increase in ionic strength. The ionic strength, however, did not drastically change the substrate-induced competition between h2A6 and h2E1 for hOR. The results demonstrate the usefulness of the coexpression system for mechanistic studies and illustrate that the interaction of monooxygenase enzymes in the microsomal membrane is regulated by the presence of substrates and b5.

Cytochrome P-450LM2 oligomers in proteoliposomes

The effects of bifunctional cross-linking reagents on the purified hexameric cytochrome P-450LM2 in an aqueous medium and on the proteoliposomal cytochrome P-450LM2 have been compared. In both cases, cross-linking is shown to result in the appearance of a range of additional protein bands in SDS electrophoretograms. The number and the positions of these bands seem to be similar in the solubilized and in the proteoliposomal cytochromes. No additional bands appear when the purified cytochrome P-450 was pretreated with 0.2%. Emulgen 913, which decomposes cytochrome P-450LM2 hexamers. The results indicate that the membrane-bound cytochrome P-450 can exist in the oligomeric (presumably hexameric) form.

Abnormal proteins as the trigger for the induction of stress responses: heat, diamide, and sodium arsenite

Thermotolerance and synthesis of heat shock proteins are induced in cells in response to a variety of environmental stresses. We examined the suggestion of Hightower (1980) that modifications of intracellular proteins may be the triggering event that induces heat shock protein synthesis and thermotolerance. We did so by modifying cellular proteins, using diamide, a sulfhydryl oxidizing agent, and dithio-bis (succinimidyl propionate), an agent that cross-links bifunctional amino groups. Both of these agents induced heat shock proteins and thermotolerance in CHO (HA-1) cells. Furthermore, we observed cross-resistance and self-tolerance with three seemingly unrelated stimuli (diamide, heat, and sodium arsenite). This observation suggests that the induction of protective responses to these stimuli is mediated by a common mechanism. The results support the hypothesis that production of abnormal proteins by various stresses induces the stress responses as well as tolerance.

Sex difference in responsiveness to Aztreonam of monooxygenase system in liver microsomes from rats

Effect of successive administration of Aztreonam on microsomal monooxygenase system was investigated in male and female Sprague-Dawley rats. The activities of benzphetamine N-demethylase, aminopyrine N-demethylase, p-nitroanisole O-demethylase and aniline hydroxylase in liver microsomes from male rats were decreased dose-dependently by Aztreonam. On the contrary, the activities in liver microsomes from female rats were slightly increased rather than decreased by the administration of Aztreonam. In addition, Aztreonam was found to decrease the specific content of microsomal cytochrome P-450 in male rats but not in female rats. The decreases in the activities observed in male rats were accompanied by a parallel decrease in the specific content of cytochrome P-450. Furthermore, the results of quantitation of P-450 (M-1), one of the male specific forms of cytochrome P-450, indicated that the administration of Aztreonam resulted in a dose-dependent decrease in the content of P-450 (M-1) in liver microsomes from male rats.

Quaternary structure of the liver microsomal cytochrome P-450

Cytochrome P-450LM2 was isolated from rabbit liver microsomes in a form which was shown to be homogeneous in AcA-22 Ultrogel and ultracentrifugation studies. The molecular mass determined by sedimentation equilibrium roughly corresponded to hexamer composed of 56 kDa monomers. Hexamer structure of the cytochrome was directly demonstrated by electron microscopic study. In the cytochrome P-450LM2 hexamer, monomers seem to be arranged in two layers (three monomers in the layer) in such a way that each monomer occupies a position at the vertices of a triangular antiprism with a 32 point group symmetry.

Carbon tetrachloride and 2-isopropyl-4-pentenamide-induced inactivation of cytochrome P-450 leads to heme-derived protein adducts

When CCl4 was incubated with rat liver microsomes from phenobarbital-treated rats in an aerobic or anaerobic atmosphere, over 69% of the heme moiety of cytochrome P-450 was destroyed. At least 45% of the degraded heme under both reaction conditions was accounted for as heme-derived products irreversibly bound to microsomal proteins. Furthermore, 33% of the irreversibly bound products were bound specifically to a 54-kDa form of cytochrome P-450. A structurally different compound, 2-isopropyl-4-pentenamide, also destroyed the heme moiety of cytochrome P-450 and produced heme-derived adducts of microsomal proteins that accounted for 28% of the destroyed heme. These results represent a novel mechanism for the destruction of cytochromes P-450 by xenobiotics.