Associate editor: D. KupferOrganization of multiple cytochrome P450s with NADPH-cytochrome P450 reductase in membranes
Introduction
Cytochrome P450, the terminal component of an electron transport chain found in the endoplasmic reticulum, has been shown to be important in the metabolism of drugs, pollutants, solvents, and other foreign compounds as well as of endogenous lipid-soluble substrates. The enzyme system takes lipid-soluble substrates and converts them to more water-soluble products by insertion of an oxygen atom into the substrate molecule. Since the identification and reconstitution of the components of this electron transport chain, there has been an interest in the organization of constituent proteins in the endoplasmic reticulum. The focus of this review is to discuss the characteristics of the interaction of P450 not only with the flavoprotein NADPH-cytochrome P450 reductase (reductase) but also with other P450 enzymes.
Section snippets
Organization of proteins in the endoplasmic reticulum
The primary components, cytochrome P450, flavoprotein NADPH-cytochrome P450 reductase, and phospholipid (PL), were first identified in the seminal studies by Lu et al. (1969). NADPH-supported catalytic activity requires both protein components, with P450 and reductase forming a 1:1 (M/M) functional complex Miwa & Lu, 1984, Miwa et al., 1979. Other protein components, such as cytochrome b5 and NADH-cytochrome b5 reductase, also interact with the P450 system (Hildebrandt & Estabrook, 1971), where
Cytochrome P450 aggregation
Because of their presence in the microsomal membrane and their hydrophobic character, there is a potential for P450 enzymes to form P450-P450 complexes. However, it is important to be able to differentiate between P450s forming functionally important complexes and nonspecific complexes that occur due to the hydrophobic nature of the proteins. One of the difficulties in assessing the importance of such interactions is that P450s tend to aggregate in solution.
Interactions between multiple P450 enzymes
The potential for P450 enzymes to interact both in solution and in the lipid membrane raises the possibility that both homomeric and heteromeric P450-P450 interactions can influence the function of these enzymes. Despite the interest in how the proteins comprising the P450 system are organized, very few studies have addressed the question of whether one P450 can influence the catalytic characteristics of another P450. Early studies by West and Lu (1972) examined the effect of addition of P450
Conclusions
The above data clearly demonstrate that P450s can function differently in a complex reconstituted system when compared with the simple binary systems. Furthermore, the results demonstrate that at least some of the interactions among these proteins involve the formation of heteromeric P450 complexes that can have a significant influence on the catalytic function of these microsomal electron transport proteins, having definite implications for drug metabolism and the bioactivation of compounds in
Acknowledgements
This work was funded, in part, by grants from the National Institute of Environmental Health Sciences (R01 ES04344), institutional support from the Research Enhancement Fund program at LSU Health Sciences Center School of Medicine, and support from the Stanley S. Scott Cancer Center. R.W.K. was supported in part by the Stanley S. Scott Cancer Center Fellows Program.
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