Association of Cytochromes P450 1A2 and 2B4: are the Interactions between Different p450 Species Involved in the Control of the Monooxygenase Activity and Coupling?

The membranes of endoplasmic reticulum contain a number of co-existing isoforms of cytochrome P450. These multiple P450 species compete for the partners, namely NADPHcytochrome P450 reductase (CPR) and cytochrome b5and hence have to be considered as a members of a single ensemble. Moreover, these different P450 isozimes also appear to interact with each other. Despite of numerous evidences on the oligomerization of P450s both in solution (Dean and Gray, 1982; Wendel et al., 19; Tsuprun et al., 1986) and in the membranes (Greinert et al., 1982; Kawato et al., 1982; Schwartz et al., 1990, Alston et al., 1991), the functional significance of this phenomenon remains obscure. However, it is likely to cause several perplexing features of microsomal P450s, such as biphasic kinetics of their reduction by NADPH and dithionite (Karyakin and Davydov, 1985), multiphasic kinetics of interactions with carbon monoxide (Davydov et al., 1986) and notable allosteric behavior of several P450 isoforms (Korzekwa et al., 1998). A remarkable observation in this context we made studying the barotropic behavior of P450 2B4 (CYP2B4) (Davydov et al., 1992,1995). We have found that only about 65-70% of the ferrous carbonyl complex of this oligomeric protein in solution is exposed to pressure-induced P450-*P420 inactivation. The same non-uniform barotropic behavior was also observed for the oligomers of ferric CYP2B4, where only about 30-35% of the hemoprotein participates in the substrate binding and related spin transitions, being, at the same time, insensitive to the pressure-induced inactivation (Davydov et al., 1995). As these irregularities disappear at the P450 monomerization in the presence of detergent, we suggest them to reflect some peculiarities of the oligomer architecture resulting in inequality of the subunits in conformation and/or orientation.