Organization of cytochrome P450 system components in the endoplasmic reticulum

Abstract

P450s are membrane‐bound proteins found in the endoplasmic reticulum (ER). The ER is known to be heterogeneous, containing both ordered and disordered regions. Ordered microdomains are more tightly packed and not readily solubilized with non‐ionic detergents and are called detergent resistant membranes (DRM). The goal of this study is to determine if these microdomains affect P450 localization. The localization of P450 system proteins was examined in rabbit liver microsomes after incubation with 1% Brij 98, followed by sucrose gradient centrifugation. Unlike CYP1A2 (which localizes in the DRM), CYP2E1 proteins localized in non‐DRM and CYP2B4 was found to be equally distributed between both domains. Importantly, for P450 to function, it is necessary for this heme protein to form a complex with its electron donor CPR; however, CPR is found largely in the DRM. Consequently, for CYP2E1 to function, a mechanism for complex formation is required, such as translocation of either CYP2E1 or CPR between the lipid domains, or the interaction of CYP2E1 with the small amount of residual CPR localized in the disordered region, which would limit its activity. These results demonstrate that the components of the P450 system are organized in specific membrane regions, and not randomly distributed throughout in the ER membrane. Supported by ES004344