Hemin-mediated restoration of allylisopropylacetamide-inactivated CYP2B1: a role for glutathione and GRP94 in the heme–protein assembly

Administration of the cytochrome P450 (P450) suicide inactivator allylisopropylacetamide (AIA) to phenobarbital (PB)-pretreated rats results in rapid and marked inactivation of several liver endoplasmic reticulum (ER)-bound P450s. A few of these such as CYP2B1, inactivated due to AIA-mediated prosthetic heme N-alkylation, can be structurally and functionally restored nearly completely by exogenous hemin in vivo or in vitro. Such in vitro hemin-mediated reassembly is unsuccessful with purified AIA-inactivated CYP2B1 and, as shown herein, is not very effective even when heme is incubated with just the corresponding liver microsomes that contain the reconstitutable CYP2B1 protein, thereby implicating a requirement for additional factors provided by the intact liver cell homogenates, ER, and/or cytosol. Using various approaches that include high-performance liquid chromatographic fractionation of the liver cytosolic subfraction as well as chemical and immunological probes such as the Hsp90/GRP94-specific inhibitor geldanamycin (GA) and polyclonal anti-GRP94 antibodies, respectively, we now demonstrate that the in vitro hemin-mediated reassembly of heme-stripped microsomal CYP2B1 requires GSH as well as the ER chaperone GRP94, but not the cytosolic chaperone heat shock protein 90. It remains to be determined whether GSH acts directly or indirectly, via a putative ER thiol reductase, to maintain the conserved active site cysteine–thiol (Cys436 in CYP2B1) in a reduced state, competent for heme binding and repair.