Cumene hydroperoxide-mediated inactivation of cytochrome P450 2B1. Identification of an active site heme-modified peptide

Cumene hydroperoxide (CuOOH)-mediated inactivation of cytochromes P450 (P450) results in the degradation of their prosthetic heme to products that alkylate the apoprotein. Indirect approaches suggest that this alkylation occurs at the active site. in order to identify the specific apoprotein site(s) alkylated, purified 3H- or 14C-heme-labeled P450 2B1 was incubated with CuOOH and subjected to lysyl endopeptidase-C digestion. Two major peaks (L1 and L2) containing 3H- or 14C-labeled peptides were detected by reverse-phase high pressure liquid chromatography of the digest. L1 contained the highest specific radioactivity and after Tricine-sodium dodecyl sulfate-polyacrylamide gel electrophoresis yielded 3 peptide bands (M(r) approximately 3,500 (P1), 5,000 (P2), and 7,000 (P3)). Although all 3 bands were found radiolabeled, the yield of P1 was higher than that of P2 or P3. Amino acid sequence analysis of the first 13 N-terminal residues of P1 revealed the sequence RICLGEGIARNEL, corresponding to residues 434-446 of the reported 2B1 sequence. A species with the molecular mass of 3771 +/- 1 Da was detected in preliminary electrospray mass spectrometric analysis of L1. Since the theoretical average mass of the predicted peptide (residues 434-466) is 3721.99 Da, the additional 49 +/- 1 Da are considered to be contributed by the alkylating heme fragment. This alkylated 2B1 sequence contains not only Cys436, the conserved residue that provides the SH ligand for heme, but also other highly conserved residues, and therefore corresponds to the heme-sandwiching helix L of P450cam. To our knowledge, this is the first report to localize CuOOH-induced heme alkylation of 2B1 to its active site.