Characterization of an Additional Splice Acceptor Site Introduced into CYP4B1 in Hominoidae during Evolution.

CYP4B1 belongs to the cytochrome P450 family 4, one of the oldest P450 families whose members have been highly conserved throughout evolution. The CYP4 monooxygenases typically oxidize fatty acids to both inactive and active lipid mediators, although the endogenous ligand(s) is largely unknown. During evolution, at the transition of great apes to humanoids, the CYP4B1 protein acquired a serine instead of a proline at the canonical position 427 in the meander region. Although this alteration impairs P450 function related to the processing of naturally occurring lung toxins, a study in transgenic mice suggested that an additional serine insertion at position 207 in human CYP4B1 can rescue the enzyme stability and activity. Here, we report that the genomic insertion of a CAG triplet at the intron 5-exon 6 boundary in human CYP4B1 introduced an additional splice acceptor site in frame. During evolution, this change occurred presumably at the stage of Hominoidae and leads to two major isoforms of the CYP4B1 enzymes of humans and great apes, either with or without a serine 207 insertion (insSer207). We further demonstrated that the CYP4B1 enzyme with insSer207 is the dominant isoform (76%) in humans. Importantly, this amino acid insertion did not affect the 4-ipomeanol metabolizing activities or stabilities of the native rabbit or human CYP4B1 enzymes, when introduced as transgenes in human primary cells and cell lines. In our 3D modeling, this functional neutrality of insSer207 is compatible with its predicted location on the exterior surface of CYP4B1 in a flexible side chain. Therefore, the Ser207 insertion does not rescue the P450 functional activity of human CYP4B1 that has been lost during evolution.

Identification of amino acid determinants in CYP4B1 for optimal catalytic processing of 4-ipomeanol.

Mammalian CYP4B1 enzymes are cytochrome P450 mono-oxygenases that are responsible for the bioactivation of several exogenous pro-toxins including 4-ipomeanol (4-IPO). In contrast with the orthologous rabbit enzyme, we show here that native human CYP4B1 with a serine residue at position 427 is unable to bioactivate 4-IPO and does not cause cytotoxicity in HepG2 cells and primary human T-cells that overexpress these enzymes. We also demonstrate that a proline residue in the meander region at position 427 in human CYP4B1 and 422 in rabbit CYP4B1 is important for protein stability and rescues the 4-IPO bioactivation of the human enzyme, but is not essential for the catalytic activity of the rabbit CYP4B1 protein. Systematic substitution of native and p.S427P human CYP4B1 with peptide regions from the highly active rabbit enzyme reveals that 18 amino acids in the wild-type rabbit CYP4B1 protein are key for conferring high 4-IPO metabolizing activity. Introduction of 12 of the 18 amino acids that are also present at corresponding positions in other human CYP4 family members into the p.S427P human CYP4B1 protein results in a mutant human enzyme (P+12) that is as stable and as active as the rabbit wild-type CYP4B1 protein. These 12 mutations cluster in the predicted B-C loop through F-helix regions and reveal new amino acid regions important to P450 enzyme stability. Finally, by minimally re-engineering the human CYP4B1 enzyme for efficient activation of 4-IPO, we have developed a novel human suicide gene system that is a candidate for adoptive cellular therapies in humans.