Functional characterization of medaka CYP3A38 and CYP3A40: kinetics and catalysis by expression in a recombinant baculovirus system.

Phylogenic analysis of the teleost genomic lineages has demonstrated the precedent for multiple genome duplications. Among many of the genes duplicated, cytochrome P450 genes have undergone independent diversification, which can be traced to a single ancestral gene. In teleosts, cytochrome P450s, from all major families, have been identified. Among these, the CYP3A family has been cloned in several teleost species and demonstrated to contain multiple paralogs differing in gene expression patterns and tissue distribution. Herein we characterized the catalytic and kinetic activities of two medaka CYP3A paralogs (CYP3A38 and CYP3A40) with benzyloxyresorufin (BFC), a fluorescent 3A-selective substrate, and testosterone, a known metabolic substrate for CYP3A enzymes. Recombinant CYP3A was produced using the baculovirus expression vector system in Spodoptera frugiperda (Sf9) and Trichoplusia ni (Tn5) insect cells and accounted for up to 24% of total cellular protein. Following addition of a heme-albumin conjugate to log phase cells, spectral P450 content reached a maximum of 560 and 2350 pmol/mg microsomal protein for CYP3A38 and CYP3A40, respectively. Incubations containing recombinant CYP3A, human NADPH-cytochrome P-450 oxidoreductase reductase, human cytochrome b5, and a NADPH generation system catalyzed the dealkylation of BFC and hydroxylation of testosterone with a high degree of stereoselectivity. However, efficiencies and specificities were significantly different between the two isoforms. Km and Vmax activities based on BFC-catalysis were 0.116 and 0.363 muM, and 7.95 and 7.77 nmol/min/nmol P450 for CYP3A38 and CYP3A40, respectively. CYP3A38 preferentially catalyzed testosterone hydroxylation at the 6beta-, 2beta- and 16beta-positions with minor hydroxylation at other positions within the steroid nucleus. Testosterone catalysis with CYP3A40 was limited predominantly to the 6beta- and 2beta-positions. Putative identification of CYP3A substrate recognition sites (SRS) 1-6 indicates that 12 of the 49 amino acid differences between CYP3A38 and CYP3A40 OFRs occur in SRS regions previously known to be associated with steroid hydroxylation. We suggest that differences in kinetics and catalytic activities are a result of amino acid substitutions in SRS regions 1, 3 and 5 within the CYP3A38 and CYP3A40 protein sequence.