Purification of two isozymes of rat liver microsomal cytochrome P450 with testosterone 7α-hydroxylase activity☆

Cytochrome P450a was purified to electrophoretic homogeneity from liver microsomes from immature male Long-Evans rats treated with Aroclor 1254. Rabbit polyclonal antibody raised against cytochrome P450a cross-reacted with cytochromes P450b, P450e, and P450f (which are structurally related to cytochrome P450a). The cross-reacting antibodies were removed by passing anti-P450a over an N-octylamino-Sepharose column containing these heterologous antigens. The immunoabsorbed antibody recognized only a single protein (i.e., cytochrome P450a) in liver microsomes from immature male rats treated with Aroclor 1254 (i.e., the microsomes from which cytochrome P450a was purified). However, the immunoabsorbed antibody recognized three proteins in liver microsomes from mature male rats, as determined by Western immunoblot. As expected, one of these proteins (Mr 48,000) corresponded to cytochrome P450a. The other two proteins did not correspond to cytochromes P450b, P450e, or P450f (as might be expected if the antibody were incompletely immunoabsorbed), nor did they correspond to cytochromes P450c, P450d, P450g, P450h, P450i, P450j, P450k, or P450p. One of these proteins was designated cytochrome P450m (Mr ~49,000), the other cytochrome P450n (Mr ~50,000). Like cytochrome P450a, cytochrome P450n was present in liver microsomes from both male and female rats. However, whereas cytochrome P450a was detectable in liver microsomes from 1-week-old rats, cytochrome P450n was barely detectable until the rats were at least 3 weeks old. Furthermore, in contrast to cytochrome P450a, the levels of cytochrome P450n did not decline appreciably with age in postpubertal male rats. Cytochrome P450m was detectable only in liver microsomes from postpubertal (>4 week-old) male rats. Cytochromes P450m and P450n were isolated from liver microsomes from mature male rats and purified to remove cytochrome P450a. When reconstituted with NADPH-cytochrome P450 reductase and lipid, cytochrome P450n exhibited little testosterone hydroxylase activity, whereas cytochrome P450m catalyzed the 15α-, 18-, 6β-, and 7α-hydroxylations of testosterone at 10.8, 4.6, 2.0, and 1.9 nmol/nmol P450/min, respectively. The ability of cytochrome P450m to catalyze the 7α-hydroxylation of testosterone was not due to contamination with cytochrome P450a, which catalyzed this reaction at ~25 nmol/nmol P450a/min. Cytochrome P450m also converted testosterone to several minor metabolites, including androstenedione and 15β-, 14α-, and 16α-hydroxytestosterone. The identity of the metabolites formed by cytochrome P450m was based on cochromatography with authentic standards resolved by HPLC. Cytochromes P450a and P450m had identical N-terminal amino acid sequences for the first 20 residues, except that residue 19 was leucine in cytochrome P450a but phenylalanine in cytochrome P450m. This high degree of amino acid sequence homology explains in part the immunochemical relatedness of cytochromes P450a and P450m. The results of this study indicate that cytochromes P450a, P450m, and P450n are immunochemically related proteins present in rat liver microsomes, and that two of these proteins, namely cytochromes P450a and P450m, are structurally related isozymes that can catalyze the 7α-hydroxylation of testosterone. The results of this study also indicate that cytochromes P450a, P450m, and P450n are independently regulated as a function of the age and sex of rats.