Cytochrome P450p (IIIA1) has been purified from rat liver microsomes by several investigators, but in all cases the purified protein, in contrast to other P450 enzymes, has not been catalytically active when reconstituted with NADPH-cytochrome P450 reductase and dilauroylphosphatidylcholine. We now report the successful reconstitution of testosterone oxidation by cytochrome P450p, which was purified from liver microsomes from troleandomycin-treated rats. The rate of testosterone oxidation was greatest when purified cytochrome P450p (50 pmol/ml) was reconstituted with a fivefold molar excess of NADPH-cytochrome P450 reductase, an equimolar amount of cytochrome b5, 200 micrograms/ml of a chloroform/methanol extract of microsomal lipid (which could not be substituted with dilauroylphosphatidylcholine), and the nonionic detergent, Emulgen 911 (50 micrograms/ml). Testosterone oxidation by cytochrome P450p was optimal at 200 mM potassium phosphate, pH 7.25. In addition to their final concentration, the order of addition of these components was found to influence the catalytic activity of cytochrome P450p. Under these experimental conditions, purified cytochrome P450p converted testosterone to four major and four minor metabolites at an overall rate of 18 nmol/nmol P450p/min (which is comparable to the rate of testosterone oxidation catalyzed by other purified forms of rat liver cytochrome P450). The four major metabolites were 6 beta-hydroxytestosterone (51%), 2 beta-hydroxytestosterone (18%), 15 beta-hydroxytestosterone (11%) and 6-dehydrotestosterone (10%). The four minor metabolites were 18-hydroxytestosterone (3%), 1 beta-hydroxytestosterone (3%), 16 beta-hydroxytestosterone (2%), and androstenedione (2%). With the exception of 16 beta-hydroxytestosterone and androstenedione, the conversion of testosterone to each of these metabolites was inhibited greater than 85% when liver microsomes from various sources were incubated with rabbit polyclonal antibody against cytochrome P450p. This antibody, which recognized two electrophoretically distinct proteins in liver microsomes from troleandomycin-treated rats, did not inhibit testosterone oxidation by cytochromes P450a, P450b, P450h, or P450m. The catalytic turnover of microsomal cytochrome P450p was estimated from the increase in testosterone oxidation and the apparent increase in cytochrome P450 concentration following treatment of liver microsomes from troleandomycin- or erythromycin-induced rats with potassium ferricyanide (which dissociates the cytochrome P450p-inducer complex). Based on this estimate, the catalytic turnover values for purified, reconstituted cytochrome P450p were 4.2 to 4.6 times greater than the rate catalyzed by microsomal cytochrome P450p.