Steroid Metabolism by Rabbit Olfactory-Specific P450 2G1

Cytochrome P450 2G1 (2G1), which is uniquely expressed in the olfactory mucosa in mammals, may have important physiological functions. In the present study, we have examined the catalytic activity of rabbit 2G1 toward a number of steroid sex hormones, including androstenedione, estradiol, progesterone, testosterone, and 5α-dihydrotestosterone; the purified cytochrome is active toward all of these compounds in a reconstituted enzyme system with turnover numbers of 1.84, 0.34, 1.46, 1.04, and 0.84, respectively, at a substrate concentration of 5 μM. In the presence of cytochrome b₅, the turnover numbers are 1.58, 0.66, 1.66, 2.74, and 1.34, respectively. Estradiol is converted to the 2-hydroxy compound (major product) and 4-hydroxy compound (minor product) by 2G1, and progesterone is converted to the 16α-hydroxy derivative as well as the corresponding keto compound as a secondary product. The same products are formed in olfactory microsomal suspensions as major metabolites of progesterone, and the reactions are inhibited strongly by anti-2G1 IgG. In a reconstituted system, 2G1 has an apparent K_m of 2.0 μM and a V_max of 1.8 nmol/min/nmol P450 for the formation of the 16α-hydroxyprogesterone. Of particular interest, 2G1-catalyzed progesterone metabolism is effectively inhibited by the boar pheromones, 5α-androst-16-en-3-one and 5α-androst-16-en-3α-ol, and to a lesser extent by a variety of odorant compounds as well as by known P450 inhibitors, including ketoconazole and α-naphthoflavone. The broad substrate specificity and relatively high catalytic efficiency of 2G1 in sex steroid metabolism suggest a role for this unique P450 isozyme in the maintenance of steroid hormone homeostasis in the olfactory mucosa.