Variation in hepatic microsomal cytochrome P-450 1 concentration among untreated rabbits alters the efficiency of estradiol hydroxylation.
The metabolism of 17 beta-estradiol was examined using both rabbit liver microsomes and highly purified forms of rabbit liver microsomal cytochrome P-450. The predominant microsomal metabolite of 17 beta-estradiol is the 2-hydroxylated product. 2-Hydroxyestradiol is also the principal metabolite in reconstitution experiments in which P-450 1 exhibits the greatest Vmax, ca. 6 mol min-1 mol P-450 1(-1), vs less than 0.6 mol min-1 mol P-450(-1) for forms 2, 3b-, 3b+, 3c, 4, and 6. In addition P-450 1 has the lowest Km, ca. 2 microM. This suggested that microsomes which differ in their content of P-450 1 would also differ in the kinetic parameters characterizing the 2-hydroxylation of 17 beta-estradiol. Microsomes containing low amounts of P-450 1, less than 0.1 nmol/mg protein, exhibit a low-efficiency (Vmax/Km) 2-hydroxylase activity. Microsomes containing elevated concentrations of P-450 1, greater than 0.3 nmol/mg protein, exhibit a substrate dependence suggestive of an additional high-efficiency enzyme. The latter is specifically inhibited by a monoclonal antibody that recognizes P-450 1. These results indicate that the elevated expression of P-450 1 in microsomes leads to a marked increase in the apparent first-order rate constant for the 2-hydroxylation of 17 beta-estradiol, as it does for the 21-hydroxylation of progesterone. This should have a marked effect on the metabolism of these two steroid hormones at concentrations that are likely to occur in vivo.
D000818:Animals; D019363:Cytochrome P-450 CYP1A1; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D004958:Estradiol / Q000378:metabolism*; D006900:Hydroxylation; D066298:In Vitro Techniques; D007527:Isoenzymes / Q000378:metabolism; D008862:Microsomes, Liver / Q000201:enzymology* / Q000378:metabolism; D011817:Rabbits; D013250:Steroid Hydroxylases / Q000378:metabolism
D007527:Isoenzymes; D004958:Estradiol; D003577:Cytochrome P-450 Enzyme System; D013250:Steroid Hydroxylases; D019363:Cytochrome P-450 CYP1A1; C020120:estrogen 2-hydroxylase