Hepatic Microsomal Biotransformation of 1,3,5( 10),16-Estratetraen-3-01 to 16,17-Epiestriol and Estriol via 16cx,l7a- and 16P,17P-Epoxy-1,3,5(10)-estratrien-3-ols*

1,3,5(10),16-Estratetraen-3-01 (estratetraenol) that has previotisly been found in the urine of women in late pregnancy and proved to be biosynthesized in human placenta from 4,16-androstadien-3-one, a hepatic metabolite of testosterone, was converted under aerobic conditions by female rat liver microsomes into estratetraenol 16a,l7a-epoxide, estratetraenol 16P,17P-epoxide, 16,17-epiestriol, and estriol. The biological reaction was mediated by two enzymes in a sequential manner, one of which was monooxygenase (P-450) requiring molecular oxygen and NADPH as the cofactor and yielded both 16a,l7+epoxide and 16/$17/&epoxide from estratetraenol in the ratio lO:l, and the other epoxide hydratase catalyzing hydrolysis of the 16a,17aepoxide stereospecifically to 16,17-epiestriol and of the 16P,17P-epoxide to 16,17-epiestriol and estriol in the ratio 1:8. The rate of enzymatic hydrolysis of the 16a,l7a-epoxide was 23 times as fast as that of the 16/3,17/3-epoxide. Enzymatic fission of the epoxide rings proceeded completely in a trans-opening manner, and none of the 16,17-cis-estriols, 16-epiestriol and 17-epiestriol, were detected in the microsomal systems containing the epoxides or estratetraenol as substrates. Similarities of hepatic microsomal estratetraenol epoxidase to P-450 for drug oxidations and of estratetraenol16,17epoxide hydratase to drug epoxide hydratase were shown by using various inhibitors. The epoxidation reaction was inhibited not only with carbon monoxide, but also other drug-oxidizing enzyme inhibitors such as SKF 525-A, metyrapone, and 7,8-benzoflavone, and hydrolysis of both epoxides with 3,3,3-trichloropropene &a-oxide. Significance of the hepatic microsomal biotransformation of estratetraenol as a new biosynthetic pathway for estriol and stereochemistry of the enzymatic epoxidation and epoxide hydrolysis are discussed.