UGT2B23, a novel uridine diphosphate-glucuronosyltransferase enzyme expressed in steroid target tissues that conjugates androgen and estrogen metabolites

Glucuronidation is widely accepted as a mechanism involved in the catabolis m and elimination of steroid hormones from the body. However, relatively li ttle is known about the enzymes involved, their specificity for the differe nt steroids, and their site of expression and action. To characterize the p athway of steroid glucuronidation, a novel uridine diphosphate glucuronosyl transferase (UGT) enzyme was cloned and characterized. A 1768-bp complement ary DNA, encoding UGT2B23 was isolated from a monkey liver Library. Stable expression of UGT2B23 in human HK293 cells and Western blot analysis demons trated the presence of a 51-kDa protein. The UGT2B23 transferase activity w as tested with 62 potential endogenous substrates and was demonstrated to b e active on 6 steroids and the bile acid, hyodeoxycholic acid. Kinetic anal ysis yielded apparent Michaelis constant (Km) values of 0.9, 13.5, 1.6, and 5.7 mu M for the conjugation of androsterone (ADT), 3 alpha-Diol, estriol, and 4-hydroxyestrone, respectively. RT-PCR analysis revealed that UGT2B23 transcript is expressed in several tissues, including the prostate, mammary gland, epididymis, testis, and ovary. Primary structure analysis shows tha t UGT2B23 is in the same family of enzymes as the previously characterized monkey isoforms UGT2B9 and UGT2B18, which are active on hydroxyandrogens. T he characterization of UGT2B23 as a functional enzyme active on 3 alpha-hyd roxysteroids, and its expression in extrahepatic tissues, indicate that it may potentially play an important role in estrogen and androgen catabolism in peripheral steroid target tissues.