Isolation and characterization of the UGT2B28 cDNA encoding a novel human steroid conjugating UDP-glucuronosyltransferase

UDP-glucuronosyltransferase (UGT) enzymes belonging to the UGT2B subfamily catalyze the transfer of glucuronic acid to a large number of endogenous co mpounds, particularly steroids, to facilitate their elimination from target cells. A novel human UGT2B cDNA of 1666 bp was isolated and encodes a 529- amino acid protein named UGT2B28 type I. Glucuronidation assays demonstrate d that UGT2B28 type I catalyzes the conjugation of endogenous and exogenous compounds. The tissue distribution of UGT2B28 revealed the expression of t he type I transcript in the liver, breast, and LNCaP cells. Two other UGT2B cDNAs were isolated, and sequence analysis led to the identification of tw o truncated UGT2B28 species. UGT2B28 type II differs from type I by a delet ion of 308 bp in the cofactor binding domain, whereas UGT2B28 type III lack s 351 bp in the putative substrate binding domain. All UGT2B28 isoforms are encoded by a single UGT2B28 gene which has a genomic organization similar to that of the other UGT2B genes characterized thus far. Although no substr ates could be identified for the shorter isoforms, the three subtypes were shown to be located in the endoplasmic reticulum and the perinuclear membra ne, demonstrating that the missing domains are not required for the subcell ular localization of these UGT2B proteins. However, all the domains remain necessary for observing glucuronidation activity. The expression of UGT2B28 type I in the breast and liver suggests a role of this enzyme in the andro gen and estrogen metabolism in these tissues.