CHARACTERIZATION OF A CLONED HUMAN DIHYDROTESTOSTERONE ANDROSTANEDIOL UDP-GLUCURONOSYLTRANSFERASE AND ITS COMPARISON TO OTHER STEROID ISOFORMS

A human cDNA, UDPGTh-3, encoding a dihydrotestosterone/5alpha-androstane-3alpha,17beta-diol UDP-glucuronosyltransferase (transferase) has been isolated and characterized. The nucleotide sequence of UDPGTh-3 encodes a 530 amino acid protein with a typical membrane insertion-signal peptide, a membrane-anchoring domain, and three potential asparagine-linked glycosylation sites. Alignment shows that this encoded isozyme is 96% identical to an apparent estriol-metabolizing isoform, HLUG4 [Coffman, B. L., et al., (1990) Arch. Biochem. Biophys. 281, 170-175]. The udpgth-3 isozyme is 78% identical to two other steroid isoforms, HLUG25 (udpgth-1) [Jackson, M. R., et al. (1987) Biochem. J. 242, 581-588; Ritter, J. K., et al. (1992) Biochemistry 31, 3409-3414] and udpgth-2 [Ritter, J. K., et al. (1990) J. Biol. Chem. 265, 7900-7906]. udpgth-2 and udpgth-1 metabolized parallel substrates (stereospecific estriols, 3,4-catechol estrogens, and the bile salt hyodeoxycholate), except that udpgth-2 was 100-fold more effective than udpgth-1. The mRNA encoding udpgth-3 is 2.4 kb in size and is present in liver, prostate, and testis; the mRNA encoding udpgth-2 is located in liver and kidney, whereas that for udpgth-1 is liver-specific. Each of the liver mRNA species encoding udpgth-3, udpgth-2, or udpgth-1 was induced 2.5-3-fold by phenobarbital treatment of the Erythrocebus patas monkey. In 16 human liver mRNA samples, the message encoding udpgth-3 was generally uniformly expressed and that for udpgth-1 exhibited wide variations in its level, whereas that for udpgth-2 was barely detectable in nine samples and not detectable in the others. Three samples contained no message for either isoform. Substrate turnover by udpgth-3 is ranked as follows: phenolphthalein > 5alpha-androstane-3alpha,17,beta-diol > 5alpha-dihydrotestosterone = 4-hydroxybiphenyl > phenolsulfonphthalein (phenol red) > phenolphthalin. Genes encoding udpgth-3, udpgth-2, and udpgth-1 mapped to human chromosome 4 with genomic DNA from human/mouse and human/hamster somatic cell hybrids; the genes encoding udpgth-1 and udpgth-2 mapped specifically to band 4q28. udpgth-3 exhibited similar K(m) values both for 5alpha-dihydrotestosterone (10 muM) and for its metabolite, 5alpha-androstane-3alpha-17beta-diol (12.5 muM). Although the role of glucuronidation in the regulation of 5alpha-dihydrotestosterone levels is not known, the location of the message for this isoform in target tissues, testis and prostate, indicates that the isoform is, most likely, important in the control of hormonal levels and, thus, in 5alpha-dihydrotestosterone action. Furthermore, a critical role for udpgth-3 is suggested in light of the absence of its messenger RNA but the presence of that for four other transferase isoforms examined in the liver of a patient with benign prostate hyperplasia, a condition associated with depressed glucuronidation of dihydrotestosterone.