UDP-glucuronosyltransferase (UGT) that catalyzes phase II biotransformation reactions in which lipophilic substrates are conjugated with glucuronic acid to increase the metabolite's water solubility, thereby facilitating excretion into either the urine or bile (PubMed:15472229, PubMed:18674515, PubMed:18719240, PubMed:23756265, PubMed:23288867, PubMed:24641623). Essential for the elimination and detoxification of drugs, xenobiotics and endogenous compounds (PubMed:23756265). Catalyzes the glucuronidation of endogenous estrogen hormones such as estradiol and estrone (PubMed:15472229, PubMed:18719240, PubMed:23288867). Contributes to bile acid (BA) detoxification by catalyzing the glucuronidation of BA substrates, which are natural detergents for dietary lipids absorption (PubMed:23756265). Involved in the glucuronidation of calcidiol, which is the major circulating form of vitamin D3, essential for the regulation of calcium and phosphate homeostasis (PubMed:24641623). Involved in the glucuronidation of the AGTR1 angiotensin receptor antagonists losartan, candesartan and zolarsartan, which can inhibit the effect of angiotensin II (PubMed:18674515). Lacks UDP-glucuronosyltransferase (UGT) activity but acts as a negative regulator of isoform 1. glucuronate acceptor + UDP-alpha-D-glucuronate = acceptor beta-D-glucuronoside + H(+) + UDP 17beta-estradiol + UDP-alpha-D-glucuronate = 17beta-estradiol 3-O-(beta-D-glucuronate) + H(+) + UDP 17beta-estradiol + UDP-alpha-D-glucuronate = 17beta-estradiol 17-O-(beta-D-glucuronate) + H(+) + UDP 17alpha-estradiol + UDP-alpha-D-glucuronate = 17alpha-estradiol 3-O-(beta-D-glucuronate) + H(+) + UDP estrone + UDP-alpha-D-glucuronate = estrone 3-O-(beta-D-glucuronate) + H(+) + UDP chenodeoxycholate + UDP-alpha-D-glucuronate = chenodeoxycholoyl-24-O-(beta-D-glucuronate) + UDP deoxycholate + UDP-alpha-D-glucuronate = deoxycholoyl-24-O-(beta-D-glucuronate) + UDP lithocholate + UDP-alpha-D-glucuronate = lithocholoyl-24-O-(beta-D-glucuronate) + UDP hyodeoxycholate + UDP-alpha-D-glucuronate = hyodeoxycholoyl-24-O-(beta-D-glucuronate) + UDP hyocholate + UDP-alpha-D-glucuronate = hyocholoyl-24-O-(beta-D-glucuronate) + UDP calcidiol + UDP-alpha-D-glucuronate = calcidiol 25-O-(beta-D-glucuronide) + H(+) + UDP losartan + UDP-alpha-D-glucuronate = losartan-2-N-beta-D-glucuronide + UDP candesartan + UDP-alpha-D-glucuronate = candesartan-2-N-beta-D-glucuronide + UDP UDP-alpha-D-glucuronate + zolasartan = UDP + zolarsartan-2-N-beta-D-glucuronide Some kinetic parameters were assessed using commercial enzymes, which may represent a mix of both active and inactive protein forms, and therefore modify the kinetic values. Homodimer (PubMed:17179145). Homooligomer (Probable). Interacts with UGT1A1, UGT1A4, UGT1A6, UGT1A7, UGT1A8, UGT1A9 and UGT1A10 to form heterodimers (PubMed:17179145). Isoform 1 interacts with isoform 2/i2 suggesting that oligomerization is involved in negative regulation of transferase activity by isoform 2. Isoform 1 also interacts with respective i2 isoforms of UGT1A1, UGT1A4, UGT1A6, UGT1A7, UGT1A8, UGT1A9 and UGT1A10 (PubMed:20610558). Expressed in liver, kidney, colon, esophagus and small intestine. Expressed in liver, kidney and colon. Not expressed in esophagus and small intestine. UGT1A3 isoform is part of the UGT1A complex locus which displays alternative use of promoters, first exons and terminal exons. The locus is defined by 13 first exons, which are alternatively spliced to 3 other common exons and 2 alternative terminal exons 5. From the 27 possible mRNA isoforms, 9 produce functionally active polypeptides (UGT1A1, 1A3, 1A4, 1A5, 1A6, 1A7, 1A8, 1A9 and 1A10) called isoforms 1 (i1). Use of an alternative exon 5 (5b) as terminal exon is leading to 9 additional alternatively spliced products termed isoforms i2 and which lack transferase activity. Belongs to the UDP-glycosyltransferase family.