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:12181437, PubMed:15472229, PubMed:18004206, PubMed:18004212, PubMed:18719240, PubMed:19830808, PubMed:23288867). Essential for the elimination and detoxification of drugs, xenobiotics and endogenous compounds (PubMed:12181437, PubMed:18004206, PubMed:18004212). Catalyzes the glucuronidation of endogenous estrogen hormones such as estradiol, estrone and estriol (PubMed:15472229, PubMed:18719240, PubMed:23288867). Involved in the glucuronidation of bilirubin, a degradation product occurring in the normal catabolic pathway that breaks down heme in vertebrates (PubMed:17187418, PubMed:18004206, PubMed:19830808). Also catalyzes the glucuronidation the isoflavones genistein, daidzein, glycitein, formononetin, biochanin A and prunetin, which are phytoestrogens with anticancer and cardiovascular properties (PubMed:18052087, PubMed:19545173). Involved in the glucuronidation of the AGTR1 angiotensin receptor antagonist losartan, a drug which can inhibit the effect of angiotensin II (PubMed:18674515). Involved in the biotransformation of 7-ethyl-10-hydroxycamptothecin (SN-38), the pharmacologically active metabolite of the anticancer drug irinotecan (PubMed:12181437, PubMed:18004212, PubMed:20610558). Lacks UGT glucuronidation 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 2-hydroxyestrone + UDP-alpha-D-glucuronate = 2-hydroxyestrone 3-O-(beta-D-glucuronate) + H(+) + UDP 2-hydroxy-17beta-estradiol + UDP-alpha-D-glucuronate = 2-hydroxy-17beta-estradiol 3-O-(beta-D-glucuronate) + H(+) + UDP 2-methoxy-17beta-estradiol + UDP-alpha-D-glucuronate = 2-methoxy-17beta-estradiol 3-O-(beta-D-glucuronate) + H(+) + UDP 17alpha-estradiol + UDP-alpha-D-glucuronate = 17alpha-estradiol 3-O-(beta-D-glucuronate) + H(+) + UDP 16beta,17beta-estriol + UDP-alpha-D-glucuronate = 16beta,17beta-estriol 16-O-(beta-D-glucuronate) + H(+) + UDP losartan + UDP-alpha-D-glucuronate = losartan-2-N-beta-D-glucuronide + UDP prunetin + UDP-alpha-D-glucuronate = prunetin-4'-O-beta-D-glucuronide + UDP SN-38 + UDP-alpha-D-glucuronate = H(+) + SN-38 O-beta-D-glucuronide + UDP 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 UGT1A3, 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 (PubMed:17187418, PubMed:20610558). Isoform 1 also interacts with respective i2 isoforms of UGT1A3, UGT1A4, UGT1A6, UGT1A7, UGT1A8, UGT1A9 and UGT1A10 (PubMed:20610558). Expressed in liver, colon and small intestine. Not expressed in kidney, esophagus and skin. Expressed in liver, colon, small intestine and kidney. Not expressed in esophagus and skin. Genetic variation in UGT1A1 defines the bilirubin serum levels quantitative trait locus 1 (BILIQTL1) [MIM:601816]. Variation in serum bilirubin is associated with altered cardiovascular disease risk and drug metabolism. The disease is caused by variants affecting the gene represented in this entry. The disease may be caused by variants affecting the gene represented in this entry. The defect has been ascribed to various breast milk substances, but the component or combination of components that is responsible remains unclear. Defects of UGT1A1 are an underlying cause of the prolonged unconjugated hyperbilirubinemia associated with breast milk. One or more components in the milk may trigger the jaundice in infants who have such mutations. Mutations are identical to those detected in patients with Gilbert syndrome, a risk factor of neonatal non-physiologic hyperbilirubinemia and a genetic factor in fasting hyperbilirubinemia. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. UGT1A1 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. Glucuronosyltransferase entry