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 (By similarity). Essential for the elimination and detoxification of drugs, xenobiotics and endogenous compounds (By similarity). Catalyzes the glucuronidation of endogenous estrogen hormones such as estradiol, estrone and estriol (By similarity). Involved in the glucuronidation of bilirubin, a degradation product occurring in the normal catabolic pathway that breaks down heme in vertebrates (By similarity). Also catalyzes the glucuronidation the isoflavones genistein, daidzein, glycitein, formononetin, biochanin A and prunetin, which are phytoestrogens with anticancer and cardiovascular properties (By similarity). Involved in the glucuronidation of the AGTR1 angiotensin receptor antagonist losartan, a drug which can inhibit the effect of angiotensin II (By similarity). Involved in the biotransformation of 7-ethyl-10-hydroxycamptothecin (SN-38), the pharmacologically active metabolite of the anticancer drug irinotecan (By similarity). 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 Homodimers. Homooligomer. Interacts with UGT1A3, UGT1A4, UGT1A6, UGT1A7, UGT1A8, UGT1A9 and UGT1A10 to form heterodimers. UGT1A1 is one of the isoforms produced at the UGT1A complex locus. The UGT1A complex locus produces different isoforms based on alternative use of promoters, first exons and terminal exons. Highly expressed in liver and at lower levels in colon, kidney, stomach and intestine. By dioxin. Belongs to the UDP-glycosyltransferase family.