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. Essential for the elimination and detoxification of drugs, xenobiotics and endogenous compounds. Catalyzes the glucuronidation of endogenous estrogen hormones such as estradiol and estrone. Also catalyzes the glucuronidation of the isoflavones genistein, daidzein, glycitein, formononetin, biochanin A and prunetin, which are phytoestrogens with anticancer and cardiovascular properties. Involved in the glucuronidation of the AGTR1 angiotensin receptor antagonist caderastan, a drug which can inhibit the effect of angiotensin II. Involved in the biotransformation of 7-ethyl-10-hydroxycamptothecin (SN-38), the pharmacologically active metabolite of the anticancer drug irinotecan. Also metabolizes mycophenolic acid, an immunosuppressive agent. glucuronate acceptor + UDP-alpha-D-glucuronate = acceptor beta-D-glucuronoside + H(+) + UDP 2-hydroxy-17beta-estradiol + UDP-alpha-D-glucuronate = 2-hydroxy-17beta-estradiol 3-O-(beta-D-glucuronate) + H(+) + UDP 4-hydroxy-17beta-estradiol + UDP-alpha-D-glucuronate = 17beta-estradiol 4-O-(beta-D-glucuronate) + H(+) + UDP 2-hydroxyestrone + UDP-alpha-D-glucuronate = 2-hydroxyestrone 3-O-(beta-D-glucuronate) + H(+) + UDP 4-hydroxyestrone + UDP-alpha-D-glucuronate = estrone 4-O-(beta-D-glucuronate) + H(+) + UDP prunetin + UDP-alpha-D-glucuronate = prunetin-5-O-beta-D-glucuronide + UDP candesartan + UDP-alpha-D-glucuronate = candesartan O-beta-D-glucuronoside + UDP SN-38 + UDP-alpha-D-glucuronate = H(+) + SN-38 O-beta-D-glucuronide + UDP mycophenolate + UDP-alpha-D-glucuronate = H(+) + mycophenolate 7-O-beta-D-glucuronide + UDP Homodimer. Homooligomer. Interacts with UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A7, UGT1A8 and UGT1A10 to form heterodimers. UGT1A9 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 stomach and kidney. Belongs to the UDP-glycosyltransferase family. Truncated N-terminus.