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:18674515). Essential for the elimination and detoxification of drugs, xenobiotics and endogenous compounds (PubMed:18674515). Involved in the glucuronidation of the AGTR1 angiotensin receptor antagonist zolarsatan, a drug which can inhibit the effect of angiotensin II (PubMed:18674515). 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 UDP-alpha-D-glucuronate + zolasartan = UDP + zolarsartan-1-N-beta-D-glucuronide Homodimer (By similarity). Homooligomer (By similarity). Interacts with UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A7, UGT1A8, UGT1A9 and UGT1A10 to form heterodimers (By similarity). 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, UGT1A3, UGT1A4, UGT1A6, UGT1A7, UGT1A8, UGT1A9 and UGT1A10 (By similarity). Isoform 1 and isoform 2 are expressed in colon and small intestine. Neither isoform is expressed in liver, kidney or esophagus. UGT1A5 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.