[Isoform 1]: 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). . [Isoform 2]: Lacks UGT glucuronidation activity but acts as a negative regulator of isoform 1. . This is the function of ENSG00000288705 (UGT1A5, UDP glucuronosyltransferase family 1 member A5).