Glucuronidation conjugation utilizes UDP-glucuronosyltransferases (UGTs; EC 18.104.22.168) to catalyze a wide range of diverse endogenous and xenobiotic compounds. Glucuronidation is the major pathway in phase II metabolism and accounts for approximately 35% of drug conjugation. UGTs are microsomal membrane-bound and catalyze the transfer of a glucuronate group of uridine diphosphoglucuronate (UDPGA, a co-substrate) to the functional group of specific substrates. UDPGA is synthesized from glucose-1-phosphate (G1P). G1P is required for glycolysis and is present in high concentrations in the cell, making it is unlikely to be a limiting factor in UDPGA synthesis. UDP is added to G1P to form UDP-glucose which is then dehydrogenated to form UDPGA. The basic reaction is
UDP-Glucuronate + acceptor -> UDP + acceptor-beta-D-glucuronide
The effect of this conjugation is to confer polarity to the substrate which can then be easily excreted in urine or bile. Functional groups acted on include hydroxyl, carboxylate, amino and sulfate groups. There are 2 families of UGTs, UGT1 and UGT2 which are further sub-divided into 3 subfamilies, UGT1A, UGT2A and UGT2B. There are more than 26 different isozymes in humans, of which 18 are functional proteins. They are composed of 527-530 residues and have a molecular weight of 50-57KDa.
The UGT1 family comprises of 9 proteins (UGT1A1, 1A3-1A10) but only 5 have been isolated in humans. Example substrates which are glucuronidated are acetaminophen by UGT1A6 and bilirubin by UGT1A1. Members of the UGT2 subfamily are each encoded by their own genes, in contrast to UGT1As which are encoded at the UGT1 locus. Example substrates are morphine conjugation by UGT2B7 and androgenic steroid conjugation by UGT2B17.
Xenobiotics conjugated with glucuronic acid can be substrates for beta-glucuronidase, an enzyme common in gut microflora. This enzyme can release the parent or phase I metabolite which can be reabsorbed. It can then either re-exert it's original effects or be conjugated by glucuronic acid again. This cycle is called enterohepatic circulation
and can delay the elimination of the xenobiotic.