D-Glucuronic acid 1-phosphate is an end product of the UDP-glucuronic acid pathway. Formation of free glucuronic acid from UDP-glucuronic acid can be considered as the first step in the synthesis of vitamin C, a pathway that occurs in most vertebrates, although not in guinea pigs and primates, including humans. Free glucuronic acid can also be converted to pentose phosphate intermediates via the 'pentose pathway'. The latter is interrupted in subjects with pentosuria, who have a deficiency in l-xylulose reductase (EC 1.1.1.10, an enzyme that belongs to the short-chain dehydrogenase/reductase family) and excrete abnormal amounts of l-xylulose. Some xenobiotics stimulate the formation of vitamin C in animals and enhance the excretion of l-xylulose in humans with pentosuria and have shown that aminopyrine, metyrapone and other xenobiotics cause an almost instantaneous increase in the conversion of UDP-glucuronic acid to glucuronic acid. It is usually stated that glucuronic acid formation from UDP-glucuronic acid is the result of two successive reactions comprising the hydrolysis of UDP-glucuronic acid to glucuronic acid 1-phosphate and UMP by nucleotide pyrophosphatase (E-NPP3, EC 3.6.1.9), followed by dephosphorylation of glucuronic acid 1-phosphate. Glucuronidation is responsible for conjugating potentially toxic lipophilic compounds with glucuronic acid, thereby producing molecules with greater aqueous solubility that is excreted more readily into urine and bile. The rate at which any compound may be glucuronidated depends on the concentration and activity of the UDP-glucuronosyltransferases as well as the concentration of the cofactor UDP-glucuronic acid. UDP-glucuronic acid is formed after oxidation of UDP-glucose by UDP-glucose dehydrogenase (UGDH, EC 1.1.1.22) with NAD as the electron acceptor. UDP-glucuronic acid may then be either used as the glucuronic acid donor for xenobiotic conjugation reactions by UDPglucuronosyltransferases (GlcAT-P, EC 2.4.1.17), or degraded to D-glucuronic acid 1-phosphate after the phosphodiester bond is cleaved by E-NPP3. E-NPP3 is the same enzyme that further reduces D-Glucuronic acid 1-phosphate to free D-glucuronic acid. Decreases in UDP-glucuronic acid concentration may be due to reduced availability of UDP-glucose or decreased UGDH activity or to increased activities of GlcAT-P or E-NPP3. Exposure to volatile anesthetics reduces hepatic UDP-glucuronic acid concentrations, and alters the rate of conjugation of compounds such as acetaminophen, bilirubin, diethylstilbestrol, iopanoic acid and valproic acid in a non-sex-dependent fashion in experimental mice. The depletion of UDP-glucuronic acid by anesthetics is caused by altered activity of microsomal E-NPP3. (PMID: 2167093, 16689937, 1276).