Glucuronate, the precursor of vitamin C, is directly formed from UDP-glucuronate in liver.
The conversion of UDP-glucuronate to glucuronate, usually thought to proceed by way of glucuronate 1-phosphate, is a site for short-term regulation of vitamin C synthesis by metyrapone and other xenobiotics in isolated rat hepatocytes. Our purpose was to explore the mechanism of this effect in cell-free systems. Metyrapone and other xenobiotics stimulated, by approximately threefold, the formation of glucuronate from UDP-glucuronate in liver extracts enriched with ATP-Mg, but did not affect the formation of glucuronate 1-phosphate from UDP-glucuronate or the conversion of glucuronate 1-phosphate to glucuronate. This and other data indicated that glucuronate 1-phosphate is not an intermediate in glucuronate formation from UDP-glucuronate, suggesting that this reaction is catalysed by a 'UDP-glucuronidase'. UDP-glucuronidase was present mainly in the microsomal fraction, where its activity was stimulated by UDP-N-acetylglucosamine, known to stimulate UDP-glucuronosyltransferases by enhancing the transport of UDP-glucuronate across the endoplasmic reticulum membrane. UDP-glucuronidase and UDP-glucuronosyltransferases displayed similar sensitivities to various detergents, which stimulated at low concentrations and generally inhibited at higher concentrations. Substrates of glucuronidation inhibited UDP-glucuronidase activity, suggesting that the latter is contributed by UDP-glucuronosyltransferase(s). Inhibitors of beta-glucuronidase and esterases did not affect the formation of glucuronate, arguing against the involvement of a glucuronidation-deglucuronidation cycle. The sensitivity of UDP-glucuronidase to metyrapone and other stimulatory xenobiotics was lost in washed microsomes, even in the presence of ATP-Mg, but it could be restored by adding a heated liver high-speed supernatant or CoASH. In conclusion, glucuronate formation in liver is catalysed by a UDP-glucuronidase which is closely related to UDP-glucuronosyltransferases. Metyrapone and other xenobiotics stimulate UDP-glucuronidase by antagonizing the inhibition exerted, presumably indirectly, by a combination of ATP-Mg and CoASH.
Linster
Carole L
CL
Laboratory of Physiological Chemistry, Université Catholique de Louvain and the Christian de Duve Institute of Cellular Pathology, Brussels, Belgium.
Van Schaftingen
Emile
E
D000255:Adenosine Triphosphate / Q000378:metabolism; D000632:Aminopyrine / Q000378:metabolism; D000818:Animals; D000894:Anti-Inflammatory Agents, Non-Steroidal / Q000378:metabolism; D000975:Antioxidants / Q000378:metabolism*; D001205:Ascorbic Acid / Q000378:metabolism*; D002474:Cell-Free System; D002724:Chlorobutanol / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000378:metabolism; D005965:Glucuronates / Q000096:biosynthesis*; D005966:Glucuronidase / Q000378:metabolism; D014453:Glucuronosyltransferase / Q000378:metabolism; D008099:Liver / Q000378:metabolism*; D008297:Male; D008797:Metyrapone / Q000378:metabolism; D008862:Microsomes, Liver / Q000378:metabolism; D011310:Preservatives, Pharmaceutical / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D014535:Uridine Diphosphate Glucuronic Acid / Q000378:metabolism*; D015262:Xenobiotics / Q000378:metabolism
D000894:Anti-Inflammatory Agents, Non-Steroidal; D000975:Antioxidants; D004791:Enzyme Inhibitors; D005965:Glucuronates; D011310:Preservatives, Pharmaceutical; D015262:Xenobiotics; D000632:Aminopyrine; D014535:Uridine Diphosphate Glucuronic Acid; D000255:Adenosine Triphosphate; D014453:Glucuronosyltransferase; D005966:Glucuronidase; D002724:Chlorobutanol; D001205:Ascorbic Acid; D008797:Metyrapone