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首页> 外文期刊>The FEBS journal >Glucuronate, the precursor of vitamin C, is directly formed from UDP-glucuronate in liver
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Glucuronate, the precursor of vitamin C, is directly formed from UDP-glucuronate in liver

机译：葡萄糖醛酸，维生素C的前体，是由肝脏中的UDP-葡萄糖醛酸直接形成的

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摘要

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 [Linster CL & Van Schaftingen E (2003) J Biol Chem 278, 36328-36333]. 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 I-phosphate from UDP-glucuronate or the conversion of glucuronate I-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 micro-somal 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, glucuronate formation in liver supernatant or CoASH. In conclusion. 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.
机译：UDP-葡萄糖醛酸酯向葡萄糖醛酸酯的转化，通常被认为是通过葡萄糖醛酸1-磷酸进行的，是甲吡酮和其他异生素在离体大鼠肝细胞中短期调节维生素C合成的部位[Linster CL＆Van Schaftingen E（ 2003）J Biol Chem 278，36328-36333]。我们的目的是探索无细胞系统中这种效应的机制。 Metyrapone和其他异生素在富含ATP-Mg的肝脏提取物中刺激了UDP-葡萄糖醛酸形成葡萄糖醛酸约三倍，但不影响UDP-葡萄糖醛酸形成葡萄糖醛酸I-磷酸或葡萄糖醛酸I-磷酸盐的转化。葡萄糖醛酸。该数据和其他数据表明葡萄糖醛酸1-磷酸酯不是由UDP-葡萄糖醛酸酸酯形成葡萄糖醛酸酯的中间体，表明该反应是由“ UDP-葡萄糖醛酸苷酶”催化的。 UDP-葡萄糖醛酸苷酶主要存在于微粒体部分，其活性受到UDP-N-乙酰葡萄糖胺的刺激，UDP-N-乙酰葡萄糖胺通过增强UDP-葡萄糖醛酸酸酯在内质网膜上的运输而刺激UDP-葡萄糖醛酸糖基转移酶。 UDP-葡糖醛酸糖苷酶和UDP-葡糖醛酸糖基转移酶对各种去污剂表现出相似的敏感性，这些去污剂在低浓度下被刺激而在高浓度下通常被抑制。葡萄糖醛酸糖苷化的底物抑制了UDP-葡萄糖醛酸糖苷酶的活性，这表明后者是由UDP-葡萄糖醛酸糖基转移酶引起的。 β-葡萄糖醛酸苷酶和酯酶的抑制剂不影响葡萄糖醛酸的形成，这与葡萄糖醛酸化-去葡萄糖醛酸化循环的参与有关。即使在存在ATP-Mg的情况下，清洗后的微粒体也失去了UDP-葡萄糖醛酸苷酶对甲吡酮和其他刺激性异源生物的敏感性，但可以通过在肝脏上清液或CoASH中加入加热的肝脏高速，葡萄糖醛酸形成来恢复其敏感性。结论。由UDP-葡糖醛酸糖苷酶催化，其与UDP-葡糖醛酸糖基转移酶密切相关。 Metyrapone和其他异种生物通过拮抗ATP-Mg和CoASH的间接作用而抑制UDP-葡萄糖醛酸酶。

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