Merrill A H, Henderson J M
Department of Biochemistry, Emory University School of Medicine, Atlanta, Georgia 30322.
Ann N Y Acad Sci. 1990;585:110-7. doi: 10.1111/j.1749-6632.1990.tb28047.x.
The B6 vitamers (pyridoxine, pyridoxamine, and pyridoxal) are primarily metabolized in liver to pyridoxal 5'-phosphate (PLP) and the deadend catabolite 4-pyridoxic acid. We have built on the elegant early work of Snell and others to describe the activities of the human liver enzymes responsible for vitamin B6 metabolism and to develop a model of the relative rates of these interconversions in vivo. This model is consistent with changes in plasma B6 after a load, the clearance of different vitamers (e.g., pyridoxine versus pyridoxal), and with the low plasma PLP in patients with cirrhosis. Because cirrhotics were found to be capable of PLP synthesis, we have used oral supplementation with pyridoxine to restore plasma PLP to the normal range, and have evaluated the effects of this intervention on amino acid metabolism. No significant differences were observed in plasma or urinary clearance of methionine (or cystathionine) after an oral load, nor in amino acid clearance from circulation after a protein load for cirrhotic patients before and after restoration of normal plasma PLP. Hence, the abnormal metabolism of vitamin B6 does not appear to be an important factor in the deranged amino acid metabolism in this disease. Nonetheless, this approach may be generally useful in assessing the importance of PLP in other abnormalities.
维生素B6的几种形式（吡哆醇、吡哆胺和吡哆醛）主要在肝脏中代谢为磷酸吡哆醛（PLP）和终末分解代谢产物4-吡哆酸。我们在斯内尔等人早期出色工作的基础上，描述了负责维生素B6代谢的人体肝脏酶的活性，并建立了体内这些相互转化相对速率的模型。该模型与负荷后血浆B6的变化、不同维生素B6形式（如吡哆醇与吡哆醛）的清除情况以及肝硬化患者血浆PLP水平较低相一致。由于发现肝硬化患者能够合成PLP，我们采用口服补充吡哆醇的方法将血浆PLP恢复到正常范围，并评估了这种干预对氨基酸代谢的影响。口服负荷后，肝硬化患者血浆或尿液中甲硫氨酸（或胱硫醚）的清除情况，以及恢复正常血浆PLP前后蛋白质负荷后循环中氨基酸的清除情况，均未观察到显著差异。因此，维生素B6代谢异常似乎不是该疾病中氨基酸代谢紊乱的重要因素。尽管如此，这种方法可能普遍有助于评估PLP在其他异常情况中的重要性。
