On the chemistry and biochemistry of 3-mercaptopyruvic acid, the alpha-keto acid analog of cysteine.
On the chemistry and biochemistry of 3-mercaptopyruvic acid, the alpha-keto acid analog of cysteine.
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DOI：
10.1016/s0021-9258(19)68270-0
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发表时间：
1982-01
期刊：
影响因子：
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通讯作者：
Arthur J. L. Cooper;M. T. Haber;A. Meister
中科院分区：
文献类型：
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作者：
Arthur J. L. Cooper;M. T. Haber;A. Meister
3-巯基丙酮酸通常被认为是通过L-半胱氨酸的酶促转氨基作用和L-半胱氨酸的氧化脱氨基作用形成的，已经发现3-巯基丙酮酸存在于溶液中，与环状二聚体2，5-二羟基-1，4-二噻烷-2，5-二羧酸平衡;在pH 5-7的水溶液中，该平衡明显有利于环状二聚体。然而，在中性溶液中，特别是在碱性溶液中，3-巯基丙酮酸酯经历非常快速和不可逆的转化为无环羟醛二聚体。羟醛二聚化的速率比在可比条件下丙酮酸二聚化的速率高约100倍。因此，pH 7.2的10 mM 3-巯基丙酮酸溶液在25 ℃下90分钟内失去约75%的乳酸脱氢酶反应性。目前的研究结果解释了以前的观察，3-巯基丙酮酸表现出异常的转氨与谷氨酰胺的行为。L-半胱氨酸被L-氨基酸氧化酶氧化脱氨产生高反应性亚胺，2-亚氨基-3-巯基丙酸，其可以用氨基脲定量捕获。在不存在氨基脲的情况下，从L-半胱氨酸几乎定量形成氨，α-酮酸的产率仅为10-20%，其中没有一种是3-巯基丙酮酸;然而，当在乳酸脱氢酶和NADH存在下进行氧化酶反应时，3-巯基丙酮酸可能被捕获为3-巯基乳酸。谷氨酰胺转氨酶的生理功能之一可能是将3-巯基丙酮酸转化为L-半胱氨酸，从而防止其通过进一步转化而损失。在这项工作的过程中，发现半硫缩酮（由半胱氨酸与α-酮酸反应形成）是L-氨基酸氧化酶的底物。通过红外光谱和质谱观察，发现了3-巯基丙酮酸酯在固态和汽态下存在环状二聚体的证据。
3-Mercaptopyruvate, generally believed to be formed by enzymatic transamination of L-cysteine and by the oxidative deamination of L-cysteine, has been found to exist in solution, in equilibrium with the cyclic dimer, 2,5-dihydroxy-1,4-dithiane-2,5-dicarboxylic acid; the equilibrium markedly favors the cyclic dimer in aqueous solution at pH 5-7. However, in neutral solutions, and especially in alkaline solution, 3-mercaptopyruvate undergoes very rapid and irreversible conversion to an acyclic aldol dimer. The rate of aldol dimerization is about 100 times greater than that of pyruvate dimerization under comparable conditions. Thus, 10 mM solutions of 3-mercaptopyruvate at pH 7.2 lose about 75% of their lactate dehydrogenase reactivity in 90 min at 25 degrees C. The present findings explain the previous observation that 3-mercaptopyruvate exhibits anomalous behavior in transamination with glutamine. Oxidative deamination of L-cysteine by L-amino acid oxidase yields the highly reactive imine, 2-imino-3-mercaptopropionic acid, which can be quantitatively trapped with semicarbazide. In the absence of semicarbazide, almost quantitative formation of ammonia from L-cysteine occurs with only a 10-20% yield of alpha-keto acid, none of which is 3-mercaptopyruvate; however, 3-mercaptopyruvate may be trapped as 3-mercaptolactate when the oxidase reaction is carried out in the presence of lactate dehydrogenase and NADH. One of the physiological functions of glutamine transaminase may be to convert 3-mercaptopyruvate to L-cysteine, thus preventing its loss through further transformations. In the course of this work it was found that hemithioketals (formed by reaction of cysteine with alpha-keto acids) are substrates of L-amino acid oxidase. Evidence for the cyclic dimer in the solid and vapor states of 3-mercaptopyruvate has been found through infrared and mass spectroscopic observations.