On the chemistry and biochemistry of 3-mercaptopyruvic acid, the alpha-keto acid analog of cysteine.

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.