Reduction of ?-thiopyruvic acid by lactate dehydrogenase: a kinetic study

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Abstract
In this paper a steady-state kinetic study on the system lactate dehydrogenase-β-thiopyruvate-β-thiolactate is presented and the possible mechanistic and physiological implications are discussed. At pH 7.4 the equilibrium between β-thiopyruvate and β-thiolactate, in the presence of NADH and lactate dehydrogenase is largerly shifted towards the formation of β-thiolactate as in the case of pyruvate and lactate. This can be relevant in connection with the mixed disulfide between cysteine and β-thiolactate that is observed to be present in the mammalian body fluids. The catalytic mechanism is of the bi-bi compulsory order type, and rapid equilibrium conditions for the binding of the first substrate (NADH) are shown to apply. A complex inhibition pattern of inhibitions by both substrates, however, prevents simple suggestions about the nature of the dead-end species involved.