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

In this paper a steady-state kinetic study on the system lactate dehydrogenase-beta-thiopyruvate-beta-thiolactate is presented and the possible mechanistic and physiological implications are discussed. At pH 7.4 the equilibrium between beta-thiopyruvate and beta-thiolactate, in the presence of NADH and lactate dehydrogenase is largely shifted towards the formation of beta-thiolactate as in the case of pyruvate and lactate. This can can be relevant in connection with the mixed disulfide between cysteine and beta-thiolactate that is observed to be present in the mammalian body fluids. The catalytic mechanism is of the bi-bi compulsory 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.