The Lactic Dehydrogenases of Yeast

Until about 1957 the L(+)-lactic dehydrogenase of aerobic cells was the only enzyme concerned with lactate oxidation known to occur in yeast. At that time, essentially simultaneously and independently, four laboratories discovered the existence of a second lactic dehydrogenase in yeast, which, in distinction to the L enzyme, occurred in anaerobic cells and reacted with ferricyanide but not cytochrome c (5-8). Slonimski and Tysarowski, in their initial reports (6, 9), postulated that the lactic dehydrogenase of anaerobic cells is a precursor of the cytochrome-linked L( +)-lactic dehydrogenase of aerobic cells and that under the conditions of adaptation to 02 the transformation of one enzyme into the other one occurs. Although this postulate seemed less likely when it was discovered in Slonimski’s laboratory (10) that the enzyme in anaerobic cells is specific for the n configuration of lactate, whereas the cytochrome-linked lactic dehydrogenase had been known to be specific for the L configuration, it was adopted by Nygaard (11, 12), who further postulated that o-lactic cytochrome reductase, the enzyme described in the previous paper in this series (13), is an intermediate in the conversion of the D enzyme to the L enzyme. An experimental study of the question, however, disclosed (14, 15) that a precursor-product relation among the three enzymes, in all probability, does not exist. In the course of this study it was found (1) that the enzyme in anaerobic cells is capable of oxidizing a number of n-a-hydroxy acids, and consequently the name o-oc-hydroxy acid dehydrogenase was proposed. A closely related and similarly named enzyme from rabbit kidney has been studied by Tubbs (16-19). Although major efforts have been directed, to the author’s knowledge, in four laboratories toward the isolation of the n-or-hydrosy acid dehydrogenase of yeast, until now only a very modest degree of purification has been achieved (7, 20). The main reason for this appears to be the fact that the enzyme is remarkably labile, particularly when extracted from yeast grown under certain conditions. The present paper describes isolation of the enzyme in a lOO-fold purified form, compared with the initial extract, and in a state in which it appears to be sufficiently stable for characterization. The properties of the enzyme and the question of the mechanism of its reversible inhibition by metal-chelating agents are also reported. It must be