The isolation, properties, and physiological role of lactic dehydrogenase from soybean cotyledons

L-Lactate:NAD oxidoreductase (EC. 1.1.1.27) was purified 110-fold from non-green soybean (Glycine max L. var. Canadian No. 1) cotyledons, and some of its kinetic properties were studied and compared to the properties of lactic dehydrogenases isolated from animals and microorganisms. The soybean enzyme was specific for L-lactate and NAD + but in the reverse direction reduced not only pyruvate but also hydroxypyruvate and glyoxylate in the presence of NADH, although pyruvate was shown to be the preferred substrate. Optimum activity occurred at pH 9.2 in the direction of pyruvate formation and at pH 7.0 in the reverse direction. In its response to the use of coenzyme analogues and to heat treatment it resembled closely the L-lactic dehydrogenase from Lactobacillus plantarum. Its responses to acrylamide gel electrophoresis and to sulfhydryl group inhibitors were comparable to those of similar enzymes from animal sources.The physiological role of the enzyme in germinating soybean seeds, especially during the first 30 h when anaerobic conditions obtain within the seed, was assessed by measuring its specific activity and also by measuring the rise and fall of lactic acid concentration in cotyledons over the same time period. Various aspects of the metabolism of germinating fatty seeds are discussed in relation to this and other work recently reported.