Enzymes Related to Lactate Metabolism in Green Algae and Lower Land Plants

Cell-free extracts of Chlorella pyrenoidosa contained two enzymes capable of oxidizing d-lactate; these were glycolate dehydrogenase and NAD(+)-dependent d-lactate dehydrogenase. The two enzymes could be distinguished by differential centrifugation, glycolate dehydrogenase being largely particulate and NAD(+)-d-lactate dehydrogenase being soluble. The reduction of pyruvate by NADH proceeded more rapidly than the reverse reaction, and the apparent Michaelis constants for pyruvate and NADH were lower than for d-lactate and NAD(+). These data indicated that under physiological conditions, the NAD(+)-linked d-lactate dehydrogenase probably functions to produce d-lactate from pyruvate.Lactate dehydrogenase activity dependent on NAD(+) was found in a number of other green algae and in the green tissues of a few lower land plants. When present in species which contain glycolate oxidase rather than glycolate dehydrogenase, the enzyme was specific for l-lactate rather than d-lactate. A cyclic system revolving around the production and utilization of d-lactate in some species and l-lactate in certain others is proposed.