Some Properties of Human Spermatozoal Lactate Dehydrogenase

1. The presence of a characteristic lactate-dehydrogenase isoenzyme (LD-) in human, mouse and dog testis and in human spermatozoa has been confirmed by electrophoresis on cellulose acetate and on polyacrylamide gel. 2. The human spermatozoal isoenzyme exhibits a much higher affinity for 2-oxobutyrate than any ofthe five isoenzymes found in other tissues. Km values of 0-05mM for pyruvate and 0-18mM for 2-oxobutyrate were obtained. 3. LD. differs from other lactatedehydrogenase isoenzymes in that its properties cannot be correlated with its electrophoretic mobility. It resembles LD1 in being strongly inhibited by 0-2mMoxalate and relatively resistant to 2M-urea, and in being relatively stable to heat. 4. The surprisingly high activity of LD. with 2-oxobutyrate suggests that this substance or 2-hydroxybutyrate may play a part in spermatozoal metabolism.