Organ specificity and lactate-dehydrogenase activity. Some properties of human spermatozoal lactate dehydrogenase.

1. The presence of a characteristic lactate-dehydrogenase isoenzyme (LD(x)) 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 of the five isoenzymes found in other tissues. K(m) values of 0.05mm for pyruvate and 0.18mm for 2-oxobutyrate were obtained. 3. LD(x) differs from other lactate-dehydrogenase isoenzymes in that its properties cannot be correlated with its electrophoretic mobility. It resembles LD(1) in being strongly inhibited by 0.2mm-oxalate and relatively resistant to 2m-urea, and in being relatively stable to heat. 4. The surprisingly high activity of LD(x) with 2-oxobutyrate suggests that this substance or 2-hydroxybutyrate may play a part in spermatozoal metabolism.