Kinetic properties of rabbit testicular lactate dehydrogenase isozyme.

The additional lactate dehydrogenase isozyme from rabbit testis and sperm, lactate dehydrogenase isozyme-1 (B4) from heart, and lactate dehydrogenase isozyme-5 (A4) from liver have been separated and partially purified by preparative starch gel electrophoresis. Studies performed on these fractions comprised K, and optimum substrate concentration for pyruvate, a-ketobutyrate, and lactate, inhibition by substrate and product, effect of oxamate, oxalate, urea, some citric acid cycle metabolites, pH, and heating. Results indicate that rabbit testicular lactate dehydrogenase isozyme is a distinct molecular form with peculiar catalytic properties. It showed different behavior against high substrate concentrations whether the direct or the reverse reaction was studied: striking inhibition by pyruvate, no effect with lactate. There is a remarkable correlation between these properties and metabolic requirements of mature spermatozoa.