Some kinetic aspects of sperm specific lactate dehydrogenase in mice

Purified LDH‐X, LDH‐1, and LDH‐5 isozymes from mouse tissues were compared according to several biochemical parameters. The sperm specific LDH‐X is more thermostable than the other isozymes, retaining 100% activity after 30 minutes at 65°C. Activity of LDH‐1 is reduced by 50% under these conditions while LDH‐5 is completely inactivated at this temperature within 18 minutes. The three isozymes had different Km values for both pyruvate and lactate with LDH‐X showing the highest substrate “affinities.” In addition, LDH‐X reacted significantly with both α‐hydroxy‐valerate and ketovalerate substrates. There was little or no catalytic activity of LDH‐X with other substrates, including several steroids, β‐hydroxybutyrate, α‐hydroxyisobutyrate, α‐ketoglutarate, α‐hydroxycaproate, and sorbitol.