Lactate dehydrogenase C_4 (LDH-C_4) is a tissue-specific enzyme in the mammalian testis and the only lactate dehydrogenase isozyme of sperm. Inhibitors of LDH activity were used to determine whether this enzyme plays a role in sperm capacitation, the acrosome reaction and/or fertilization. Oxamate or its derivative was used to inhibit sperm LDH activity in a medium promoting capacitation. Complete inhibition of LDH activity blocked capacitation. This effect could be reversed partially by the addition of dbcAMP or pentoxifylline to the culture medium. Western blotting showed that oxamate and N-isopropyl oxamate inhibited the tyrosine phosphorylation of proteins during the sperm capacitation process. Presumably, glycolysis is the primary energy pathway for sperm metabolism. The oxidation of reduced NAD with the conversion of pyruvate to lactate by LDH provides ATP necessary for protein kinase A (PKA) activity. Our data indicate that LDH-C_4 plays an important metabolic role in sperm capacitation.
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