Lactate dehydrogenase isoenzymes of sperm cells and tests.

1. The kinetic and metabolic properties of lactate dehydrogenase isoenzyme LDH(x) from human sperm cells and rat testes were studied. 2. LDH(x) shows a sensitivity to inhibition by stilboestrol diphosphate, urea and guanidinium chloride different from that of the LDH-H(4) and LDH-M(4) isoenzymes. 3. About 10 and 20% of the total lactate dehydrogenase activity of testes and sperm cells respectively were associated with particulate fractions. In sperm cells 11% was localized in the middle piece and 18.8% in the head fraction. LDH(x) was found in all particulate fractions of sperm cells. The middle piece contained 41.0% of total LDH(x) activity and showed high succinate dehydrogenase activity. 5. The pH-dependence of lactate/pyruvate and NAD(+)/NADH concentration ratios were estimated. Lactate dehydrogenase in sperm cells has maximal activity with NADH as coenzyme at pH7.5 and with NADPH as coenzyme at pH6.0. At pH6.0 a 10% greater oxidation of NADPH than of NADH was found. At acid pH lactate hydrogenase may function as an enzyme bringing about transhydrogenation from NADPH to NAD(+). 6. In agreement with the stoicheiometry of the lactate de- hydrogenase reaction, the lactate/pyruvate concentration ratio decreased with increasing pH. 7. The lactate/pyruvate and NAD(+)/NADH concentration ratios were estimated with glucose, fructose and sorbitol as substrates and as a function of time after addition of these substrates. During a 20min. period after the addition of the substrates, changes in lactate/pyruvate and NAD(+)/NADH concentration ratios were noticed. Increasing concentration of the substrates mentioned gave rise to asymptotic increases in lactate and pyruvate. 8. Sorbitol did not act as a substrate for LDH(x). 9. The findings described are consistent with the idea that LDH(x) is different from other known lactate dehydrogenase isoenzymes, but that it has a metabolic function similar to that of the isoenzymes of other tissues.