Metabolism of round spermatids: evidence that lactate is preferred substrate.

Glucose was utilized at a rate of 17.9 +/- 1.0 nmol X 10(6) cells-1 X h-1, with 5.2 and 9.5% accumulated as glucose and fructose 1,6-diphosphate, respectively, 79.6% transformed into lactate, 2% oxidized by the trichloroacetic acid cycle, and little metabolized by the pentose cycle. Glucose utilization increased and reached a maximum at 1 mM glucose. The ATP level was maintained by approximately 0.8 mM lactate, but not by glucose or pyruvate. Glucose decreased the ATP level to a value lower than the control level due partly to the fructose 6-phosphate-fructose 1,6-diphosphate futile cycle. In the presence of lactate (0.01-20 mM), pyruvate production was increased, but the intracellular level of pyruvate (0.2-0.3 mM) remained unchanged. Lactate dehydrogenase (LDH) activity was increased, at a maximum rate, by 0.1-0.2 mM pyruvate but was inhibited beyond 0.3 mM (Ki = 2.5 mM). The Km values of LDH for pyruvate and lactate were 24.4 +/- 4.2 microM and 2.36 +/- 0.14 mM, respectively. The importance of LDH in glucose and lactate metabolism by spermatids was described. These observations further support the hypothesis that lactate is the preferred substrate for spermatid metabolism.