Metabolism of Round Spermatids: Pyruvate cannot Maintain the ATP Level

Round spermatids (steps 1–8) were isolated from rat testes and the effect of pyruvate on their intracellular ATP level was examined. Results showed that although the spermatids consumed a considerable amounts of pyruvate, this substrate alone did not maintain their ATP level. However, their ATP level was maintained in the presence of both pyruvate and α‐ketovalerate or α‐ketobutyrate. Maintenance of the ATP level by these substrates was associated with electron trasnport and oxidative phosphorylation. α‐Ketoacid inhibited pyruvate reduction to lactate in the lactate dehydrogenase (LDH) reaction, but increased pyruvate oxidation to CO2. The NADH level in spermatids was too low to be detectable, but the NAD level remained unchanged in the presence of pyruvate and α‐ketovalerate. These results suggest that pyruvate by itself is not an adequate energy‐yielding substrate for spermatids and that a high NADH/NAD ratio may be essential for maintenance of their ATP level.