Metabolism of round spermatids: kinetic properties of pyruvate kinase

Round spermatids (steps 1-8) were isolated from rat testes and kinetic properties of pyruvate (PK) in their extract were examined. A plot of PK activity against phosphoenolpyruvate (PEP) or ADP concentration appeared sigmoidal. Km values for PEP and ADP were 0.12 and 0.29 mM, respectively. However, fructose 1,6-bisphosphate (FBP) stimulated the enzyme markedly by increasing its affinity for PEP. FBP (0.35 microM) was required for 50% activation of PK, when the PK activity was measured at 25 microM PEP and 0.2 mM ADP. In contrast, ATP (Ki = 6.5 mM) inhibited the PK activity. On the other hand, in the presence of 5 mM glucose, the level of FBP in spermatids increased markedly, while that of ATP declined rapidly. The level of ADP remained constant. When the activity of PK in spermatid extract was measured at intracellular levels of FBP, ADP and ATP, it was maximum. The results suggest that PK becomes probably fully activated when glucose is metabolized in the glycolytic pathway of spermatids. It seems unlikely that PK is the rate-limiting step in glycolysis of spermatids.