The Control of Bovine Sperm Glycolysis During Epididymal Transit

This study was undertaken to determine whether spermatozoa from the caput of the bovine epididymis glycolyze at lower rates than those from the cauda and, if so, what factors might be involved in the activation of glycolysis during epididymal maturation. Bovine caput sperm glycolyze at approximately one-third the rate of caudal sperm and convert a lower percentage of the utilized glucose to lactate. Comparison of glycolytic rates between these cell types requires kinetic, rather than end-point, analysis. This is so because both glucose utilization and lactate formation in caput sperm are characterized by a distinct lag phase of some 5-mm duration while changes in these parameters in caudal sperm describe hyperbolic plots. Since glycolytic rates in caudal sperm are inversely correlated with ATP levels or the energy change of the adenylate pool (Hoskins, 1973) we anticipated that slowly glycolyzing caput cells would be characterized by high ATP levels and a high cellular energy charge. We have found instead that washed caput sperm are characterized by low steady-state levels of ATP (10 nmoles/10[unknown] cells) and a low energy charge ratio (0.68). Rapidly glycolyzing caudal sperm contain approximately 23 nmoles of ATP/10[unknown] cells and have an energy charge ratio of 0.83. Thus, high ATP levels do not inhibit glycolysis in bovine caput sperm. In order to understand what factors, if not diminishing ATP levels, cause glycolytic rates to increase during epididymal transit we have identified those enzymes that become activated by measuring and comparing the levels of glycolytic intermediates in both caput and caudal sperm. Two enzyme sites that are more inhibited in caput than in caudal sperm have been identified. One site is at the level of hexokinase and a second lies somewhere between glyceraldehyde-3-phosphate dehydrogenase and enolase. The possibility that hexokinase is the principal glycolytic enzyme activated during epididymal transit is discussed.