Studies of metabolism of round spermatids: glucose as unfavorable substrate

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Abstract
The exposure of spermatids to glucose in the absence of pyruvate and lactate resulted in an extremely low energy charge. The adenosine 5′-triphosphate (ATP) level rapidly declined and the fructose 1,6-bisphosphate (FBP) and triose levels increased. These changes were prevented by the addition of pyruvate or lactate. The levels of ATP and FBP were inversely correlated. In cells exposed to glucose, FBP did not flow appreciably through the step of glyceraldehyde 3-phosphate dehydrogenase (GA3PDH). The lactate level did not change. However, when pyruvate or lactate was administered to cells exposed to glucose, the FBP level declined rapidly. This drop was accompanied by a commensurate increase in lactate. In these cells, pyruvate transport was suppressed, and the pyruvate taken up by these cells was mostly oxidized in the tricarboxylic acid (TCA) cycle without its being reduced to lactate. In this case, the ATP level increased, but to a level still lower than existed before exposure to glucose. Furthermore, when kinetic studies on the activity of 6-phosphofructokinase (PFK) were carried out, PFK appeared to be fully activated at intracellular levels of fructose 6-phosphate, ATP and adenosine 5′-monophosphate (AMP). These results indicate that the rate of glucose metabolism in glycolysis depends heavily on the energy charge. In cells exposed to glucose, the sugar does not flow appreciably through the glycolytic pathway due to inhibition of GA3PDH. Moreover, the ATP level cannot be recovered fully from the lowest level by the addition of pyruvate or lactate. When glucose does flow through the glycolytic pathway, it is mostly transformed into lactate rather than undergoing mitochondrial oxidation and subsequent ATP synthesis. Glucose, therefore, is not a proper energy-yielding substrate for spermatids. In addition, these studies suggest that GA3PDH is a key enzyme in glycolysis in spermatids.