Mechanism of ATP loss in nonoxidative contracting muscle.

The transition from rest to intense exercise is a challenge to cellular energetics (11, 13, 15). The metabolic fuels, i.e., the sources of ATP to sustain muscular contraction, are creatine phosphate and glycogen. Two anaerobic metabolic paths, leading to ATP generation, are catalyzed by creatine kinase and by the 12 enzymes of nonoxidative glycolysis, starting from glycogen. There is now general agreement that, unless replenished, creatine phosphate can sustain heavy muscle contraction for only 3–4 s. Thereafter, nonoxidative glycolysis becomes the main ATP source, until the onset of fatigue. This article aimed to relate the path of ATP generation during glycogen utilization as a metabolic fuel with that of ATP breakdown in nonoxidative contracting muscle. Correlation-type considerations. Table 1 shows the reactions of nonoxidative glycolysis, starting from glycogen. Since the product of each reaction becomes the substrate of the subsequent one, the 12 reactions constitute a catabolic pathway, whose summary equation, with all constituents and charges at physiological pH, is as follows: