Abstract
1. Comparison of the maximum activities of pyruvate kinase with those of phosphofructokinase in a large number of muscles from invertebrates and vertebrates indicates that, in general, in any individual muscle, the activity of pyruvate kinase is only severalfold higher than that of phosphofructokinase. This is consistent with the suggestion, based on mass-action ratio data, that the pyruvate kinase reaction is non-equilibrium in muscle. However, the range of activities of pyruvate kinase in these muscles is considerably larger than that of phosphofructokinase. This difference almost disappears if the enzyme activities from muscles that are known to possess an anaerobic ;succinate pathway' are excluded. It is suggested that, in these muscles, phosphofructokinase provides glycolytic residues for both pyruvate kinase (i.e. glycolysis) and phosphoenolpyruvate carboxykinase (i.e. the succinate pathway). This is supported by a negative correlation between the activity ratio, pyruvate kinase/phosphofructokinase, and the activities of nucleoside diphosphokinase in these muscles, since high activities of nucleoside diphosphokinase are considered to indicate the presence of the succinate pathway. 2. The effect of fructose bisphosphate on the activities of pyruvate kinase from many different muscles was studied. The stimulatory effect of fructose bisphosphate appears to be lost whenever an efficient system for supply of oxygen to the muscles is developed (e.g. insects, squids, birds and mammals). This suggests that activation of pyruvate kinase is important in the co-ordinated regulation of glycolysis in anaerobic or hypoxic conditions, when the change in glycolytic flux during the transition from rest to activity needs to be large in order to provide sufficient energy for the contractile activity. However, lack of this effect in the anaerobic muscles of the birds and mammals suggests that another metabolic control may exist for avian and mammalian pyruvate kinase in these muscles.