The activities of pyruvate carboxylase, phosphoenolpyruvate carboxylase and fructose diphosphatase in muscles from vertebrates and invertebrates.

1. The activities of pyruvate carboxylase, phosphoenolpyruvate carboxylase and fructose diphosphatase in crude homogenates of vertebrate and invertebrate muscles are reported. 2. Pyruvate carboxylase activity was present in all insect flight muscles that were investigated: in homogenates of bumble-bee flight muscle the activity was inhibited by ADP and activated by acetyl-CoA, and it was distributed mainly in the mitochondrial fraction. This is the first demonstration of pyruvate carboxylase activity in muscle. However, the activity appears to be restricted to insect flight muscle, since it was not found in other invertebrate or vertebrate muscles. 3. Since the three enzymes were never found together in the same muscle, it is concluded that these enzymes cannot provide a pathway for the synthesis of glycogen from lactate or pyruvate in muscle. Other roles for these enzymes in muscle are suggested. In particular, pyruvate carboxylase may be present in insect flight muscle for the provision of oxaloacetate to support the large increase in activity of the tricarboxylic acid cycle which occurs when an insect takes flight.