Abstract
1. 1. Protein-specific activities of metabolically important enzymes were measured in liver and skeletal muscle homogenates from three species of lizards, Sauromalus hispidus and Dipsosaurus dorsalis (Iguanidae) and Varanus gouldii (Varanidae), and laboratory rats. 2. 2. Phosphofructokinase activity was relatively high in the muscle tissue of the iguanid lizards, which rely heavily on anaerobic metabolism during activity. Other enzymes had similar activities in all species of lizards. 3. 3. The soluble (anaerobic) enzymes had equal activities in the lizards and the rat; however, the mitochondrial (aerobic) enzymes were approximately one-fifth as active in the reptiles. 4. 4. This differential activity parallels differences in organismal oxygen consumption and is probably due to a lower number of mitochondria in the reptilian tissue. The implications of this differential activity for the evolution of homeothermy are discussed.
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