Tissue cytochrome oxidase activity and body weight.

Recently there has been evidence of an increasing amount of interest in the enzyme systems or factors responsible for the level of the basal heat production in the intact animal (1, 2). Krebs (2) and von Bertalanffy and Pirozynski (3) have extended the studies of Kleiber and coworkers (4, 5) in which the Qo, of surviving tissue slices was measured and related to body size. In interspecific studies (2, 4, 5), it was noted that QO, values of slices of brain, kidney, spleen, lung, and liver of the larger animals were, in general, somewhat lower than the homologous values of the smaller species, but the QoZ of most tissues changed much less with body weight than did the basal metabolic rate. With some species, particularly rats, rabbits, and sheep (4, a), a systematic decrease of the Qo, values of liver slices with increasing size appeared evident, but parallelism over the whole series was imperfect. The Qo? for guinea pig, cat, and dog liver was found to be approximately the same in each case, while the rate of heat production varied widely. The intraspecific studies with the rat as the test animal (1, 3) provided results which indicated that a QOy which reflected changes in basal metabolic rate could he measured in musculature (diaphragm) but not in other tissues. These data do not satisfactorily explain differences in metabolic rate, but they do suggest interspecific and intraspecific differences in the activity of specific enzyme systems of homologous tissues which may be related to size or age of animals. Tipton and coworkers (6, 7) have reported that thyrotoxicosis results in increased liver cytochrome oxidase and succinic oxidase activity. This, coupled with the observations of Kunkel and Williams (8) that liver cytochrome oxidase was elevated in rats suffering from fat deficiency, suggested that this enzyme may reflect the changes in metabolic rate accompanying variations in body size. Fat deficiency is known to be accompanied by