Anaerobic Glycolysis: The Smith and Wesson of the Heterotherms

PERSPECTIVES IN BIOLOGY AND MEDICINE Volume 22 ¦ Number 4 · Summer 1979 ANAEROBIC GLYCOLYSIS: THE SMITHAND WESSON OF THE HETEROTHERMS ROLAND A. COULSON* Smith and Wesson makes all men equal. [Western Frontier Advertisement ] It is difficult to understand how animals with very low metabolic rates have survived and prospered even though they were surrounded by more powerful predators. Viewed as an oxygen-consuming machine, a record-sized king snake is less powerful than a mouse yet the snake eats mice rather than the other way around. It would be reasonable to believe that reptiles could compete with mammals by mobilizing their glycogen stores in times of emergency were it not for the fact that mammals also have glycogen which can be used for the same purpose. The coexistence of a remarkable variety of animals, cold blooded and warm blooded, is testimony to the fact that there are effective compensations for a low metabolic rate. We have considered the problem and some thoughts on metabolic rate and glycolysis, as they affect the habits of vertebrates, appear below. Aerobic Metabolism Although it is true that all vertebrates derive energy from both aerobic and anaerobic reactions, the relative contribution of the two processes varies from one animal to another. With respect to aerobic metabolism, the factors responsible for the great differences in metabolic rates have *Department of Biochemistry, Louisiana State University Medical Center, New Orleans, Louisiana 701 12. Drs. Thomas Hernandez andJack D. Herbert of Louisiana State University Medical Center were involved in the development of the theory from its beginning, and without their continuing criticism little progress would have been made. I am grateful to the Louisiana Department of Wildlife and Fisheries for their financial support and for lending us alligators for many years.© 1979 by The University of Chicago. 0031-5982/79/2204-0071$01.00 Perspectives in Biohgy and Medicine ¦ Summer 1979 j 465 received considerable attention for decades. Perhaps the most widely accepted theory involves the apparent relationship between oxygen consumption per unit mass of tissue and the surface area of the animal. According to this, since the loss of heat from the body surface is proportional to the surface area, the small animals which have a large surface area per gram lose more heat in proportion to their size than larger animals. If the body temperature is to remain constant more heat must be supplied and more oxygen must be consumed to produce it. This theory proved useful in that it enabled one to predict the metabolic rate of animals whose oxygen consumption was unknown, and when actual measurements of oxygen consumption were performed later the estimates proved reasonably accurate. The shape of different animals varies widely and minor corrections were necessary for different species. One weakness in the theory, which was sometimes ignored, was that the surface rule applied not only to warm animals that lost heat to the environment but also to the cold-blooded animals that lost, gained, or were in heat equilibrium, depending upon the ambient temperature. For example, at any one temperature the smallest alligator (30 g) has 20 times the metabolic rate of the largest (700 kg) on an equivalent weight basis, and although not enough data are yet available, it appears that the ratio of surface area to body weight is also about 20: 1 [I]. We spent years comparing the rates of catabolism of various compounds in mammals and reptiles and concluded that the velocity of metabolic reactions in vivo was in direct proportion to the metabolic rate of the animal [2]. The factors responsible for metabolic rate defied explanation . The classical Michaelis-Menten equation gives mathematical expression to the observation that the rate of a chemical reaction at any one temperature is determined by the concentration ofthe enzyme times the concentration ofthe substrate. One would be rash indeed to question the validity of an equation which has been accepted for 65 yr and which can be verified readily in any laboratory in vitro. Yet from the first, it did not seem that this equation was applicable to a living animal. Could the almost 1,000-fold difference between the metabolic rate of the smallest shrew and that of the largest...