BIOENERGETIC SCALING - METABOLIC DESIGN AND BODY-SIZE CONSTRAINTS IN MAMMALS

The cytosolic phosphorylation ratio ([ATP]/[ADP] [P-i]) in the mammali an heart was found to be inversely related to body mass with an expone nt of -0.30 (r = 0.999). This exponent is similar to -0.25 calculated for the mass-specific Oz consumption. The inverse of cytosolic free [A DP], the Gibbs energy of ATP hydrolysis (Delta G'(ATP)), and the effic iency of ATP production (energy captured in forming 3 mol of ATP per c ycle along the mitochondrial respiratory chain from NADH to 1/2 O-2) w ere all found to scale with body mass with a negative exponent. On the basis of scaling of the phosphorylation ratio and free cytosolic [ADP ], we propose that the myocardium and other tissues of small mammals r epresent a metabolic system with a higher driving potential (a higher Delta G'(ATP) from the higher [ATP]/[ADP][P-i]) and a higher kinetic g ain {(Delta V/V-max)/Delta[ADP]} where small changes in free [ADP] pro duce large changes in steady-state rates of Oz consumption. From the i nverse relationship between mitochondrial efficiency and body size we calculate that tissues of small mammals are more efficient than those of large mammals in converting energy from the oxidation of foodstuffs to the bond energy of ATP. A higher efficiency also indicates that mi tochondrial electron transport is not the major site for higher heat p roduction in small mammals. We further propose that the lower limit of about 2 g for adult endotherm body size (bumblebee bat, Estrucan shre w, and hummingbird) may be set by the thermodynamics of the electron t ransport chain. The upper limit for body size (100,000-kg adult blue w hale) may relate to a minimum Delta G'(ATP) of approximate to 55 kJ/mo l for a cytoplasmic phosphorylation ratio of 12,000 M(-1).