The free energy of glycogen-lactic acid breakdown in muscle

Abstract

It is the purpose of the present discussion to show, upon the basis of thermodynamic data obtained within the last four of five years, that the free energy of glycogen-lactic acid breakdown in muscle is considerably greater than the heat of reaction, about one and one-half to two times. It is the intention to outline merely the orders of magnitude of the various quantities involved in the evaluation of this difference. This evaluation, as will be shown, need not depend upon a knowledge of the actual heat of reaction, which is still in dispute, varying between Meyerhof's value of —180 cal. and Slater's value of —235 cal. It will depend, rather, upon the specific heat differences, or ultimately, molecular structure differences, obtaining between glycogen and lactic acid. Stated briefly, the existence of this large negative difference, designated hereafter as (ΔF — ΔH), implies that the theoretical maximum mechanical work which a muscle can perform as a consequence of this breakdown is considerably greater than the corresponding heat of reaction. The notations of Lewis and Randall (1) will be used throughout. ΔH, the heat of reaction, and ΔF, the free energy of reaction, will be negative when heat and free energy respectively are liberated. Before presenting the thermodynamic data and calculations, it will be it historical interest to point out that in 1912 A. V. Hill (2) suggested the possibility of such a difference, when he first made the observation that during anaerobic lactic acid formation in muscle the heat evolved amounted to at least three times as much as would have been predicted if the precursor were a hexose carbohydrate. He suggested, "the breakdown from this body to lactic acid may be one of those somewhat rare but by no means unknown chemical reactions which can do more mechanical work than is equivalent to their total loss of energy; by virtue of their completeness they possess the power of absorbing heat from their surroundings to do this excess of work." Meyerhof (3, 1922) reconsidered the question, and while alive to the possibility of a considerable difference, offered an opinion, based upon the Nernst heat theorem, that probably no difference did exist. In general, however, little attention has been paid to A. V. Hill's original surmise, especially since further investigation of the other hydrolysis, neutralization, and deionization reactions occurring simultaneously with the formation of lactic acid, has shifted the attention to explaining the other more immediate problem, namely, the discrepancy between the observed chemical change and the required evolution of heat.