The Relationships between Substrates and Enzymes of Glycolysis in Brain

Abstract

In the preceding paper (1) are recorded the concentrations in mouse brain of the substrates and cofactors of the EmbdenMeyerhof pathway, and the changes in concentration when the glycolytic rate is increased many fold by sudden ischemia (decapitation). In this paper, the concentrations found are examined in relationship to the activity and kinetic properties of each enzyme concerned and the glycolytic flux. To make this possible, the activities of all enzymes of glycolysis in mouse brain except triosephosphate isomerase were measured under conditions designed to simulate the pH and ionic environment of the cell, and abbreviated kinetic studies were made of each enzyme under these same conditions. Many of the steps involved are reversible reactions with low AF values. Bticher and Klingenberg (2) and others from the same laboratory (3, 4) have concluded that in liver and muscle many steps of glycolysis and of related systems are maintained close to equilibrium even during increased activity. Hess (5) has come to similar conclusions in regard to ascites tumor cells. For these reasons, and because of a major discrepancy between observed and expected substrate levels at the aldolose step, some of the equilibria have been redetermined under the simulated conditions in vivo. The combined analytical and kinetic information makes it seem likely that along the glycol.ytic pathway in mouse brain (a) no step is limited by the amount of enzyme present; (b) equilibrium is approximated at five or possibly six steps even during maximal glycolysis; (c) one step (phosphoglycerate kinase) never reaches equilibrium whereas three others (counting cu-glycerophosphate dehydrogenase) are not maintained at equilibrium during rapid glycolysis; and (d) the hexokinase and phosphofructokinase steps are the only points at which there is absolute control of glycolytic flux. Those steps which do not control flux, but which are not rapid enough to maintain equilibrium, influence the system to the extent that they raise the levels of substrates further upstream. They serve the function of a dam but not of a valve.