Biological disulfides: the third messenger? Modulation of phosphofructokinase activity by thiol/disulfide exchange

Rabbit muscle phosphofructokinase is rapidly inactivated at pH 8.0 by incubation with low concentrations of oxidized glutathione, Coenzyme A glutathione mixed disulfide, and oxidized Coenzyme A. The inactivation is first order in disulfide concentration over the concentration ranges examined (50-200 microM), and is approximately 8-fold slower at pH 7.0 than at pH 8.0. The substrates ATP and fructose 6-phosphate protect against inactivation while effector molecules such as AMP, cAMP, and citrate do not. The oxidation of the enzyme by disulfides is fully reversible. The equilibrium constant for the reaction Ered + GSSG in equilibrium Eox + GSH at pH 8.0 is 7.1 in the absence of substrates and 2.5 in the presence of 0.1 mM ATP. For comparison, the equilibrium constant for the reaction CoASH + GSSG in equilibrium CoASSG + GSH was found to be 3.1 at pH 8.0. These equilibrium constants for thiol/disulfide exchange are such that modulation of phosphofructokinase activity by thiol/disulfide exchange in vivo is feasible. The ability of the thiol/disulfide ratio in vivo to modulate the activity of the fructose 6-phosphate/fructose 1,6-diphosphate futile cycle is discussed. The possibility is considered that modulation of the thiol/disulfide ratio in vivo may serve as a "third messenger" in response to cAMP levels, and that the activity of key enzymes of glycolysis/gluconeogenesis may be regulated in response to changing thiol/disulfide ratios.