Allosteric activation of pyruvate kinase via NAD(+) in rat liver cells

In isolated rat hepatocytes, it has previously been reported that a rise in the ATP content induces a proportional increase in cytosolic NAD(+) concen tration [Devin, A., Guerin, B. & Rigoulet, M. (1997) FEBS Lett. 410, 329-33 2]. This occurs under physiological conditions such as various substrates o r different energetic states. To investigate the effect of a physiological rise in cytosolic [NAD(+)] per se on glycolysis and gluconeogenesis, an inc rease in [NAD(+)] induced by exogenous nicotinamide addition was obtained w ithout a change in redox potential, ATP/ADP ratio and ATP concentration. Us ing dihydroxyacetone as substrate, we found that an increase in cytosolic [ NAD(+)] decreases gluconeogenesis and enhances glycolysis without significa nt alteration of dihydroxyacetone consumption rate. These modifications are the consequence of an allosteric activation of pyruvate kinase via cytosol ic NAD(+) content. Thus, in addition to the well-known thermodynamic contro l of glycolysis by pyridine-nucleotide redox status, our study points to a new mechanism of glycolytic flux regulation by NAD(+) concentration at the level of pyruvate kinase activity.