Energy-dependent regulation of the steady-state concentrations of the components of the lactate dehydrogenase reaction in liver.

It has been known for some years that the components of the lactate dehydrogenase reaction in liver are maintained in a steady-state believed to be close to thermodynamic equilibrium [ 1,2]. The ratio, [lactate]/[pyruvate] has thus been considered to reflect the ‘redox state’ (the ratio of ‘free’ [NAD]/ [NADH]) within the cytoplasmic compartment of the hepatic cell [3]. Analogous investigations of mitochondrial dehydrogenase systems in liver have led to the conclusion that the mitochondrial redox state is -lOO-times more reduced than that of the cytoplasm [3]. It has been pointed out that because of these differences in redox state, the transfer of reducingequivalents from cytoplasmic to mitochondrial NAD(H) pools would be against the electrochemical potential gradient and therefore likely to be energydependent [4,5]. However, few relevant experimental observations using whole cell preparations have been described. The work presented here provides evidence for a direct involvement of energy in the maintenance of the steady-state [lactate]/[pyruvate] ratio in.isolated liver cells.