1. The ratio [ATP]/[ADP][Pi], as measured by direct determination of the three components in rat liver, was found in various nutritional states to have approximately the same value as the ratio [ATP]/[ADP][Pi] calculated from the concentrations of lactate, pyruvate, glyceraldehyde phosphate and 3-phosphoglycerate on the assumption that lactate dehydrogenase, glyceraldehyde phosphate dehydrogenase and 3-phosphoglycerate kinase are at near-equilibrium in the liver. This implies that the redox state of the NAD couple in the cytoplasm is linked to, and partially controlled by, the phosphorylation state of the adenine nucleotides. 2. The combined equilibrium constant of the glyceraldehyde 3-phosphate dehydrogenase and 3-phosphoglycerate kinase reactions at 38°C and I0.25, was found to be 5.9×10−6. 3. The fall of the [NAD+]/[NADH] ratio in starvation and other situations is taken to be the consequence of a primary fall of the [ATP]/[ADP][HPO42−] ratio.

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