The free-energy changes for the reduction of diphosphopyridine nucleotide and the dehydrogenation of l-malate and l-glycerol 1-phosphate

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Abstract
The reaction between isopropanol and diphosphopyridine nucleotide (DPN) to form acetone and reduced DPN is catalysed by crystalline prepns. of yeast alcohol dehydrogenase. About 500 times as much enzyme is required to obtain the same rate of reaction as that with the ethanol-DPN-acetaldehyde system. The equilibrium constant of the isopropanol-DPN-acetone reaction was combined with reliable free-energy data for isopropanol and acetone to calculate the free-energy change for the reduction of DPN. [DELTA]G[degree]=5.22 kcal. or 1.25 kJ. at 25[degree]. This value corresponds to an oxidation-reduction potential E[image] o of -0.320 V. at pH 7. The equilibria of the L-malic and L-glycerol-1-phosphate dehydrogenase systems were measured at low ionic strengths. These and other equilibrium data were combined with the data for DPN to obtain reliable values for other free-energy changes, [DELTA]G[degree]in kcal., E[image] o at pH 7 [long dash]reduction of triphosphopyridine nucleotide, 544, - 0.324; dehydrogenations of ethanol, 9.68, -0.204; isopropanol 5.89, -0.296; 1-lactate, 10.32, -0.190; l-malate 11.45, -0.166; L-glycerol-1-phosphate, 10.24, -0.192; d-isocitrate, 3.74, -0.332; and glyceraldehyde -3-phosphate, 5.83, -0.286.