THE OXIDATION OF d- AND l-GLYCERATE BY RAT LIVER

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Abstract
The interconversion of hydroxypyruvate and L-glycerate in the presence of nicotinamide adenine dinucleotide (NAD) and rat-liver L-lactate dehydrogenase has been demonstrated. Michaelis constants for these substrates together with an equilibrium constant have been determined and compared with those for pyruvate and L-lactate. The presence of D-glycerate dehydrogenase in rat liver has been confirmed, and the enzyme has been purified 16-20-foldfrom the supernatant fraction of a homogenate, when it is free of L-lactate dehydrogenase, with a 23-29% recovery. The enzyme catalyzes the interconversion of hydroxypyruvate and D-glycerate in the presence of either NAD or nicotinamide adenine dinucleotide phosphate with almost equal efficiency. D-Glycerate dehydrogenase also catalyzes the reduction of glyoxylate, but is distinct from L-lactate dehydrogenase in that it fails to act on pyruvate, D-lactate or L-lactate. The enzyme is strongly dependent on free thiol groups, as shown by inhibition with p-chloromercuribenzoate, and in the presence of NaCl the reduction of hydroxypyruvate is activated. Michaelis constants for these substrates of D-glycerate dehydrogenase and an equilibrium constant for the NAD-catalyzed reaction have been calculated. An explanation for the lowered Vmax with D-glycerate as compared with DL-glycerate for the rabbit-kidney D-a-hydroxy acid dehydrogenase has been proposed.