Coenzyme specificity of mammalian liver d‐glycerate dehydrogenase

Abstract

d ‐Glycerate dehydrogenase (glyoxylate reductase) was partially purified from rat liver by anion‐ and cation‐exchange chromatography. When assayed in the direction of d ‐glycerate or glycolate formation, the enzyme was inhibited by high (≥0.5 mM), unphysiological concentrations of hydroxypyruvate or glyoxylate much more potently in the presence of NADPH than in the presence of NADH. However, the dehydrogenase displayed a much greater affinity for NADPH ( K m < 1 μM) than for NADH ( K m = 48–153 μM). Furthermore, NADP was over 1000‐fold more potent than NAD in inhibiting the enzyme competitively with respect to NADH. NADP also inhibited the reaction competitively with respect to NADPH whereas NAD, at concentrations of up to 10 mM had no inhibitory effect. When measured by the formation of hydroxypyruvate from d ‐glycerate, the enzyme also displayed a much greater affinity for NADP than for NAD. These properties indicate that liver d ‐glycerate dehydrogenase functions physiologically as an NADPH‐specific reductase. In agreement with this conclusion, the addition of hydroxypyruvate or glyoxylate to suspensions of rat hepatocytes stimulated the pentose‐phosphate pathway. The coenzyme specificity of d ‐glycerate dehydrogenase is discussed in relation to the biochemical findings made in d ‐glyceric aciduria and in primary hyperoxaluria type II ( l ‐glyceric aciduria).