The importance of glyoxylate and other glycine precursors in the hepatic and renal conjugation of benzoate in normal and hyperammonemic mice

Abstract

Benzoate conjugation, represented by hippurate synthesis, was measured in hepatocytes isolated from normal and sparse-fur (spf) mutant mice, with X-linked ornithine transcarbamylase deficiency, to compare the effects of glyoxylate and piridoxylate (a hemiacetal of glyoxylate and pyridoxine), substituted for glycine. Various amino acid precursors of glycine described in the literature, including serine, threonine, glutamine, and glutamate, were studied in a similar manner. The role of glyoxylate and piridoxylate was also assessed in the renal cortex, in comparison with liver homogenates from normal and hyperammonemic mice. The results indicate the importance of glyoxylate and piridoxylate to completely substitute for glycine (96–115%) in isolated hepatocytes of spf/Y mice, as compared with 53–69% (p < 0.05) in normal +/Y controls. The mean value of amino acid precursors to substitute for glycine in spf mice was serine 51%, threonine 29% (p < 0.05), and glutamine 9%. In normal mice, only serine (21%) (p < 0.01) partly substituted for glycine, whereas threonine, glutamine and glutamate gave negative values of net hippurate synthesis. The specific activity of renal cortex for hippurate synthesis from glycine, glyoxylate and piridoxylate was 3–4 times that of liver homogenates (p < 0.01 – < 0.001). A scheme for the transamination of glyoxylate by alanine is presented. Besides alanine, the excess of glycine, serine, and threonine is readily deaminated in the body to take part in gluconeogenic reactions, thus contributing to hyperammonemia. The cumulative effect of benzoate conjugation to drain these ammoniagenic precursors through glycine may be the basis of its therapeutic effect in hyperammonemia.Key words: glycine, glyoxylate, sodium benzoate, hippurate synthesis, spf mice, hepatocytes.