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Effect of Benzoate on Glycine Metabolism in Rats Fed Branched-Chain -Ketoacids

Institute of Physiological Chemistry, University of Erlangen-Nuremberg, Fahrstrasse 17, 8520 Erlangen, West Germany

Male albino rats (110–120 g) were fed for 9 days an amino acid diet low in nitrogen (nitrogen, 1.05%) devoid of valine, leucine and isoleucine and supplemented with branched-chain -ketoacids (9.4%) (BCKA diet). Pair-fed controls received an isocaloric diet (AA diet), which contained the three branched-chain amino acids (1.4%) instead of the -ketoacids (nitrogen, 1.2%). A third group was fed a standard diet. The animals were divided into three groups of 12 animals, and 6 animals of each group were injected with benzoate (210 µmol/100 g) for 9 days. On day 10 of the experiment the activity of the glycine cleavage system and glycine concentrations were measured in liver; glycine, serine, ammonia, urea, glutamine and glutamate concentrations were determined in plasma on day 10 and determinations of urinary urea, ammonia, glutamine, glutamate, hippuric acid and benzoate were carried out on days 0 and 10 of the experiment. Feeding BCKA led to an increase of plasma and hepatic glycine levels. Activity of hepatic glycine synthesis was reduced in the BCKA-fed group, whereas compared to the AA-fed group no increase in the decarboxylation activity was observed. Injection of benzoate led to an increase in hepatic glycine decarboxylation and glycine synthesis activity in the BCKA-fed group. Plasma glycine levels did not fall, and there was no change in any of the metabolites measured in urine and plasma.

KEY WORDS: • glycine cleavage system • glycine • branchedchain -ketoacids • liver • hippuric acid • benzoate

Manuscript received 23 March 1983.