The purpose of the present study was to determine whether the regulation of brain protein synthesis was mediated through changes in the plasma concentrations of insulin and growth hormone (GH), and whether the concentrations of amino acids in the brain and plasma regulate the brain protein synthesis when the quantity and quality of dietary protein is manipulated. Two experiments were done on three groups of aged rats given diets containing 20% casein, 5% casein or 0% casein (Experiment 1), and 20% casein, 20% gluten, or 20% gelatin (Experiment 2) for 1 d (only one 5-h period) after all rats were fed the 20% casein diet for 10 d (only 5-h feeding per day). The aggregation of brain ribosomes, the concentration in plasma GH, and the branched chain amino acids in the plasma and cerebral cortex declined with a decrease of quantity and quality of dietary protein. The concentration of plasma insulin did not differ among groups. The results suggest that the ingestion of a higher quantity and quality of dietary protein increases the concentrations of GH and several amino acids in aged rats, and that the concentrations of GH and amino acids are at least partly related to the mechanism by which the dietary protein affects brain protein synthesis in aged rats.
The brain protein synthesis is sensitive to the dietary protein; however, the role of dietary protein on biomarkers including choline acetyltransferase and nerve growth factor (NGF) for the function of cholinergic neurons remains unknown in young rats. The purpose of this study was to determine whether the quantity and quality of dietary protein affects the concentration of NGF and activity of choline acetyltransferase, and their mRNA levels in the brains of young rats. Experiments were carried out on five groups of young rats (4 weeks) given the diets containing 0, 5, 20% casein, 20% gluten or 20% gelatin for 10 days. The activity of choline acetyltransferase in the cerebral cortex and hippocampus declined gradually with a decrease in quantity and quality of dietary protein. The concentration of NGF in the cerebral cortex and the mRNA levels of choline acetyltransferase in the cerebral cortex and hippocampus did not differ among groups. However, the concentration and mRNA level of NGF in the hippocampus was significantly lower in rats fed with lower quantity of protein or lower quality of protein. In the hippocampus, the mRNA levels of NGF significantly correlated with the NGF concentration when the quantity (r = 0.704, P < 0.01) and quality (r = 0.682, P < 0.01) of dietary protein was manipulated. It was further found that a significant positive correlation existed between the NGF concentration and the activity of choline acetyltransferase in the hippocampus (dietary protein quantity, r = 0.632, P < 0.05; dietary protein quality, r = 0.623, P < 0.05). These results suggest that the ingestion of lower quantity and quality of dietary protein are likely to control the mRNA level and concentration of NGF, and cause a decline in the activity of choline acetyltransferase in the brains of young rats.
SummaryWe have shown that urinary urea excretion decreased in rats fed a low gluten diet supplemented with dietary limiting amino acids. The purpose of present study was to determine whether the addition of dietary limiting amino acids to a low gluten diet affected the synthesis and degradation of N -acetylglutamate and regulated urea synthesis. Experiments were done on two groups of rats, given diets containing 10% gluten or 10% gluten ϩ 0.5% L -lysine, 0.2% L -threonine and 0.2% L -methionine for 10 d. The urinary excretion of urea, and the liver concentration of N -acetylglutamate, and the liver activity of N -acetylglutamate synthetase decreased with the addition of dietary L -lysine, L -threonine and L -methionine. N -Acetylglutamate concentration in the liver was closely correlated with the N -acetylglutamate synthetase activity in the liver and excretion of urea. The greater degradation of N -acetylglutamate was observed in the group fed the 10% gluten ϩ L -lysine, L -threonine and L -methionine. The hepatic concentration of glutamate and plasma concentration of arginine were not related to the N -acetylglutamate concentration in the liver. These results suggest that the addition of limiting amino acids to the low gluten diet controls the synthesis and degradation of N -acetylglutamate in the liver and lowers urea synthesis. Key Words dietary limiting amino acids, urea synthesis, N -acetylglutamate synthesis, Nacetylglutamate degradation, rats Schimke ( 1 , 2 ) has suggested that the concentrations of urea-cycle intermediates were unchanged under conditions affecting the rate of urea excretion (e.g., ingestion of a high-protein diet) and concluded that the activities of various urea-cycle enzymes were regulatory factors of urea synthesis. However, many investigators have previously reported that there was an increase in urinary urea excretion without a comparable increase in the enzyme activities when a diet containing high quality protein was replaced by an isonitrogenous diet with low quality protein ( 3-6 ).We demonstrated that urinary excretion of urea increased with a 10% casein diet and still more with a 10% gluten diet as compared with a 10% whole egg protein diet ( 5 ). Gluten is known to be a lower quality protein than whole egg protein because of deficiency in lysine ( 7 ). The concentrations of threonine and sulfur amino acids in gluten are also lower than in whole egg protein. In a previous study ( 8 ), the activities of argininosuccinate synthetase and carbamylphosphate synthetase, urea cycle enzymes, were not affected by the dietary addition of L -lysine, L -threonine and L -methionine to a low gluten diet, while supplementation with these dietary limiting amino acids to a gluten diet decreased urea excretion.Substrate availability normally may limit the rate of urea synthesis ( 9 ). Urea formation has been shown to be stimulated by adding N -acetylglutamate in perfused liver ( 10-12 ) and in isolated hepatocytes ( 13 ) when substrates for urea production were present in excess. Thus, at lea...
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