Metabolism of histidine in growing rats at various dietary protein levels.

SummaryThe metabolic fate of L-[35S]cysteine was investigated in growing rats fed on diets containing graded levels of protein calorie percentages (0,5,10,15, and 30PC%) by use of purified whole egg protein at 4,100 kcal of metabolizable energy per kilogram of diet. Incor poration of radioactivity into body protein during the 12h period after intraperitoneal injection of 35S-cysteine was about 70% of the dose in the 0 to 15PC% group, but it decreased significantly in the 30PC% group, showing a break point at around 15PC% in the diet. A considerable amount of the radioactivity was recovered in the carcass soluble fraction in which the label in the taurine fraction gradually in creased with increasing dietary protein levels from 0 to 30PC%. Urinary excretion of total 35S during the 12h period was depressed in the lower PC% groups, but it increased in the 30PC% group, about 23% of the dose being recovered. More than 50% of the urinary radioactivity was present in the inorganic sulfate fraction, but less than 10% was in the taurine fraction. These results indicate that cysteine sulfur is preferentially utilized for body protein synthesis, especially in dietary protein depletion, and that the oxidation of cysteine sulfur to taurine and inorganic sulfate is elevated with higher PC% in the diet. The response pattern of cysteine sulfur metabolism to dietary protein intake resembled that of cysteine carbon metabolism which was previously reported. Key Words amino acid metabolism, cysteine sulfur, taurine, urinary sulfateCyst(e)ine is nonessential for the growth of rats and other mammals, since this amino acid can be synthesized de novo where cysteine sulfur is derived from methionine and its carbon skeleton from serine (1). Rose and Wixom (2) have reported that about one-third of the methionine requirement of the growing rat can be satisfied by cystine. Cysteine sulfur is catabolized mainly by two pathways, a) to cysteine sulfinic acid in a reaction catalyzed by cysteine dioxygenase and b) to reduced inorganic sulfur catalyzed by cysteine desulfhydrase (1, 3). The dietary 507

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Metabolic Fate of Cysteine Sulfur in Growing Rats at Various Dietary Protein Levels.

Tanaka

Takahashi

Ogura

1993

Journal of Nutritional Science and Vitaminology, J Nutr Sci Vit

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Metabolic Fates of Carbon Skeletons of Methionine, Serine, and Alanine in Growing Rats Fed Soybean Protein Diets.

Tanaka

Nakajima

Mori

et al. 1994

Journal of Nutritional Science and Vitaminology, J Nutr Sci Vit

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SummaryThe effects of dietary soybean protein isolate intakes on the metabolic fates of L-[U-14C]methionine, L-[U-14C]serine, and L-[U-14C] alanine were investigated in growing rats. In the growth experiment for 21days, body weight gain reached a plateau at more than 20 protein calorie percent (PC%) in the diet, and the protein efficiency ratio attained maximum in the 10 PC% group. Carcass and liver lipid contents increased greatly in the lower levels of dietary soybean protein, which was associated with their decrease in moisture contents. In the isotope experiments, the incorporation of 14C into the body protein 12h after the injection of [14C]methionine was extremely high (more than 80% of the dose) in the 5 to 15 PC% groups, but decreased thereafter in the higher PC% groups. The expired 14CO2 production from [14C]methionine was depressed in the lower PC% groups, and thereafter it increased with increasing levels of dietary protein, showing a break point at around 20 PC%. The carbon skeleton of [14C]alanine was extensively oxidized to 14002 , even in protein-depleted rats, while serine carbon was utilized for body protein synthesis rather than for energy production. These results indicate that methionine, which is a limiting amino acid of soybean protein for rats, is preferentially utilized for body protein synthesis especially in protein-depleted rats, and the metabolic responses of methi onine, serine, and alanine are quite different from each other.

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Metabolism of Essential Amino Acids in Growing Rats at Graded Levels of Soybean Protein Isolate.

Tanaka

Shibata

Mori

et al. 1995

Journal of Nutritional Science and Vitaminology, J Nutr Sci Vit

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SummaryThe metabolic fates of the carbon skeletons of essential amino acids were investigated in growing rats fed on diets containing graded percentages of protein calories (5,10,15,20, 30, and 40 PC%) using soybean protein isolate at 4,100 kcal of metabolizable energy per kg of diet. The incorporation of 14C into the body protein 12h after the intraperitoneal injection of labeled essential amino acids was more than 60% of the dose in the 5 to 10 PC% groups, but it began to decline gradually in the higher PC% groups. The expired 14CO2 production from labeled threonine, tryptophan, leucine, isoleucine, valine, phenylalanine, and lysine increased linearly with increasing levels of dietary soybean protein, but the rate of increase in the 14CO2 production was lower in the higher PC% groups. In comparison, the 14CO2 production from methio nine, histidine, and arginine decreased in the 5 to 15 PC% groups, and it increased linearly in the higher PC% groups. The extent of 14CO2 production varied among the essential amino acids in each dietary group. These results indicate that the carbon skeletons of essential amino acids are mainly utilized for body protein synthesis, but significant amounts of their carbons are oxidized to expired carbon dioxide for energy produc tion, and that the metabolic responses of these amino acids to dietary protein level change at around 20 PC%, where the growth rate reached its approximate maximum. The utilization of individual essential amino acids in rats fed on the soybean protein diets changed markedly as compared to those on purified whole egg protein diets, the results of which have been reported elsewhere.

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Metabolism of histidine in growing rats at various dietary protein levels.

Cited by 3 publications

References 4 publications

Metabolic Fate of Cysteine Sulfur in Growing Rats at Various Dietary Protein Levels.

Metabolic Fate of Cysteine Sulfur in Growing Rats at Various Dietary Protein Levels.

Metabolic Fates of Carbon Skeletons of Methionine, Serine, and Alanine in Growing Rats Fed Soybean Protein Diets.

Metabolism of Essential Amino Acids in Growing Rats at Graded Levels of Soybean Protein Isolate.

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