Studies on carbamoylphosphate synthetase in livers of rats fed a protein free diet supplemented with methionine and threonine and injected with N-acetylglutamic acid.

Supplementation of methionine and threonine to a protein free diet reduces urinary nitrogen excretion in rats, and it has been assumed that N-acetylglutamic acid may have a regulatory role in urea synthesis. The present paper reports the effect of supplementation of methionine and threonine on the levels of N-acetylglutamic acid and of carbamoylphosphate synthetase (EC 2.7.2.5) in liver. In spite of a significant decrease in urinary urea when methionine and threonine were supplemented to the protein free diet, the activity of carbamoylphosphate synthetase was not reduced. On the other hand, following injection of glutamic acid, N-acetylglutamic acid or N-acetylglutamine, the excretion of urinary urea of rats fed the protein free diet supplemented with methionine and threonine was significantly reduced whereas the level of carbamoylphosphate synthetase was the same in each group. Furthermore, the concentrations of N-acetylglutamic acid in liver of rats fed the protein free diet or the protein free diet supplemented with methionine and threonine were the same. Therefore, these results indicated that, when methionine and threonine were supplemented to the protein free diet, the changes in the levels of N-acetylglutamic acid and of carbamoylphosphate synthetase in liver does not contribute to the reduced body weight loss and urinary nitrogen excretion.     

Effect of methionine and threonine on the hypercholesterolemia induced by polychlorinated biphenyls in rats fed a nonprotein diet

It was previously reported that the hypercholesterolemia induced by polychlorinated biphenyls (PCB) was influenced by dietary protein quantity and quality. On the other hand, the supplementation of methionine and threonine to a nonprotein diet ameliorated the body weight loss and decreased the urinary urea excretion in rats. We examined the effect of methionine and threonine supplements on the hypercholesterolemia induced by PCB in rats fed a nonprotein diet. The administration of PCB increased plasma cholesterol concentration and the supplements of methionine and threonine to the nonprotein diet significantly accelerated the elevation of plasma level of cholesterol due to PCB feeding. Liver microsomal 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in rats fed the nonprotein diet was also elevated by PCB administration and the supplementation of methionine and threonine caused further inducing effect.     

Alteration of serum lipoprotein metabolism by polychlorinated biphenyls and methionine in rats fed a soybean protein diet

The effects of dietary supplementation of methionine to a 20% soybean protein isolate diet on serum lipoprotein profiles and secretion rate of VLDL in rats receiving polychlorinated biphenyls (PCB) were investigated. Serum cholesterol levels were higher in rats fed PCB or a methionine supplement than in controls. The effects of PCB and methionine were synergistic. The feeding of PCB resulted in more cholesterol in all fractions of serum lipoproteins tested, especially HDL (HDL1 and HDL2). Dietary supplementation of methionine primarily increased HDL cholesterol. The elevation of serum lipoprotein cholesterol due to PCB and/or methionine was significant in HDL1, which showed alpha-mobility. These results showed that methionine and PCB significantly influenced HDL metabolism. The secretion rate of VLDL was higher in rats fed PCB than in controls, but the addition of methionine to diets did not affect the secretion rate of VLDL cholesterol. This implies that PCB increased serum cholesterol partly through the stimulation of VLDL cholesterol secretion.     

Effect of dietary glycine on methionine metabolism in rats fed a high-methionine diet

The alleviation mechanism of methionine toxicity by dietary glycine was investigated in weanling rats fed a high-methionine diet. When rats were fed a 10% casein diet containing 2% methionine, the activities of methionine adenosyltransferase, cystathionine beta-synthase, and cystathionine gamma-lyase, which participate in the methionine metabolism in the transsulfuration pathway, were significantly enhanced. But the addition of 2% glycine to the high methionine diet did not cause further increase in these enzyme activities; the activities of methionine adenosyltransferase and cystathionine beta-synthase were rather decreased while cystathionine gamma-lyase activity was not altered. Methionine transaminase activity was essentially insensitive to the dietary addition of methionine and glycine. In rats fed a high methionine diet, the hepatic methionine level was significantly increased with a concomitant decrease in the levels of glycine, serine, and threonine. The addition of glycine to the high methionine diet effectively suppressed the enhancement of the hepatic methionine level and almost completely restored the glycine level, but it only partially restored the serine level and further decreased the threonine level. From these results, it is suggested that the alleviating effect of dietary glycine on methionine toxicity is primarily elicited by the restoration of the hepatic glycine level rather than by an increase in hepatic enzyme activity.     

Effect of dietary methionine and polychlorinated biphenyls on cholesterol metabolism in rats fed a diet containing soy protein isolate

The effects of supplementation of methionine to a 20% soy protein isolate diet on serum level of cholesterol. 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity, cholesterol 7 alpha-hydroxylase activity, and biliary and fecal steroids in rats with or without receiving polychlorinated biphenyls (PCB) were investigated. Supplementation of methionine and PCB did not affect the growth. Serum level of cholesterol was higher in rats fed PCB than in controls. In rats fed PCB, addition of methionine elevated serum level of cholesterol synergistically. The activity of HMG-CoA reductase was higher in rats fed the methionine-supplemented diet than in those fed the unsupplemented diet when PCB was included in the diets. Cholesterol 7 alpha-hydroxylase activity (nmol/h.100 g body weight) was higher in rats fed PCB than in controls. Biliary secretion of bile acids was higher in rats fed PCB than in controls. On the other hand, fecal excretion of bile acids decreased in PCB-treated rats, but total steroids were not affected by PCB. In rats fed PCB, the addition of methionine did not alter cholesterol 7 alpha-hydroxylase activity and biliary and fecal steroid output. The data suggest that the increase in serum level of cholesterol due to dietary addition of methionine together with PCB would be mediated through the stimulation of hepatic synthesis of cholesterol.     

Accumulation of hypotaurine in tissues and urine of rats fed an excess methionine diet.

Hypotaurine increased in some tissues, especially in muscle, and urine of rats fed methionine excess diet. The significant depression of the body weight and food intake of rats caused by excess methionine was remarkably alleviated as previous reports and hypotaurine content in muscle and urine increased further by supplement with glycine to the excess methionine diet.     

Altered hepatic metabolism of free fatty acid in rats fed a threonine-imbalanced diet

The effects of a threonine-imbalanced diet (8% casein supplemented with 0.3% methionine, TI) on the ketone body production and the secretion rate of lipids were examined in the isolated perfused rat liver. Feeding a TI diet compared to an 8% casein (C) diet resulted in an enlargement of liver, presumably due to 2-4-fold accumulation of triglyceride. Serum triglyceride likewise increased significantly in rats fed a TI diet. No significant difference was found in the other lipid components both in serum and liver. When the livers from rats fed C or TI diets were isolated and perfused in the presence of an exogenous oleate substrate, the TI diet decreased the ketone body production and conversely increased the secretion rate of triglyceride, suggesting an inverse relationship between rates of ketogenesis and triglyceride secretion. The proportion of oleate in the perfusate triglyceride obtained at the end of perfusion was comparable between the C and TI groups, whereas in the post-perfused liver it was higher in the former than in the latter, suggesting a stimulatory effect of the TI diet on the secretion of the oleate in the form of triglyceride. These results indicate that altered hepatic metabolism of long-chain free fatty acids between the pathways of oxidation and esterification is one of the causative factors for triglyceride accumulation in the liver produced by threonine imbalance.     

Effects of methionine and related compounds on plasma cholesterol level in rats fed a high cholesterol diet

The comparative effects of methionine and its structurally and metabolically related compounds on plasma cholesterol level were investigated with rats fed a high cholesterol diet. The plasma cholesterol level was significantly enhanced by the dietary addition of methyl compounds such as L-methionine, D-methionine, choline and betaine. On the other hand, the intermediary metabolites of methionine such as homocystine, cysteine and 3-methylthiopropionate reduced plasma cholesterol. S-Methyl-L-cysteine and dimethylglycine had no significant effect. The plasma cholesterol-elevating effects of methionine, betaine and histidine were all prevented, more or less, by the concurrent addition of glycine to the diet, suggesting the existence of a common mechanism for their effects. The results support a possibility that the plasma cholesterol-elevating efficacy of methionine is attributable to its methyl group.     

Beneficial effect of methionine and threonine supplements on tyrosine toxicity in rats.

The growth retardation and external pathological lesions that occur in rats fed a 10% casein diet containing 5% tyrosine could be alleviated by the supplementation of extra protein (20% casein) or 0.66% methionine plus 0.90% threonine (equivalent to the contents in a 20% casein, respectively). Liver tyrosine aminotransferase activity is elevated by ingestion of excess tyrosine, but lowered by the supplementation of extra casein of methionine plus threonine. In rats fed a high tyrosine diet supplemented with methionine plus threonine, liver p-hydroxyphenylpyruvate hydroxylase activity was not higher than that of the 10% casein group, but liver homogentisate oxidase activity increased significantly. When excess tyrosine was included in the 10% casein diet, free tyrosine concentrations in plasma, liver, muscle and brain were extremely elevated, but when the high tyrosine diet was supplemented with extra casein or methionine plus threonine, their plasma and tissues tyrosine concentrations lowered significantly. A large increase in total phenols, p-hydroxyphenylpyruvate and free tyrosine excretions in urine was produced in animals fed the high tyrosine-low protein diet, but these were lowered by the supplementation of methionine and threonine to the diet.     

Less body fat accumulation in rats fed a safflower oil diet than in rats fed a beef tallow diet

The effects on body fat accumulation of long-term feeding of high fat diets of differing fatty acid composition were studied in rats. The rats were meal-fed isoenergetic diets based on safflower oil or beef tallow for 4 mo. Each diet was freshly prepared every day throughout the experimental period. Oxygen consumption and carbon dioxide production for 6 h after meals were measured between the 50th and 54th d of the experimental period. Oxygen consumption for 3 h after meals was significantly greater in the safflower oil diet group than in the beef tallow diet group, indicating greater diet-induced thermogenesis in the former group. From the assessment of respiratory quotient, the fat oxidation rate was also higher in the former. After the experimental period (4 mo), body fat accumulation was significantly less in the rats fed safflower oil. This difference was, at least in part, ascribed to increased diet-induced thermogenesis and fat oxidation. Serum triacylglycerol level was markedly lower in the rats fed safflower oil than in those fed beef tallow. The lipoprotein lipase activities in heart and soleus muscle after meals appeared to be higher in the former than in the latter. These results suggest that the consumption of the safflower oil diet increased lipoprotein lipase activity in heart and skeletal muscle, resulting in the elevation of fat oxidation rate and the depression of serum triacylglycerol level.     

Effect of colostomy on nitrogen nutrition in the chicken fed a low protein diet plus urea

Recent interest in the role of the ceca in avian nutrition has focused on recycling of urinary nitrogen through the ceca. In the colostomized chicken, we observed appreciable decrease in utilization of dietary urea nitrogen. This is the first report demonstrating that the ceca are able to play an advantageous role in nitrogen nutrition of the chicken, because back-flow of urinary nitrogen into the ceca was completely inhibited by colostomy. When colostomized chickens were fed a diet containing urea, little urea was found in feces but the amount of urea excreted in the urine corresponded to 77.5% of urea intake. Droppings of normal control chickens fed the same diet did not contain urea. However, they contained twice as much ammonia as the urine plus feces of colostomized chickens, indicating active and great degradation of urinary urea to ammonia by microflora in the ceca of control chickens. The recycling of urinary nitrogen through the ceca may be involved in the utilization of dietary urea by the chicken.