Effects of addition of sulfur-containing amino acids and glycine to soybean protein and casein on serum cholesterol levels of rats

The effects of the supplementation of methionine (Met), cystine (Cys), and glycine (Gly) to soybean protein or casein on serum and liver lipid levels were studied in rats. Rats were fed cholesterol-free diets containing 25% soybean protein or casein supplemented with 0.75% Met, 2.5% Gly, or a combination of these two for 4 weeks. The addition of Met to soybean protein caused a significant increase in serum cholesterol and this was slightly ameliorated when Gly was given simultaneously. In rats fed casein diets, serum cholesterol tended to decrease when Gly, or Met and Gly were added. A simultaneous supplementation of Met and Gly to casein resulted in a reduction of hepatic cholesterol. Cystine added at the 0.6% level did not cause demonstrable changes in lipid concentrations except for a drop in serum triglyceride of the casein group. When 2.0% Gly was added to cholesterol-enriched diets containing 20% protein, serum cholesterol decreased significantly only when the protein source was casein and the level attained was comparable to that observed in rats fed soybean protein. Liver cholesterol was also markedly decreased by the addition of Gly to casein. The results suggest a possible role of Gly in the regulation of serum cholesterol levels by dietary protein.     

Relationship between amino acid composition of diet and plasma cholesterol level in growing rats fed a high cholesterol diet

The effects of dietary sulfur-containing amino acids and glycine on plasma cholesterol level were studied in rats fed amino acid mixture diets containing cholesterol. The relationship between the amino acid composition of dietary proteins and plasma cholesterol levels was also investigated in rats fed diets containing various kinds of protein such as casein, egg albumin, pork protein, fish protein, corn gluten, wheat gluten and soy protein. Feeding the amino acid mixture corresponding to casein led to an approximately two-fold level of plasma total cholesterol as compared with feeding the amino acid mixture corresponding to wheat gluten. It was possible to reduce the plasma cholesterol of rats fed the amino acid mixture of the casein type by increasing the proportion of cystine in the total sulfur amino acids. Inversely, the deprivation of cystine resulted in an enhancement of the plasma cholesterol of rats fed the gluten type amino acid mixture. Glycine had a tendency to resist increases in the plasma cholesterol level. A significant negative correlation was noted between plasma cholesterol levels and the content of cystine in intact dietary proteins. The results suggest that the differential effect of dietary proteins on plasma cholesterol level is mainly associated with sulfur-containing amino acids included in the protein, regardless of whether it is of animal or plant origin.     

Effects of dietary methionine, cystine, and glycine on endogenous hypercholesterolemia in hepatoma-bearing rats

The effects on hypercholesterolemia of dietary additions of cystine (Cys), methionine (Met), glycine (Gly), and a combination of Met and Gly to a 20% casein diet were studied in male Donryu rats subcutaneously implanted with an ascites hepatoma line of AH109A cells. The hepatoma-bearing rats fed the 20% casein diet lapsed into both endogenous hypertriglyceridemia and hypercholesterolemia when compared to hepatoma-free (normal) rats fed the same diet. The hypercholesterolemia was due to an elevation (3.2 fold) in the very low-density lipoprotein plus low-density lipoprotein (VLDL + LDL)-cholesterol (Ch) level. The high-density lipoprotein (HDL)-Ch level was slightly but significantly decreased. These lipoprotein changes in hepatoma-bearing rats resulted in a marked (4.5 fold) increase in the atherogenic index (AI, (VLDL + LDL)-Ch/HDL-Ch) in comparison with that of tumor-free rats. The dietary additions of 1.2% Met, 1.2% Cys, and a combination of 1.2% Met and 2.5% Gly significantly suppressed the hepatoma-induced increase in (VLDL + LDL)-Ch with no influence on the hepatoma-induced decrease in HDL-Ch, leading to a noticeable fall in AI. These results indicate that hepatoma-bearing rats are useful as an endogenously hyperlipidemic model and that some dietary amino acids are capable of improving hepatoma-induced hypercholesterolemia and abnormal serum lipoprotein profiles.     

Effect of individual amino acids on plasma cholesterol level in rats fed a high cholesterol diet

The effect of individual amino acids added to a cholesterol containing casein diet on the plasma cholesterol level of rats was systematically investigated with objective of confirming the possibility that specific amino acids are responsible for the regulation of plasma cholesterol level by dietary proteins. Half the amino acids tested were effective to some degree in influencing the plasma cholesterol level when amino acids were added singly to the diet at a 5% level. However, only sulfur containing amino acids had significant effects when added to the diet at a 1% level; Met enhanced and Cys reduced the plasma cholesterol level. In addition, the results of an experiment with amino acid mixture diets with different Met, Cys, and/or Gly contents indicated the importance of sulfur amino acids and also of Gly in plasma cholesterol regulation. The results obtained here support the notion that the plasma cholesterol level of rats can be influenced by specific amino acids or amino acid compositions of dietary proteins, at least under the experimental conditions used.     

Effects of nutrition on plasma, liver and muscle amino acids in scalded rats

This study set out to investigate the effect of three different parenterally administered diets on the free amino acid (AA) levels in the plasma, muscle, and liver of scalded rats. Diet I consisted of AA (1.4 g/100 g weight) and a high glucose dose (6 g/100 g weight), diet II consisted of AA and a low glucose dose (1.4 g/100 g weight) and in diet III only a low glucose dose was infused. Parenteral nutrition was started on the 3rd day posttraumatically. Sampling was performed on the 7th day posttraumatically. Nitrogen balances were significantly different in all three groups, being lowest in group III. Scalded rats fed isonitrogenously, but with different amounts of glucose showed only minor changes in AA concentrations. However scalded rats fed with a nitrogen-free diet exhibited significantly reduced total muscle and liver AA levels. These decreased AA levels were due to a drop of glycine in the muscle tissue (74%) and liver (49%). Contrary to the clinical catabolic situation in scalded and starved rats, it was not intracellular glutamine but glycine which was considerably influenced by catabolism and starvation.     

Effects of dietary methionine, cystine and potassium sulfate on serum cholesterol and urinary ascorbic acid in rats fed PCB

Liver weight, liver and urinary ascorbic acid levels and serum cholesterol concentration were higher in rats fed polychlorinated biphenyls (PCB) than in controls. The influences of methionine, cystine and potassium sulfate on these metabolic responses were studied. Methionine or equivalent moles of cystine or potassium sulfate were added to a basal diet containing 10% soy protein isolate. The basal diet contained 0.3% of total sulfur-containing amino acids (S-AAs). When methionine was added to the basal diet, maximum gain in body weight was obtained with 0.5% of dietary S-AAs, while the highest values in serum cholesterol and urinary ascorbic acid were obtained with 0.8% of dietary S-AAs in rats fed PCB. Dietary addition of cystine had little effect on body weight gain. Nevertheless, in rat fed PCB, urinary ascorbic acid and serum cholesterol were significantly higher in rats fed the cystine-supplemented diet than in those fed the unsupplemented diet. Addition of potassium sulfate had no effect on body weight gain, urinary ascorbic acid or serum cholesterol. These results suggest that more S-AAs are required for the highest metabolic response to PCB than for maximum growth, and the higher requirement for S-AAs cannot be replaced by inorganic sulfate.     

Effects of methyl-group acceptors on the regulation of plasma cholesterol level in rats fed high cholesterol diets

The effects of methyl-group acceptors such as glycine, guanidinoacetic acid, and nicotinamide on cholesterol metabolism and phosphatidylcholine(PC) biosynthesis were investigated with rats fed a 25% casein diet containing cholesterol with or without methionine supplement. The effect of ethanolamine, an indirect methyl-group acceptor via phosphatidylethanolamine(PE) formation, was also compared with those of methyl-group acceptors. The methyl-group acceptors and ethanolamine decreased or tended to decrease plasma total cholesterol level when added to the 25% casein diet. These compounds also significantly depressed the methionine-induced enhancement of plasma cholesterol level. The activity of PE N-methyltransferase was decreased by the addition of glycine, guanidinoacetic acid, and nicotinamide, but not ethanolamine, to the reaction mixture when assayed using the postmitochondrial fraction of liver homogenate, suggesting that PE N-methyltransferase activity can be depressed by glycine N-methyltransferase, guanidinoacetic acid N-methyltransferase, and nicotinamide N-methyltransferase systems. The PE N-methyltransferase activity in liver microsomes, however, did not decrease in response to the dietary addition of methyl-group acceptors. The in vitro incorporation of [CH3-14C]methionine into PC of liver slices was also significantly inhibited by the addition of glycine and nicotinamide, but not guanidinoacetic acid and ethanolamine, to the incubation medium. It is suggested that methyl-group acceptors can decrease plasma cholesterol level at least in part through the depression of PC biosynthesis via PE N-methylation pathway, and that the mechanism for the plasma cholesterol-lowering effect of ethanolamine is different from that of methyl-group acceptors.     

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.     

Effects of casein or soybean protein on plasma cholesterol level in jejunectomized or ileectomized rats

The contribution of enterohepatic circulation of cholesterol and bile acids in the hypocholesterolemic activity of soybean protein isolate (SPI) was compared with the activity of casein. Intact, sham-operated, jejunectomized or ileectomized adult rats were fed a cholesterol-free, purified diet containing either 20% casein or 20% SPI for 7 or 10 d. For the subsequent 7 d the diets were reversed. In intact rats the plasma cholesterol concentration (p-chol) was significantly higher when the casein diet was fed than when the SPI diet was fed. Within 3 d after the diet crossover, p-chol in each group of intact rats reached rapidly the same level as that in its previous counterpart. These rapid inherent responses of p-chol to the casein and SPI diets remained unchanged even when the jejunum or the ileum was resected, and p-chol in jejunectomized or ileectomized rats was similar to that found in sham-operated rats. The extent of the change in p-chol induced by exchanging dietary proteins was almost the same among intact, sham-operated, jejunectomized and ileectomized rats. These findings indicate that the interruption of enterohepatic circulation of cholesterol and bile acids is not the major factor involved in the differential effects of dietary proteins on p-chol.     

Dietary taurine or glycine supplementation reduces plasma and liver cholesterol and triglyceride concentrations in rats fed a cholesterol-free diet

The effects of dietary supplementation of taurine or glycine, the two amino acids involved in bile acid conjugation in the liver, on plasma and hepatic lipid concentrations were evaluated in rats fed a cholesterol-free diet. Three groups of male rats (140150g) were fed a cholesterol-free diet (CFD), a taurine-supplemented diet (TSD; CFD + 1.5% taurine) or a glycine-supplemented diet (GSD; CFD + 1.5% glycine) for 5 weeks. There was no significant difference in organ weights and cumulative body weight gain between groups at the end of the experimental period. Plasma triglyceride level was significantly lower in rats fed the TSD (53% decrease, P<0.001) compared to those fed the CFD. Both TSD and GSD significantly lowered the plasma levels of total cholesterol (40% decrease in TSD, p<0.001 and 27% decrease in GSD, p<0.001, respectively) and LDL-plus VLDL-cholesterol (50% decrease in TSD, p<0.05 and 39% decrease in GSD, p<0.01, respectively) compared to the values for CFD. Liver cholesterol concentration was not significantly influenced by the dietary supplementation of taurine or glycine. However, both TSD and GSD showed significantly lower hepatic triglyceride concentrations (43% and 53% decreases in TSD and GSD, p<0.001, respectively), and elevated hepatic free fatty acid levels (77% increases in both groups, p<0.001) compared to the values for CFD. These results suggest the possible roles of dietary taurine or glycine as hypocholesterolemic and/or hypotriglyceridemic agents.     

Addition of arginine but not glycine to lysine plus methionine-enriched diets modulates serum cholesterol and liver phospholipids in rabbits.

Previous experiments from our laboratory showed that in rabbits fed an amino acid diet corresponding to 30% casein, enrichment of the diet with L-lysine and L-methionine caused a marked increase in serum total and LDL cholesterol levels as well as a substantial body weight loss. Both effects were partially prevented by supplementation with L-arginine. The present studies were designed to extend this earlier observation by assessing the role of different dietary amino acids in modulation of cholesterolemic responses and body weights. In the first experiment, the original lysine and methionine-enriched diet was supplemented with glycine in an attempt to modify methionine metabolism, and thus to reduce body weight loss. In addition, the mechanism of action of lysine and methionine was investigated by quantitation of major liver phospholipids. The results showed that glycine addition had no effect on weight loss or hypercholesterolemia, nor did it alter plasma levels of homocyst(e)ine, an intermediate in methionine metabolism. However, enrichment of the diet with lysine and methionine (with or without glycine) significantly increased liver levels of phosphatidylcholine and the ratio of phosphatidylcholine to phosphatidylethanolamine, apparently through increased enzymatic conversion. These changes were consistent with higher lipoprotein levels and thus may explain the hypercholesterolemia. A second experiment showed that similar effects on body weights and serum cholesterol could be obtained by adding lysine and methionine to a diet containing amino acids equivalent to only 15% casein, or 15% intact casein. This approach is more physiologic and also reduces the expense of experiments designed to study the effects of lysine and methionine in more detail.     

Dietary sulfur amino acid effects on fasting plasma cysteine/cystine redox potential in humans

Objective

Oxidation of plasma cysteine/cystine (Cys/CySS) redox potential (E_hCySS) has been associated with risk factors for cardiovascular disease in humans. Cys and CySS are derived from dietary sulfur amino acids (SAA), but the specific effects of SAA depletion and repletion on Cys/CySS redox indices are unknown. The present study examined the effect of dietary SAA intake level on free Cys, free CySS, and E_hCySS in human plasma under fasting conditions.

Methods

Healthy individuals aged 18-36 y (n = 13) were equilibrated to foods providing the RDA for SAA and then fed chemically defined diets without SAA (0 mg·kg^-1·d^-1; n = 13) followed by SAA at levels approximating the mean (56 mg·kg^-1·d^-1; n = 8) or 99th percentile (117 mg·kg^-1·d^-1; n = 5) intake levels of Americans. Fasting plasma samples were collected daily during 4-d study periods and analyzed for free Cys, free CySS, and the E_hCySS.

Results

The SAA-free diet significantly (P < 0.05) decreased plasma-free Cys concentrations and oxidized E_hCySS values after 4 d of SAA depletion. With SAA repletion at 56 mg·kg^-1·d^-1, plasma-free Cys increased significantly and values for E_hCySS became more reduced. Administration of a diet providing a higher dose of SAA (117 mg·kg^-1·d^-1) resulted in a significantly higher level of free Cys and a more reduced E_hCySS.

Conclusions

These results show that free Cys and Cys/CySS redox potential (E_hCySS) in fasting plasma are affected by dietary SAA intake level in humans. Significant changes occur slowly over 4 d with insufficient SAA intake, but rapidly (after 1 d) with repletion.     

Chronic arginine aspartate supplementation in runners reduces total plasma amino acid level at rest and during a marathon run

BACKGROUND: Athletes consume arginine and/or aspartate as potential nutritional ergogenics. Their metabolic effects are controversial and there is some evidence that ingestion of large doses of single amino acids can adversely affect the nitrogen balance or induce an amino acid imbalance. Nevertheless, the general metabolic influence of an arginine aspartate supplementation during a prolonged exercise bout has not yet been investigated. AIM OF THE STUDY: The aim of this study was, therefore, to investigate the general metabolic impact of a chronic supplementation with arginine aspartate in endurance-trained athletes at rest and during a marathon run. METHODS: Fourteen endurance-trained runners participated in this field study which was carried out according to a double-blind crossover design. 15 g of arginine aspartate or a carbohydrate-based placebo were supplemented daily for 14 days before a marathon run. Blood samples for analysis of metabolites and hormones were collected shortly before the run, after 31 km, at the end of the run, and after a recovery period of two hours. Additionally, the respiratory exchange ratio was determined during the run. RESULTS: The plasma level of carbohydrate (glucose, lactate, pyruvate) and fat metabolites (fatty acids, glycerol, beta-hydroxybutyrate), cortisol, insulin, ammonia, lactate dehydrogenase, and creatine kinase as well as the respiratory exchange ratio were unaffected by the supplementation. In contrast, the plasma level of somatotropic hormone, glucagon, urea, and arginine were significantly increased, and the level of most of the remaining plasma amino acids as well as their sum was significantly reduced. CONCLUSIONS: There was no obvious metabolic benefit derived from the chronic supplementation with arginine aspartate. And since furthermore the consequences of a reduction of the total plasma amino acid level are not known, the practice of using single amino acid supplements as potential ergogenics should be critically reevaluated.     

Effects of arginine and lysine addition to casein and soya-bean protein on serum lipids, apolipoproteins, insulin and glucagon in rats

1. The effect of variation in arginine: lysine on the relative cholesterolaemic effects of dietary soya-bean protein and casein was studied. Male rats received semi-purified diets containing soya-bean protein isolate or casein supplemented respectively with varying amounts of lysine or arginine for 40 d and blood samples were taken after a 5 h fast. 2. Neither the addition of arginine to casein nor lysine to soya-bean protein modified the intrinsic effect of these proteins on serum cholesterol. 3. Serum triglyceride levels tended to rise with increasing amounts of lysine supplementation. The opposite trend was obtained with arginine supplementation. 4. Casein caused an increase in the concentration of serum insulin, but not glucagon. The glucagon level was increased proportionately with increasing amounts of arginine, while the addition of lysine showed no effect. The effects of added amino acids on serum insulin were inconclusive. 5. There was a parallel increase in serum apo E and glucagon in response to arginine supplementation, while lysine supplementation increased serum apo E. 6. Thus, arginine: lysine was more effective in regulating serum triglyceride than serum cholesterol. Insulin was associated with different effects of these proteins on serum lipids.