Methionine deficiency in rats fed soy protein induces hypercholesterolemia and potentiates lipoprotein susceptibility to peroxidation

Association of Homocysteine, Methionine, and MTHFR 677C>T Polymorphism With Rate of Cardiovascular Multimorbidity Development in Older Adults in Sweden

IMPORTANCE

Strong evidence links high total serum homocysteine (tHcy) and low methionine (Met) levels with higher risk of ischemic disease, but other cardiovascular (CV) diseases may also be associated with their pleiotropic effects.

OBJECTIVES

To investigate the association of serum concentrations of tHcy and Met with the rate of CV multimorbidity development in older adults and to explore the role of methylenetetrahydrofolate reductase (MTHFR) 677C>T polymorphism in this association.

DESIGN, SETTING, AND PARTICIPANTS

The Swedish National Study on Aging and Care in Kungsholmen is a cohort study of randomly selected individuals aged 60 years or older. The present study included data on 1969 individuals with complete information and without CV diseases at baseline, collected from the baseline examination (2001-2004) to the fourth follow-up (2013-2016). Data analysis was conducted from January to May 2019.

EXPOSURES

Concentrations of tHcy and Met were measured from nonfasting venous blood samples. The Met:tHcy ratio was considered a possible indicator of methylation activity. MTHFR status was dichotomized as any T carriers vs noncarriers.

MAIN OUTCOME AND MEASURES

The number of CV diseases at each wave was ascertained based on medical interviews and records, laboratory test results, and drug data. Linear mixed models were used to study the association of baseline tHcy and Met levels and the rate of CV multimorbidity development, adjusting for sociodemographic characteristics, CV risk factors, chronic disease burden, and drug use.

RESULTS

Of 1969 participants, most were women (1261 [64.0%]), with a mean (SD) age of 70.9 (9.8) years; 1703 participants (86.6%) had at least a high school level of education. Baseline measurements of serum tHcy, Met, and the Met:tHcy ratio were associated with the rate of CV disease accumulation (tHcy: β = 0.023 per year; 95% CI, 0.015 to 0.030; P < .001; Met: β = -0.007 per year; 95% CI, -0.013 to -0.001; P = .02; Met:tHcy ratio: β = -0.017 per year; 95% CI, -0.023 to -0.011; P < .001). The association between low Met concentrations and the rate of CV multimorbidity development was restricted to the group with CT/TT alleles of MTHFR (β = 0.023 per year; 95% CI, 0.006 to 0.041; P = .009). Results remained largely significant when individual CV diseases were removed from the total count 1 at a time (eg, ischemic heart disease, tHcy: β = 0.023 per year; 95% CI, 0.013 to 0.027; P < .001; Met: β = -0.006 per year; 95% CI, -0.011 to -0.0003; P = .04; Met:tHcy ratio: β = -0.015 per year; 95% CI, -0.020 to -0.009; P < .001).

CONCLUSIONS AND RELEVANCE

In this study, high tHcy and low Met levels were associated with faster CV multimorbidity development in older age. The interactive association of Met concentrations and MTHFR polymorphism, together with the association found for the Met:tHcy ratio, point toward the relevance of impaired methylation in the pathogenesis of CV aging.

l-Methionine activates Nrf2-ARE pathway to induce endogenous antioxidant activity for depressing ROS-derived oxidative stress in growing rats

BACKGROUND

Methionine is an essential sulfur-containing amino acid. To elucidate the influence of l-methionine on activation of the nuclear factor erythroid 2-related factor 2-antioxidant responsive element (Nrf2-ARE) antioxidant pathway to stimulate the endogenous antioxidant activity for depressing reactive oxygen species (ROS)-derived oxidative stress, male Wistar rats were orally administered l-methionine daily for 14 days.

RESULTS

With the intake of l-methionine, Nrf2 was activated by l-methionine through depressing Keap1 and Cul3, resulting in upregulation of ARE-driven antioxidant expression (glutamate cysteine ligase catalytic subunit, glutamate cysteine ligase modulatory subunit, glutathione synthase (GS), catalase (CAT), superoxide dismutase (SOD), heme oxygenase 1, NAD(P)H:quinone oxidoreductase 1, glutathione reductase (GR), glutathione S-transferase (GST), glutathione peroxidase (GPx)) with increasing l-methionine availability. Upon activation of Nrf2, glutathione synthesis was increased through upregulated expression of methionine adenosyltransferase, S-adenosylhomocysteine hydrolase, cystathionine β-synthase, cystathionine γ-lyse, glutamate cysteine ligase (GCL) and GS, while hepatic expressions of methionine sulfoxide reductases (MsrA, MsrB2, MsrB3) and hepatic enzyme activities (CAT, SOD, GCL, GR, GST, GPx) were uniformly stimulated with increasing consumption of l-methionine. As a result, hepatic content of ROS and MDA were effectively reduced by l-methionine intake.

CONCLUSION

The present study demonstrates that methionine availability plays a critical role in activation of the Nrf2-ARE pathway to induce an endogenous antioxidant response for depressing ROS-derived oxidative stress, which is primarily attributed to the stimulation of methionine sulfoxide reductase expression and glutathione synthesis. © 2019 Society of Chemical Industry.

Long-Term Feeding of Soy Protein Attenuates Choline Deficient-Induced Adverse Effects in Wild Type Mice and Prohibitin 1 Deficient Mice Response More Sensitively

Non-alcoholic fatty liver disease (NAFLD) is the most common chronic liver disease, however the exact cause of NAFLD remains unknown. Methionine, an essential amino acid, is the first limiting amino acid of soy protein, and its deficiency is suggested to cause hepatocyte damage and NAFLD. The objective of this study is to examine the changes in NAFLD susceptibility with soy protein consumption and deterioration due to prohibitin 1 (PHB1) deficiency, an important protein in hepatic mitochondrial function. In this study, liver-specific phb1 +/- mice and wild-type mice were fed a normal diet, choline-deficient diet (CDD), or soy protein diet without choline (SPD) for 16 weeks. Using hematoxylin and eosin staining, we showed that SPD attenuates symptoms of hepatocyte damage and lipid accumulation induced by CDD in mouse liver. The liver damage in mice fed the SPD was alleviated by decreasing lipogenic markers and by increasing anti-inflammatory markers. Furthermore, mRNA expression of genes involved in hepatic methionine metabolism was significantly lower in liver-specific phb1 +/- mice fed with a SPD compared with wild-type mice fed with a SPD. These data suggest a CDD can cause non-alcohol related liver damage, which can be attenuated by a SPD in wild-type mice. These phenomena were not observed in liver-specific phb1 +/- mice. It may therefore be concluded that SPD attenuates CDD-induced liver damage in wild-type mice, and that PHB1 deficiency blocks the beneficial effects of SPD against CDD-induced liver damage.

Methionine Augments Antioxidant Activity of Rice Protein during Gastrointestinal Digestion

To elucidate the influence of methionine, which is an essential sulfur-containing amino acid, on the antioxidant activity of rice protein (RP), methionine was added to RP (RM). The addition of methionine to RM0.5, RM1.0, RM1.5, RM2.0, and RM2.5 was 0.5-, 1.0-, 1.5-, 2.0-, and 2.5-fold of methionine of RP, respectively. Using the in vitro digestive system, the antioxidant capacities of scavenging free radicals (superoxide; nitric oxide; 2,2'-azinobis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt, ABTS), chelating metal (iron), and reducing power were investigated in the hydrolysates of RP and RMs. Upon pepsin-pancreatin digestion, the weakest antioxidant capacity was produced by RP. With the addition of methionine, RMs exhibited more excellent responses to free radical scavenging activities and reducing power than RP, whereas RMs did not produce the marked enhancements in iron chelating activity as compared to RP. The present study demonstrated that RMs differently exerted the free radical scavenging activities that emerged in the protein hydrolysates, in which the strongest scavenging capacities for ABTS, superoxide, and nitric oxide were RM1.5, RM2.0, and RM2.5, respectively. Results suggested that the availability of methionine is a critical factor to augment antioxidant ability of RP in the in vitro gastrointestinal tract.

The Impact of Egg Nutrient Composition and Its Consumption on Cholesterol Homeostasis

Nutrient deficiencies and excess are involved in many aspects of human health. As a source of essential nutrients, eggs have been used worldwide to support the nutritional needs of human societies. On the other hand, eggs also contain a significant amount of cholesterol, a lipid molecule that has been associated with the development of cardiovascular diseases. Whether the increase of egg consumption will lead to elevated cholesterol absorption and disruption of cholesterol homeostasis has been a concern of debate for a while. Cholesterol homeostasis is regulated through its dietary intake, endogenous biosynthesis, utilization, and excretion. Recently, some research interests have been paid to the effects of egg consumption on cholesterol homeostasis through the intestinal cholesterol absorption. Nutrient components in eggs such as phospholipids may contribute to this process. The goals of this review are to summarize the recent progress in this area and to discuss some potential benefits of egg consumption.

Plasma methionine and risk of acute myocardial infarction: Effect modification by established risk factors

BACKGROUND AND AIMS

Methionine (Met) is an essential amino acid involved in methylation reactions and lipid metabolism. A Met-deficient diet may cause hepatic lipid accumulation, which is considered an independent risk factor for atherosclerosis. However, the prospective relationship between circulating Met and incident acute myocardial infarction (AMI) is unknown.

METHODS

We studied the associations of plasma Met and incident AMI in 4156 patients (77% men; median age 62 years) with stable angina pectoris, among whom the majority received lipid lowering therapy with statins. Risk associations were estimated using Cox-regression analyses.

RESULTS

Plasma Met was negatively related to age, serum levels of total cholesterol, low-density lipoprotein cholesterol (LDL-C) and apolipoprotein (apo) B at baseline (all p≤0.05). During a median follow-up of 7.5 years, 534 (12.8%) patients experienced an AMI. There was no overall association between plasma Met and incident AMI; however, plasma Met was inversely associated with risk among patients with high as compared to low levels of serum LDL-C or apo B 100 (multivariate adjusted HRs per SD [95% CI] 0.84 [0.73-0.96] and 0.83[0.73-0.95], respectively; p-interaction ≤0.02). Trends towards an inverse risk relationship were also observed among those younger than 62 years and patients without diabetes or hypertension.

CONCLUSIONS

Low plasma Met was associated with increased risk of AMI in patients with high circulating levels of atherogenic lipids, but also in subgroups with presumably lower cardiovascular risk. The determinants of Met status and their relation with residual cardiovascular risk in patients with coronary heart disease should be further investigated.

Animal Proteins as Important Contributors to a Healthy Human Diet

Adequate protein intake is critical for health and development. Generally, protein of animal origin is of higher quality for humans owing to its amino acid pattern and good digestibility. When administered in mixtures it can enhance the quality of plant proteins, but its availability is often low in low-income communities, especially in young children, the elderly, and pregnant and lactating women, who have increased requirements and in whom high-quality protein also stimulates (bone) growth and maintenance. Although high protein intake was associated with increased type 2 diabetes mellitus risk, milk and seafood are good sources of branched chain amino acids and taurine, which act beneficially on glucose metabolism and blood pressure. However, high consumption of protein-rich animal food is also associated with adverse health effects and higher risk for noncommunicable diseases, partly related to other components of these foods, like saturated fatty acids and potential carcinogens in processed meat but also the atherogenic methionine metabolite homocysteine. In moderation, however, animal proteins are especially important for health maintenance in vulnerable persons.

In vitro antioxidant activity of rice protein affected by alkaline degree and gastrointestinal protease digestion

BACKGROUND

To elucidate whether and how alkali treatment, which is a common process for rice protein (RP) extraction, affects antioxidant activity of RP, the different degree of alkali (from 0.1% to 0.4% of NaOH) was used to extract RP (RP-1, RP-2, RP-3, RP-4).

RESULTS

The antioxidant capacities of scavenging free radicals [2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid] diammonium salt, ABTS; 1,1-diphenyl-2-picrylhydrazyl, DPPH), chelating metals (iron, copper) and reducing power investigated in the hydrolysates of RPs (RP-1, RP-2, RP-3, RP-4) during in vitro pepsin-pancreatin digestion were effectively affected by alkali treatment. The present study demonstrated that the weakest antioxidant responses to ABTS radical-scavenging activity, DPPH radical-scavenging activity, iron chelating activity, copper chelating activity and reducing power were produced by RP-4 extracted by the highest alkali proportion (0.4% NaOH).

CONCULSION

The present study indicates that antioxidant capacity of RP could be more readily depressed by strict alkali degree and affected by gastrointestinal proteases. Results suggest that alkali extraction is a vital process to regulate the antioxidant activity of RP through modifying the compositions of amino acids, which are dependent on alkali magnitude. © 2016 Society of Chemical Industry.

Soybean flour induces a greater increase of the antioxidant defenses in rats fed with a normocaloric diet compared with a hypercaloric diet

BACKGROUND

Soybeans, due to their antioxidant properties, present beneficial health effects. The objective was to evaluate if replacing casein with soy flour, modifies antioxidant defenses in rat liver, compared to animals that continued being fed with casein based diets (normocaloric and hypercaloric).

RESULTS

Four groups of rats were used: CC (control casein), CS (control soy), HC (hypercaloric casein) and HS (hypercaloric soy). Malondialdehyde, in serum and liver, did not present differences. In liver, when comparing CS vs. CC: increased superoxide dismutase 1 (P < 0.001), catalase (P < 0.01) and glutathione reductase (P < 0.05) activities, the total glutathione (P < 0.001) and reduced glutathione (P < 0.05) content and decreased oxidized glutathione content (P < 0.05). In HS vs. HC: increased carbonyl groups (P < 0.01) and superoxide dismutase 1 activity (P < 0.05), and decreased glutathione peroxidase activity (P < 0.01), total glutathione (P < 0.05) and oxidized glutathione content (P < 0.001). In HS vs. CS: decreased glutathione reductase activity (P < 0.01), total glutathione (P < 0.001) and reduced glutathione (P < 0.01) content, and increased oxidized glutathione content (P < 0.05).

CONCLUSION

Replacing casein by soybean flour improves antioxidant defenses, mainly in normocaloric diets.

Hypolipidemic effect of dietary water-soluble protein extract from chicken: impact on genes regulating hepatic lipid and bile acid metabolism

BACKGROUND

Amount and type of dietary protein have been shown to influence blood lipids. The present study aimed to evaluate the effects of a water-soluble fraction of chicken protein (CP) on plasma and hepatic lipid metabolism in normolipidemic rats.

METHODS

Male Wistar rats were fed either a control diet with 20 % w/w casein as the protein source, or an experimental diet where casein was replaced with CP at 6, 14, or 20 % w/w for 4 weeks.

RESULTS

Rats fed CP had markedly reduced levels of triacylglycerols (TAG) and cholesterol in both plasma and liver, accompanied by stimulated hepatic mitochondrial fatty acid oxidation and carnitine palmitoyltransferase 2 activity in the 20 % CP group compared to the control group. In addition, reduced activities and gene expression of hepatic enzymes involved in lipogenesis were observed. The gene expression of sterol regulatory element-binding transcription factor 1 was reduced in the 20 % CP-fed rats, whereas gene expression of peroxisome proliferator-activated receptor alpha was increased. Moreover, 6, 14, and 20 % CP-fed rats had significantly increased free carnitine and acylcarnitine plasma levels compared to control rats. The plasma methionine/glycine and lysine/arginine ratios were reduced in 20 % CP-treated rats. The mRNA level of ATP-binding cassette 4 was increased in the 20 % CP group, accompanied by the increased level of plasma bile acids.

CONCLUSIONS

The present data suggest that the hypotriglyceridemic property of a water-soluble fraction of CP is primarily due to effects on TAG synthesis and mitochondrial fatty acid oxidation. The cholesterol-lowering effect by CP may be linked to increased bile acid formation.

Hydrolyzed protein supplementation improves protein content and peroxidation of skeletal muscle by adjusting the plasma amino acid spectrums in rats after exhaustive swimming exercise: a pilot study

Background

This study was designed to evaluate the effects of hydrolyzed protein supplementation upon skeletal muscle total protein and peroxidation in rats following exhaustive swimming exercise.

Methods

Twenty-four rats were randomized to 4 experimental groups (n = 6 per group): control group fed standard diet without exercise (SD), exercise (EX), exercise plus standard diet for 72 hours (EX + SD), and exercise plus standard diet supplemented with hydrolyzed protein (2 g/kg/d) for 72 hours (EX + HP). Immediately following exercise, the EX group was euthanized for collecting plasma and skeletal muscle samples. The EX + SD and EX + HP groups were fed their respective diets for 72 hour still plasma and skeletal muscle collection. Skeletal muscle samples were used to measure levels of total protein (TP), malondialdehyde (MDA), and protein carbonyl (PC). Plasma samples were used to analyze the amino acids spectrum.

Results

Compared with the EX + SD, EX + HP presented the significantly increased TP (P = 0.02) and decreased MDA and PC levels (P = 0.035). MDA was negatively correlated with the methionine levels. Moreover, EX + HP maintained higher levels of plasmaleucine, isoleucine, and methionine than EX + SD, which may be associated with the increased skeletal muscle TP levels observed (P < 0.05).

Conclusions

These results collectively suggest that hydrolyzed protein supplementation can improve skeletal muscle TP and ameliorate peroxidation damage in rats subjected to exhaustive exercise stress, which may be, at least in part, related with the maintenance of plasma leucine, isoleucine, and methionine levels.

Methionine and choline regulate the metabolic phenotype of a ketogenic diet

Low-carbohydrate ketogenic diets are commonly used as weight loss alternatives to low-fat diets, however the physiological and molecular adaptations to these diets are not completely understood. It is assumed that the metabolic phenotype of the ketogenic diet (KD) is caused by the absence of carbohydrate and high fat content, however in rodents the protein content of KD affects weight gain and ketosis. In this study we examined the role of methionine and choline in mediating the metabolic effects of KD. We have found that choline was more effective than methionine in decreasing the liver steatosis of KD-fed mice. On the other hand, methionine supplementation was more effective than choline in restoring weight gain and normalizing the expression of several fatty acid and inflammatory genes in the liver of KD-fed mice. Our results indicate that choline and methionine restriction rather than carbohydrate restriction underlies many of the metabolic effects of KD.

Rice protein improves oxidative stress by regulating glutathione metabolism and attenuating oxidative damage to lipids and proteins in rats

AIMS

To evaluate the effects of rice protein (RP) on glutathione metabolism and oxidative damage.

MAIN METHODS

Seven-week-old male Wistar rats were fed diets containing casein and RP without cholesterol for 3weeks. Plasma and liver lipid levels, hepatic accumulation of total glutathione (T-GSH), oxidized glutathione (GSSG), reduced glutathione (GSH), malondialdehyde (MDA) and protein carbonyl (PCO) were measured. In the liver, the total antioxidative capacity (T-AOC), mRNA levels of glutamate cysteine ligase catalytic subunit (GCLC) and glutamate cysteine ligase modulatory subunit (GCLM), and the activities of hepatic catalase (CAT), total superoxide dismutase (T-SOD), γ-glutamylcysteine synthetase (γ-GCS), glutathione S-transferase (GST), glutathione reductase (GR) and glutathione peroxidase (GSHPx) were also measured.

KEY FINDINGS

T-AOC, GCLC and GCLM mRNA levels, antioxidative enzyme activities (T-SOD and CAT) and glutathione metabolism related enzyme activities (γ-GCS, GST, GR and GSHPx) were effectively stimulated by RP feeding compared to casein, and RP significantly reduced the hepatic accumulation of MDA and PCO in rats. These results indicate that lipid-lowering activity was induced by RP feeding.

SIGNIFICANCE

The present study demonstrates that RP improves oxidative stress primarily through enzymatic and non-enzymatic antioxidative defense mechanisms, reflected by enhancing the antioxidative status and attenuating the oxidative damage to lipids and proteins. These results suggest that RP can prevent hyperlipidemia in part through modifying glutathione metabolism, and sulfur amino acids may be the main modulator of this antioxidative mechanism.

Activation of Nrf2 is required for up-regulation of the π class of glutathione S-transferase in rat primary hepatocytes with L-methionine starvation

Numerous genes expression is regulated in response to amino acid shortage, which helps organisms adapt to amino acid limitation. The expression of the π class of glutathione (GSH) S-transferase (GSTP), a highly inducible phase II detoxification enzyme, is regulated mainly by activates activating protein 1 (AP-1) binding to the enhancer I of GSTP (GPEI). Here we show the critical role of nuclear factor erythroid-2-related factor 2 (Nrf2) in up-regulating GSTP gene transcription. Primary rat hepatocytes were cultured in a methionine-restricted medium, and immunoblotting and RT-PCR analyses showed that methionine restriction time-dependently increased GSTP protein and mRNA expression over a 48 h period. Nrf2 translocation to the nucleus, nuclear proteins binding to GPEI, and antioxidant response element (ARE) luciferase reporter activity were increased by methionine restriction as well as by l-buthionine sulfoximine (BSO), a GSH synthesis inhibitor. Transfection with Nrf2 siRNA knocked down Nrf2 expression and reversed the methionine-induced GSTP expression and GPEI binding activity. Chromatin immunoprecipitation assay confirmed the binding of Nrf2 to the GPEI. Phosphorylation of extracellular signal-regulated kinase 2 (ERK2) was increased in methionine-restricted and BSO-treated cells. ERK2 siRNA abolished methionine restriction-induced Nrf2 nuclear translocation, GPEI binding activity, ARE-luciferase reporter activity, and GSTP expression. Our results suggest that the up-regulation of GSTP gene transcription in response to methionine restriction likely occurs via the ERK-Nrf2-GPEI signaling pathway.

The effect of methionine supplementation of the AIN-93G semi-synthetic diet on the levels of homocysteine and lipids in experimental rats

OBJECTIVES

The studies were carried out on 36 growing albino Wistar rats. PARTICIPANTS/MEASUREMENTS: The animals were randomly divided into six equinumerous groups (six rats per group), and were fed six different diets for 42 days. The control group (I) was fed with AIN-93G semi-synthetic diet, whereas groups II-VI were fed with AIN-93G semi-synthetic diet supplemented with: 2, 4, 8, 16 and 32 g of methionine/kg diet, respectively. There were assessed enzymatically, in rats' blood serum, the contents of homocysteine, total cholesterol, HDL fraction and triacyloglicerols. In addition, the LDL+VLDL cholesterol content was calculated.

RESULTS

The methionine content of the diet was found to be highly positively correlated with the homocysteine content (r = 0.981) and negatively correlated with the triacylglycerols content (r = -0.916) of the experimental animals' blood serum.

CONCLUSION

In the blood serum of rats fed the highest-methionine diet (32 g methionine/kg diet), the homocysteine content was significantly higher, as were the levels of total cholesterol and its HDL fraction, while the triacylglycerols content was lower as compared to the values obtained for rats fed other diet types.

Effects of a ferment soy product on the adipocyte area reduction and dyslipidemia control in hypercholesterolemic adult male rats

BACKGROUND

Available data on the effects of a fermented soy product enriched with Enterococcus faecium and Lactobacillus Jugurti on circulating lipids and adiposity are not completely settled. This study aimed to observe the effects of a fermented soy product enriched with Enterococcus faecium and Lactobacillus Jugurti on central obesity and dyslipidemia control in Wistar adult male rats.

METHODS

Over a period of 8 weeks, animals had "ad libitum" food intake and water consumption as well as body weight and food consumption was monitored. The animals were assigned to four different experimental groups: Control Group (C); Control + Fermented Product Group (CPF); Hypercholesterolemic diet group (H); and Hypercholesterolemic + Fermented Product Group (HPF). The HPF and CPF groups received an intragastric administration of 1 ml of fermented product daily. After the experimental period the animals were killed by decapitation, blood was collected to measure cholesterol, triglycerides and HDL-cholesterol plasma concentration. Adipocyte circumference, lipolysis and lipogenis rates were measures using epididymal and retroperitoneal white adipose tissues.

RESULTS

The results demonstrated that 1 ml/day/rat of the fermented soy product promoted important benefits such as reduced cholesterolemia in hypercholesterolemic diet group and the adipocyte circumference in both control and hypercholesterolemic diet group.

CONCLUSION

The fermented soy product enriched with Enterococcus faecium and Lactobacillus Jugurti decreased circulating lipids levels and reduced adipocyte area in rats.

Activation of skeletal muscle protein breakdown following consumption of soyabean protein in pigs

Diets with protein of inferior quality may increase protein breakdown in skeletal muscle but the experimental results are inconsistent. To elucidate the relationship, pigs were fed isoenergetic and isonitrogenous diets based on soyabean-protein isolate or casein for 15 weeks, with four to six animals per group. A higher plasma level of urea (2.5-fold the casein group value, P=0.01), higher urinary N excretion (2.1-fold the casein group value, P=0.01), a postabsorptive rise in the plasma levels of urea, 3-methylhistidine and isoleucine in soyabean protein-fed pigs suggested recruitment of circulatory amino acids by protein breakdown in peripheral tissues. Significant differences between dietary groups were detected in lysosomal and ATP-dependent proteolytic activities in the semimembranosus muscle of food-deprived pigs. A higher concentration of cathepsin B protein was found, corresponding to a rise in the cathepsin B activity, in response to dietary soyabean protein. Muscle ATP-stimulated proteolytical activity was 1.6-fold the casein group value (P=0.03). A transient rise in the level of cortisol (2.9-times the casein group value, P=0.02) occurred in the postprandial phase only in the soyabean group. These data suggest that the inferior quality of dietary soyabean protein induces hormonally-mediated upregulation of muscle protein breakdown for recruitment of circulatory amino acids in a postabsorptive state.

The influence of dietary vitamin E, fat, and methionine on blood cholesterol profile, homocysteine levels, and oxidizability of low density lipoprotein in the gerbil

A 90-day feeding study with gerbils was conducted to evaluate the influence of dietary vitamin E levels (25 mg/kg diet, 75 mg/kg, 300 mg/kg, and 900 mg/kg), two levels of dietary methionione (casein or casein+L-methionine (1% w/w)) and two sources of lipid (soybean oil [20%] or soybean oil [4%]+coconut oil [16%, 1:4 w/w]) upon serum lipids (total cholesterol, HDL-cholesterol, LDL-cholesterol). In addition, this study examined the effects of diet-induced hyperhomocysteinemia and supplemental dietary vitamin E on the oxidation of low density lipoproteins. Tissue vitamin E (heart, liver, and plasma) demonstrated a dose response (P< or =0.001) following the supplementation with increasing dietary vitamin E (25, 75, 300, and 900 mg/kg). In addition, tissue vitamin E levels were found to be higher (P< or =0.001) in those animals receiving a combination of coconut oil+soybean oil as compared to the group receiving soybean oil solely. Blood cholesterol profiles indicated an increase (P< or =0.001) in total cholesterol and LDL cholesterol by the influence of saturated fat and supplemental methionine. Low-density lipoprotein cholesterol profile demonstrated a reduction (P< or =0.001) at the higher dietary vitamin E levels (300 and 900 mg/kg) as compared to the 25 mg/kg and 75 mg/kg dietary vitamin E. Plasma protein carbonyls were not influenced by dietary vitamin E nor by supplemental methionine intake. In vitro oxidation of LDL showed that vitamin E delayed the lag time of the oxidation phase (P< or =0.001) and reduced total diene production (P< or =0.001). On the contrary, supplemental methionine decreased (P< or =0.001) the delay time of the lag phase, whereas total diene production was increased (P< or =0.001). Plasma lipid hydroperoxides were significantly reduced (P< or =0.05) with supplemental dietary vitamin E, whereas supplemental L-methionine (1%) resulted in a significant (P< or =0.05) increase in lipid plasma hydroperoxide formation. Plasma homocysteine was elevated (P< or =0.001) with supplemental dietary L-methionine (1%) as well as the inclusion of dietary saturated fat. The present data showed that 1) a combination of dietary lipids (saturated and unsaturated fatty acids) as well as vitamin E and methionine supplementation altered blood cholesterol lipoprotein profiles; 2) in vitro oxidation parameters including LDL (lag time and diene production) and plasma hydroperoxide formations were affected by vitamin E and methionine supplementation; and 3) plasma homocysteine concentrations were influenced by supplemental methionine and the inclusion of dietary saturated fat.

Dietary protein modifies hepatic gene expression associated with oxidative stress responsiveness in growing pigs

Understanding the basis for differences in nutrient requirements and for nutrient effects on health and performance requires an appreciation of the links between nutrition and gene expression. We developed and applied molecular probes to characterize diet-associated postabsorptive hepatic gene expression in growing pigs chronically fed protein-restricted diets based on either casein (CAS) or soy protein isolate (SPI). Eighty-eight expressed sequence tags (ESTs) were identified on the basis of diet-related changes in expression, by using an mRNA differential display method. Expression profiling based on transcription analysis by real-time reverse transcriptase-polymerase chain reaction showed that the SPI diet significantly changed the pattern of gene expression as compared with the CAS diet and allowed identification of coregulated genes. The expression of six genes involved in the metabolism of stress response (glutathione S-transferase, peptide methionine sulfoxide reductase, apolipoprotein A-I, organic anion transport polypeptide 2, calnexin, heat shock transcription factor 1) exhibited significant changes in the transcription level and indicated an increased oxidative stress response in pigs fed the SPI diet. Hierarchical clustering of gene expression data of all 33 ESTs analyzed across 14 pigs fed the two different diets resulted in clustering of genes related to the oxidative stress response with genes related to the regulation of gene expression and neuronal signaling.

Soy proteins and cardiovascular disease

The soybean diet is the most potent dietary tool for hypercholesterolemia. The United States Food and Drug Administration recently approved the health claim for its role in reducing the risk of coronary disease. The hypocholesterolemic effect is directly correlated to the patient's cholesterolemia, with minimal or no reductions occurring at cholesterol of 6 mmol/L or less, and the most benefit occurring in patients with cholesterol of greater than 7 mmol/L. Hypotheses on the mechanism of action include soy fiber, isoflavones (phytoestrogens), and the protein itself. Although there is no evidence for the effect of fiber, studies with ethanol-extracted soy (devoid of isoflavones) indicated a loss of effect, but the extract itself (isoflavone rich) has no hypocholesterolemic activity. In humans, soy protein activates the low-density lipoprotein (LDL) receptor pathway. Recent data suggest that soy protein subunits, particularly 7S, directly activiate LDL receptors in the human liver, thus providing a novel mechanism of plasma cholesterol reduction different from currently available diets and hypolipidemic drugs.

Dietary protein level and origin (casein and highly purified soybean protein) affect hepatic storage, plasma lipid transport, and antioxidative defense status in the rat

The effects of different proportions (10, 20, and 30%) of dietary casein or highly purified soybean protein on lipid metabolism were studied in growing Wistar rats. Hepatic, plasma and lipoprotein lipid, and protein concentrations, plasma thiobarbituric acid-reactive substance (TBARS) levels, and resistance of red blood cells against free-radical attack were determined after a 4-wk dietary regimen. Compared with the 20% casein diet, the 20% soybean protein diet exhibited similar cholesterolemia but lower plasma triacylglycerol concentrations and very-low-density lipoprotein (VLDL) particle number, as measured by diminished contents of VLDL-triacylglycerol, VLDL-protein, and VLDL-apolipoprotein (Apo) B (B-100 and B-48). The soybean protein diet raised high-density lipoprotein (HDL)(2-3) particle number, as measured by enhanced concentrations of HDL(2-3) cholesterol, HDL-phospholipid, and HDL-ApoA-I. Increasing casein or soybean protein level (from 10 to 30%) in the diet involved higher VLDL-ApoB (B-100 and B-48), indicating an increase in the number of VLDL particles. Feeding the 30% casein or 30% soybean protein diet enhanced LDL-HDL(1) cholesterol contents. Despite similar HDL(2-3)-ApoA-I levels, the 30% casein diet enhanced the HDL(2-3) mass and its cholesterol concentrations. In contrast, feeding either the 10 or 30% soybean protein diet significantly lowered HDL(2-3) cholesterol and ApoA-I levels. These effects on cholesterol distribution in lipoprotein fractions occurred despite unchanged total cholesterol concentrations in plasma. Feeding 20% soybean protein versus 20% casein involved lower plasma TBARS concentrations. Decreasing casein or soybean protein levels in the diet were associated with higher plasma TBARS concentrations and had a lower resistance of red blood cells against free-radical attack. The present study shows that dietary protein level and origin play an important role in lipoprotein metabolism and the antioxidative defense status but do not affect total cholesterol concentrations in plasma.

Cholesterol-lowering effects of soybean, potato and rice proteins depend on their low methionine contents in rats fed a cholesterol-free purified diet

Rats were fed cholesterol-free purified diets containing casein, rice (RP), potato (PP) or soybean (SP) proteins having different amounts of methionine (25.9, 21.3, 16.2 and 10.9 g methionine/kg, respectively). Each protein was fed at 250 g/kg diet for 14 d. Growth rates of rats were the same in all groups. Serum total cholesterol concentrations were lower in rats fed SP, PP and RP than in those fed casein. Fecal bile acid plus neutral steroid excretion was significantly higher in rats fed the RP, PP and SP diets compared with those fed casein. There was a significant negative correlation between serum cholesterol concentration and fecal total steroid excretion (r = -0.490, P = 0.01). However, a stronger positive correlation was observed between serum cholesterol concentration and dietary methionine concentration (r = 0.674, P = 0.0003) or methionine:glycine ratios (r = 0.656, P = 0.0005). In a separate experiment in rats fed diets containing amino acid mixtures simulating the RP, PP and SP diets, serum total cholesterol concentrations were lower than in rats fed simulated casein. Fecal total steroid excretion was the same in all groups. A strong correlation was found between serum cholesterol concentration and dietary methionine concentration (r = 0.743, P = 0.0002) or the methionine:glycine ratio (r = 0.685, P = 0.0009) in rats fed the amino acid mixtures. Finally, we examined the hypocholesterolemic effects of 250 g SP or casein/kg diet with or without supplementation with 0.3 g/100 g sodium taurocholate (TC). Supplementation with TC did not alter the hypocholesterolemic effect of SP. These results support the view that RP, PP and SP lower serum cholesterol concentration in a similar manner.