Screening of soy protein-derived hypotriglyceridemic di-peptides in vitro and in vivo.

BACKGROUND: Soy protein and soy peptides have attracted considerable attention because of their potentially beneficial biological properties, including antihypertensive, anticarcinogenic, and hypolipidemic effects. Although soy protein isolate contains several bioactive peptides that have distinct physiological activities in lipid metabolism, it is not clear which peptide sequences are responsible for the triglyceride (TG)-lowering effects. In the present study, we investigated the effects of soy protein-derived peptides on lipid metabolism, especially TG metabolism, in HepG2 cells and obese Otsuka Long-Evans Tokushima fatty (OLETF) rats. RESULTS: In the first experiment, we found that soy crude peptide (SCP)-LD3, which was prepared by hydrolyze of soy protein isolate with endo-type protease, showed hypolipidemic effects in HepG2 cells and OLETF rats. In the second experiment, we found that hydrophilic fraction, separated from SCP-LD3 with hydrophobic synthetic absorbent, revealed lipid-lowering effects in HepG2 cells and OLETF rats. In the third experiment, we found that Fraction-C (Frc-C) peptides, fractionated from hydrophilic peptides by gel permeation chromatography-high performance liquid chromatography, significantly reduced TG synthesis and apolipoprotein B (apoB) secretion in HepG2 cells. In the fourth experiment, we found that the fraction with 0.1% trifluoroacetic acid, isolated from Frc-C peptides by octadecylsilyl column chromatography, showed hypolipidemic effects in HepG2 cells. In the final experiment, we found that 3 di-peptides, Lys-Ala, Val-Lys, and Ser-Tyr, reduced TG synthesis, and Ser-Tyr additionally reduced apoB secretion in HepG2 cells. CONCLUSION: Novel active peptides with TG-lowering effects from soy protein have been isolated.

Structured triacylglycerol containing behenic and oleic acids suppresses triacylglycerol absorption and prevents obesity in rats.

BACKGROUND: Dietary 1(3)-behenoyl-2,3(1)-dioleoyl-rac-glycerol (BOO) has been reported to inhibit pancreatic lipase activity in vitro and suppress postprandial hypertriacylglycerolemia in humans. In the present study, the anti-obesity activities of BOO and its inhibitory effects on lymphatic triacylglycerol (TAG) absorption were investigated in rats. METHODS: In Experiment 1, rats were fed either BOO or soybean oil (SO) diet for 6 weeks. In the BOO diet, 20% of SO was replaced with an experimental oil rich in BOO. In Experiments 2 and 3, rats cannulated in the thoracic duct were administered an emulsions containing trioleoylglycerol (OOO) or an oil mixture (OOO:BOO, 9:1). Tri[1-14C]oleoylglycerol (14C-OOO) was added to the emulsions administered in Experiment 3. RESULTS: No observable differences were detected in food intake or body weight gain between the BOO and SO groups in Experiment 1. Plasma and liver TAG concentrations and visceral fat weights were significantly lower in the BOO group than in the SO group. The apparent absorption rate of fat was significantly lower in the BOO group than in the SO group. In Experiment 2, the lymphatic recovery of oleic and behenic acids was significantly lower at 5 and 6 h after BOO administration than after OOO administration. In Experiment 3, the lymphatic recovery of 14C-OOO was significantly lower at 5 and 6 h after BOO administration than after OOO administration. CONCLUSIONS: These results suggest that BOO prevents deposition of visceral fat and hepatic TAG by lowering and delaying intestinal absorption of TAG.

Intake of soy protein isolate alters hepatic gene expression in rats.

Soy protein isolate (SPI) can elicit various physiological effects such as cholesterol lowering and antiobesity effects. To examine whether hepatic gene expression is altered by SPI intake, rats were fed an SPI or casein diet for 8 weeks. After 8 weeks of feeding, liver weight and plasma triglyceride and cholesterol levels were significantly lower in the SPI group than in the casein group. Hepatic gene expression was investigated using DNA microarrays. The expression profiles and statistical analysis showed clear and significant differences between the SPI and casein groups (p < 0.05); in the SPI group, 63 genes were up-regulated and 57 genes were down-regulated, most involved in various physiological functions such as lipid metabolism, antioxidant activity, transcriptional regulation, and energy metabolism. Especially in lipid metabolism, the down-regulated genes are related to fatty acid synthesis and the up-regulated genes are related to cholesterol synthesis and steroid catabolism. These results suggest that SPI intake could maintain homeostasis primarily by modulating lipid and energy metabolism.

Ethanol washing does not attenuate the hypocholesterolemic potential of soy protein.

OBJECTIVE: A feeding study in rats investigated the principal active component for the hypocholesterolemic effect of soy protein isolate (SPI) by comparing the effect before and after ethanol washing. METHODS: Five-week-old male Sprague-Dawley rats were fed cholesterol-enriched AIN-93G diets containing 20% casein (CAS), 20% SPI, 20% ethanol-washed SPI (EWS), 18.4% EWS plus 1.6% ethanol extract (EE), or 20% CAS plus 1.6% EE for 2 wk. RESULTS: Plasma cholesterol concentrations in rats fed EWS and SPI were comparable and were significantly lower than those in rats fed CAS. The addition of EE to EWS and CAS did not influence plasma cholesterol level. Fecal steroid excretion of the three SPI groups was higher than that of the two CAS groups. The addition of EE to EWS and CAS showed a tendency to increase acidic steroid and decrease neutral steroid. CONCLUSIONS: In this experiment which used identifiable protein preparations, a significant fraction of the cholesterol-lowering effect of SPI in rats was attributed to its protein component but not to the ethanol-extractable minor constituents including isoflavones.

Isoflavone-free soy protein prepared by column chromatography reduces plasma cholesterol in rats.

To know whether isoflavones are responsible for the hypocholesterolemic effect of soy protein, the effect on plasma cholesterol of isoflavone-free soy protein prepared by column chromatography was examined in rats. Five-week-old male Sprague-Dawley rats were fed cholesterol-enriched AIN-93G diets containing either 20% casein (CAS), 20% soy protein isolate (SPI), 20% isoflavone-free SPI (IF-SPI), 19.7% IF-SPI + 0.3% isoflavone-rich fraction (isoflavone concentrate, IC), or 20% CAS + 0.3% IC for 2 weeks. Plasma total cholesterol concentrations of rats fed SPI and IF-SPI were comparable and were significantly lower than that of rats fed CAS. The addition of IC to the CAS and IF-SPI did not influence plasma cholesterol level. Fecal steroid excretion of the three SPI groups was higher than that of the two CAS groups, whereas the addition of IC showed no effect. Thus, a significant fraction of the cholesterol-lowering effect of SPI in rats can be attributed to the protein content, but the isoflavones and other minor constituents may also play a role.

Soy protein functionality and nutrigenomic analysis.

Soy protein has various biological functions, such as cholesterol-lowering effect, anti-obese, in addition to that nutritive value, and may relieve some lifestyle-related diseases, cancer, osteoporosis, menopausal disorder and so on. Because of the cholesterol-lowering effect of soy protein had been studied extremely and internationally for mechanism and clinical proof, the Ministry of Health and Welfare in Japan had approved soy protein product as a food for special health use (FOSHU) in 1994, and also the FDA in the United States also approved the food labeling for that in 1999. Although, some reports suggested that soy protein changes in expression of the genes concerned with lipid metabolism in liver and in adipose tissue, there are few reports that it is concerned with the comprehensive influence of the intake of soy protein on gene expression. The gene expression of liver in soy protein fed animal has been analyzed with the Affymetrix DNA-microarray. About 33% of 8000 genes in soy fed animals differed compared with those of casein fed. After cluster analysis, significant differences were observed in the gene cluster concerned with lipid metabolism, also in the gene related to energy metabolism, transcription factor, and anti-oxidization enzymes.

Malonyl isoflavone glucosides are chiefly hydrolyzed and absorbed in the colon.

Malonyl isoflavone glucosides are water-soluble derivatives of soybean hypocotyls. This study compared the hydrolysis and absorption of malonyl isoflavone glucosides and nonmalonyl isoflavone glucosides in rats. Plasma concentrations of isoflavones were measured after oral administration of malonyl isoflavone glucosides or isoflavone glucosides. After fasting, the duodenum, jejunum, ileum, and colon were excised, and homogenates were prepared. The extent of hydrolysis of each glucoside by each intestinal homogenate was measured. Plasma levels of isoflavone aglycones, such as daidzein and glycitein, were higher in rats administered malonyl isoflavone glucosides than in those administered isoflavone glucosides. The area under the curve of daidzein in plasma of rats administered malonyl isoflavone glucosides was also significantly greater than that in those administered isoflavone glucosides. A transport experiment using Caco-2 cells suggested that degradation of malonyl glucosides to aglycones is necessary for intestinal absorption. Malonyl isoflavone glucosides were hydrolyzed only in the colon, whereas hydrolysis of isoflavone glucosides occurred in the jejunum, ileum, and colon. These results indicated more effective absorption of malonyl isoflavone glucosides than of nonmalonyl isoflavone glucosides. Moreover, effective absorption of malonyl isoflavone aglycones in the colon contributed to the significant increase in plasma isoflavone levels.

Effects of two-month consumption of 30 g a day of soy protein isolate or skimmed curd protein on blood lipid concentration in Russian adults with hyperlipidemia.

Recently the American Heart Association has reported that favorable effects of soy protein on blood lipids were characteristic only for high amounts of soy protein and not observed for an intake less than 30 g/d. However, the period of the studies with the smaller amount was 4-6 wk and we thought a longer study was necessary for the conclusion. The death rate by heart disease is very high in Russia; therefore, we have done this study in Russian subjects with hyperlipidemia. Prior to the study we tried to find a favorable method for subjects to take 30 g protein a day from soybean protein isolate (SPI) or skimmed curd protein (SMP) and decided to use Russian style cookies. Thirty subjects with hyperlipidemia were recruited; however, due to the 5-mo long study 28 of them (19 females and 9 males aged 50+/-2 y) could complete the trial. They were randomly assigned to two groups and were given either cookie for 2 mo separated by a month-long washout interval in a cross-over design. Fasting blood samples were drawn before and after the dietary treatments. Fasting blood samples at 1 mo were also measured as a health check and to observe the trends of the blood parameters in the middle of the study period. Serum samples were used for the lipid and other biochemical measurements. Every month for 3 non-consecutive days, energy and nutrient intakes were assessed and physical activity was estimated by pedometer. With the consumption of SPI for 2 mo, concentrations of total-cholesterol changed from 280+/-7 to 263+/-8 mg/dL (-6.5%, p=0.0099), HDL-cholesterol from 57.4+/-2.5 to 62.6+/-2.9 mg/dL (+9%, p=0.0047), non-HDL-cholesterol (total-cholesterol-HDL-cholesterol) from 223+/-7 to 201+/-8 mg/dL (-11%, p=0.0023) and triglycerides from 204+/-23 to 173+/- 19 mg/dL (-18%, p=0.022). There were no significant changes with SMP (p>0.05). Thus, administration of 30 g SPI a day for 2 mo confirmed its favorable effects on serum lipids in Russians with hyperlipidemia.

Effects of soybean beta-conglycinin on hepatic lipid metabolism and fecal lipid excretion in normal adult rats.

beta-Conglycinin decreased blood triacylglycerol (TAG) levels in male Wistar adult rats. Liver mitochondrial carnitine palmitoyltransferase activity in the beta-conglycinin-fed group significantly increased as against the casein-fed group. Hepatic fatty acid synthase activity in the beta-conglycinin group significantly decreased as against that of the casein-fed group. Fecal fatty acid excretion in the beta-conglycinin group was significantly higher than in the casein group.

Divergent effects of soy protein diet on the expression of adipocytokines.

Adipose tissue secretes various bioactive molecules called adipocytokines. Dysregulation of adipocytokines plays an important role in the development of atherosclerotic vascular diseases. Consumption of vegetable protein reduces the risks of atherosclerotic vascular diseases. Here, we investigated the effects of 10-day dietary soy protein isolate (SPI) on the regulation of adipocytokines in Wistar rats. No significant difference in body weight was observed between SPI and Casein (animal protein) group. Expression of adipose PAI-1 was lower and expression and plasma concentration of adiponectin were higher in SPI than Casein group. Triglyceride content was lower and fatty acid synthase mRNA level in adipose tissue was lower in SPI than Casein group. Although SREBP-1 mRNA expression was decreased in the liver of SPI group, adipose SREBP and PPARgamma mRNA levels remained unchanged. Our data suggest that dietary SPI alters the gene expressions in adipose tissue and has beneficial effects on the expression of adipocytokines.