We examined the lipid-lowering potential of rice protein and especially the effects of rice protein on key enzymes involved in the lipolytic and lipogenic pathways of triglyceride metabolism. Our findings demonstrated that the triglyceride-lowering action of RP was associated with depressing fatty acid synthesis and stimulating triglyceride hydrolysis, suggesting that rice protein might possess a potential of anti-adiposity.
The most frequently suggested mechanism responsible for the lipid-lowering effect of plant protein is the interference with enterohepatic circulation, leading to an inhibition of hepatic secretion of lipids into circulation, which is closely associated with hepatic lipogenesis and lipoprotein production [17, 26–28]. Yang et al. suggested that rice protein could depress the VLDL assembly and secretion to result in the decreased hepatic secretion of triglyceride and cholesterol into circulation, hence hypocholesterolemia and hypotriglyceridemia . In the present study, results further confirmed and expanded this view. After 2 weeks feeding period, in addition to a significant reduction in hepatic accumulation of total lipids, cholesterol and triglyceride induced by RP-feeding, both RP-A and RP-E were found to be effective in depressing the activities of various enzymes involved in the regulation of fatty acids synthesis, showing the drastic reductions in the activities of fatty acid synthase, glucose 6-phosphate dehydrogenase and malate dehydrogenase. Furthermore, data obtained here indicated that there was a significant positive correlation between the activity of fatty acid synthase and the hepatic accumulation of triglyceride (r = 0.7939, P < 0.05), as well as the concentration of plasma VLDL-C (r = 0.7248, P < 0.05). As a results, a significant positive correlation between the activity of fatty acid synthase and the deposit fat (r = 0.8739, P < 0.05) was observed in this study. Thus, the decreased biosynthesis of fatty acids, in turn, will reduce the production of VLDL particles into circulation, hence leading to lower plasma triglyceride level and less deposit fat in the body. In accordance with the previous findings , results suggest that the lowering-effects on accumulation of lipids and deposit fat in rats were in part caused by the downregulation of activities of lipogenic enzymes, which also involved with the inhibition of hepatic assembly and secretion of VLDL.
Lipogenesis and lipolysis are two major processes for maintaining triglyceride homeostasis through regulatory triglyceride storage and triglyceride mobilization [16, 17, 30]. During lipolytic reaction, LPL is a rate-limiting enzyme in the provision of free fatty acid to muscles (utilization) and the adipose tissue (storage), thereby leading to weight loss or obesity . Thus, the enzymolysis action of LPL may be an important factor participating in the accumulation of triglyceride.
Of interest was the finding in this study that the decreased concentrations of triglyceride and free fatty acid in the plasma were found in RP groups, despite the observed increment of the activities of LPL and HL induced by RP-A and RP-E. Also, compared with CAS, we did not observe the plasma VLDL-elevating effect of RP-feeding. Thus, the question arises why the stronger stimulation of triglyceride hydrolysis and the more concentration of free fatty acids, which were induced by higher activity of LPL and HL, could not result in the excess accumulation of triglyceride and free fatty acid in plasma. The precise mechanism by which RP affects triglyceride metabolism is not fully understood, but the mechanism responsible for the effect of LPL and HL on the removal of VLDL from circulation by hepatic uptake should be taken into account [16–18]. Besides its hydrolytic activity, LPL and HL might interact with lipoproteins to anchor them the vessel wall and facilitate lipoprotein particle uptake by the liver. Particularly, LPL and HL are also important in HDL metabolism contributing to the transfer of surface lipid to small HDL after lipolysis [17, 18]. In addition, it is noteworthy that the decreased fatty acid synthesis in this study was unable to completely compensate for the marked depletion of hepatic triglyceride for the hydrolysis and the secretion into circulation via VLDL. To support this view, Yang et al. provided direct evidence by the perfused rat liver under the same experimental condition that hepatic triglyceride secretion into circulation as well as hepatic triglyceride secretion into VLDL were effectively depressed by RP-feeding, showing a decrease of the availability of hepatic triglyceride for VLDL secretion into circulation . Clearly, the finding observed in this study suggests that, in addition to the proposed mechanism of inhibition of fatty acid synthesis and the stimulation of triglyceride oxidation, the uptake lipoprotein may participate in the triglyceride-lowering action of RP, where HDL is the major plasma lipoprotein.
Also, compared with CAS, excess accumulation of deposit fat was not observed in RP groups, indicating less free fatty acid was used for storage in rats fed RP-A and RP-E, despite the stimulation of hydrolysis of triglyceride. To explain this phenomenon, some studies suggest that weight loss selectively increase adipose tissue LPL activity as an attempt to maintain lipid stores, resulting in a diminish in fat mass [18, 31]. Thus, in light with this view, results indicated that lower deposit fat and weight loss in rats under the present experimental condition was closely associated with higher activity of LPL. Furthermore, a significant negative correlation between the activity of LPL and the weight of abdominal fat (r = -0.8761, P < 0.05), as well as the concentration of plasma triglyceride (r = -0.8410, P < 0.05) was found in rats fed cholesterol-enriched diets. Taken together, results obtained in this study clearly suggest that rice protein may possess a vital function in improving lipids level and adiposity.
The causes of the lipid-lowering action of rice protein, especially resistant to adiposity, appear to be multi-factorial. It has been demonstrated that the biological utilization of a protein is primarily dependent on its digestibility by gastric, pancreatic, and intestinal peptidases [32, 33], providing the insight that the digestibility of rice protein may be a major factor to influence lipid metabolism. To support this view, in the present study, the lower intestinal absorption of fat and triglyceride caused by higher indigestible proportion in the intestine was found in RP-feeding, showing a significant negative correlation between the apparent protein digestibility and the fecal excretion of total fat (r = -0.8824, P < 0.05), as well as the fecal triglyceride excretion (r = -0.9165, P < 0.05). As a results, a significant positive correlation between the apparent protein digestibility and the deposit fat (r = 0.8567, P < 0.05) as well as the plasma TG concentration (r = 0.8627, P < 0.05) was observed in this study. In addition, the inhibition of activities of ALT and AST in plasma was also investigated RP-A and RP-E, indicating that RP-feeding could efficaciously improve liver histology and result in less fatty infiltration in hepatocytes as compared to CAS. On the other hand, the amino acid compositions can be influenced by the extraction method. The contents of the first limiting amino acid-Lys and the second limiting amino acid-Thr in RP-A and RP-E were lower than those in CAS. As a result, the higher ratio of Arg/Lys, which can regulate the digestibility of rice protein , was found in RP-A (2.56) and in RP-E (3.15) than that in CAS (0.44). Thus, the body-fat lowering response to RP may be in part attributable to their lower digestibility. Clearly, the precise mechanism remains to be clarified in further studies.
Here, it must be noted that the association of changes in the metabolism of branched chain amino acids (BCAAs) with alteration of lipids level was observed under the present experimental condition. A significant positive correlation between the plasma level of BCAAs and the weight of body fat (r = 0.7316, P < 0.05), as well as the gain in body weight (r = 0.7270, P < 0.05) was investigated in this study. The observed changes induced by RP-E tended to be the lowest level of plasma BCAAs among the experimental groups (RP-E: 466.5 nmol/mL; RP-A: 482.5 nmol/mL; CAS: 575.2 nmol/mL), showing the similar tendency with plasma triglyceride (r = 0.7958, P < 0.05). Indeed, an improved understanding of the mechanism underlying obesity-related rises in plasma BCAAs is important. It has been demonstrated that obesity-related elevations in plasma leucine are associated with alterations in enzymes involved in BCAA metabolism . In addition, raises in plasma BCAAs due to a block in mitochondrial branched chain amino acid transaminase (BCATm) have been also associated with improvement in glucose metabolism . Thus, an idea, which is consistent with previous studies, is that a reduction in plasma concentrations of BCAAs induced by RP-feeding might represent a possible mechanism for improving body weight control and resistant adiposity in rats fed cholesterol-enriched diets. Clearly, additional studies are required to confirm this view.