Elsevier

Life Sciences

Volume 91, Issues 11–12, 5 October 2012, Pages 389-394
Life Sciences

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

https://doi.org/10.1016/j.lfs.2012.08.003Get rights and content

Abstract

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 3 weeks. 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.

Introduction

Oxidative stress is one of the major risk factors for developing hyperlipidemia. Diet plays an important role in regulating oxidative stress to prevent hyperlipidemia (Avula and Fernandes, 1999, Coyle et al., 2008, Gorinstein et al., 2005, Lee, 2006). Accordingly, the suppression of oxidative stress may be a useful target for new therapies in preventing hyperlipidemia.

Oxidative stress can be caused by oxidative damage to lipids and proteins. In addition to malondialdehyde (MDA), which is generally used as a marker of lipid peroxidation, the oxidation process may be accelerated by the formation and accumulation of carbonylated protein (Pirinccioglu et al., 2010). Recent studies have shown that protein carbonylation may be involved in various disease states, and protein carbonyls (PCO) may serve as biomarkers of oxidative stress (Dalle-Donne et al., 2003). Thus, lipid peroxidation and protein oxidative damage may indicate a higher risk for developing hyperlipidemia, suggesting that effective dietary antioxidants should be not only reactive oxygen species scavengers but also reactive carbonyl scavengers.

Glutathione (GSH) is an antioxidant that is ubiquitous in mammals, and it plays important roles in several detoxification reactions and in the suppression of lipid peroxidation (Anderson, 1998). The liver represents the major site of GSH metabolism, in which GSH and its related enzymes comprise an antioxidant defense system that protects against oxidative stress. Further, as a non-essential nutrient, GSH can be synthesized in the body from several amino acids (l-cysteine, l-glutamic acid and glycine), suggesting that the amino acid profile of the diet may be a major contributor to GSH metabolism responsible for an antioxidative defense.

Rice is a staple cereal that is widely consumed around the world (Kishine et al., 2008, Ohtsubo and Nakamura, 2007, Tran et al., 2005). There has been a growing emphasis on the improvement of the physiological functions of rice (Tran et al., 2004, Yang et al., 2012a), and an association between rice protein consumption and reduced plasma and liver lipid levels has been extensively demonstrated in some studies (Yang et al., 2007, Yang et al., 2012b, Yang and Kadowaki, 2009). However, the precise mechanism by which rice protein affects lipid metabolism has not been fully established. Specifically, there is not yet a comprehensive understanding of the link between the modulation of oxidative status and the consumption of rice protein. Accordingly, evidence describing how rice protein can improve oxidative stress to regulate lipid metabolism is lacking.

The present study, therefore, was conducted to focus on the regulatory effects of rice protein on GSH metabolism and lipid and protein oxidative damage status. The key questions addressed are: (1) whether there is a link between lipid-lowering action and antioxidative stress and the consumption of rice protein and (2) whether rice protein can improve oxidative stress in growing rats fed cholesterol-free diets.

Section snippets

Protein sources

Rice protein (RP) from Oryza sativa L. cv. Longjing 26 (Rice Research Institute of Heilongjiang Academy of Agricultural Sciences, Jiamusi, China) and casein (CAS) (Gansu Hualing Industrial Group, Gansu, China) were used as the dietary protein sources in the present study. RP was prepared by the alkaline extraction method (Yang et al., 2011a, Yang et al., 2012b).

Animals and diets

The present experiment was performed in compliance with the Guidelines of the Committee for Animal Experimentation of Harbin Medical

Body weight, food intake and liver weight

Body weight gain was significantly reduced in rats fed RP compared with CAS (P < 0.05) (Table 2). No significant change in food intake was found between RP and CAS, suggesting that the dietary protein did not affect food intake during the growing period.

As shown in Table 2, less liver swelling was observed in rats fed RP (P < 0.05) than in those fed CAS, as suggested by previous studies (Yang et al., 2007, Yang et al., 2011a).

Plasma lipids, lipoprotein profiles and hepatic lipids

As shown in Table 2, RP significantly reduced the plasma TC

Discussion

We examined the antioxidative potential of rice protein, and especially the effect of rice protein on the antioxidant defense mechanism, through the regulation of glutathione metabolism in growing rats fed cholesterol-free diets. The present study demonstrated that the lipid-lowering activity of the rice protein was closely associated with increases in the gene expression of rate-limiting enzymes in glutathione biosynthesis and the activities of hepatic antioxidative enzymes, as well as a

Conclusion

The present study demonstrates that rice protein can improve oxidative stress, leading to an enhancement of the antioxidative defense mechanism. The lipid-lowering activity induced by rice protein is attributed in part to its stimulation of antioxidative enzyme activities and consequent reduction of the oxidative damage to lipid and protein. These results suggest that rice protein can prevent hyperlipidemia through the modification of glutathione metabolism, in which sulfur amino acids may be

Conflict of interest statement

There are no conflicts of interest to report.

Acknowledgments

This work was supported in part by the National Natural Science Foundation of China (31071526), National Natural Science Foundation of Heilongjiang Province (C201027), Science Innovation Foundation of Harbin (2009RFLXN013), China Postdoctoral Science Foundation Funded Project (200902391, 20080440864) and Heilongjiang Postdoctoral Fund (LBH-Z08143).

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