|ISSN No. 1606-7754 Vol.16 No.1 April 2008|
Effect of rosmarinic acid on insulin sensitivity, glyoxalase system and oxidative events in liver of fructose-fed mice.
Balasubramanian Vanithadevi, Carani Venkataraman Anuradha
Department of Biochemistry and Biotechnology, Annamalai University, Annamalai Nagar -608 002, Tamil Nadu, India.
The study investigates the effects of rosmarinic acid (RA) on insulin sensitivity, protein glycation and oxidative events in fructose-fed mice, a model of insulin resistance (IR). Experiments were performed in four groups of animals administered either fructose diet or starch diet with and without RA administration. Insulin sensitivity indices were computed at the end of the treatment period. Redox homeostasis in liver was determined by assaying lipid peroxidative markers and antioxidants in the liver. Glyoxalase system and protein damage were assessed by assaying aldehydes, glyoxalase I and II, protein carbonyls, total thiols and nitrosothiols. Protein glycation was studied by measuring glycated hemoglobin, fructosamine and advanced glycation end products. Mitochondrial function was assessed by assaying succinate dehydrogenase and calcium ATPase. Fructose administration caused glycation of proteins, changes in metabolic parameters, inactivation of the glyoxalase system and depletion of antioxidants. Oxidative stress and reduced mitochondrial function were observed. Administration of RA to fructose-fed mice mitigated the above alterations. The data suggest that metabolic and redox disturbances in this dietary model of IR could be mitigated by RA. The antioxidant action of RA could be one of the contributing mechanisms for the improvement of insulin sensitivity.
Key words: Antioxidants, fructose, glycation, glyoxalase, insulin resistance, rosmarinic acid.
Chronic changes in carbohydrate composition of the diet have an impact on intrahepatic milieu specifically by inducing adaptative changes in hepatic glucose metabolism that characterize obesity and type 2 diabetes. For instance, rats administered a high fructose diet develop impaired glucose tolerance and mild obesity secondary to a defect in insulin action1. Exposure of the liver to a high fructose load increases hepatic gluconeogenesis, reduces the ability of insulin to suppress hepatic glucose production, and stimulates lipogenesis and triglyceride accumulation.2
Studies have emphasized that fructose feeding also facilitates oxidative and nitrosative damage in the liver.3,4 Enhanced reactive oxygen species (ROS) production, a defect in nitric oxide (NO•) production and oxygen radical mediated NO• inactivation have been documented in rats fed a high fructose diet.5,6 We have recently demonstrated the increased accumulation of nitrotyrosylated proteins in fructose-fed rat.7
The glyoxalase system consists of 2 enzymes, glyoxalase I and glyoxalase II. Glyoxalase I catalyses the formation of S-D-lactoylglutathione by the reaction between methylglyoxal (MG) and glutathione (GSH) while glyoxalase II catalyses the hydrolysis of S-D-lactoylglutathione to D-lactic acid and GSH. Glyoxal may be formed as degradation products of autoxidation, of glucose or from glucose adducts to proteins.8 Glyoxal is more reactive than glucose and can react non-enzymatically with proteins forming crosslinks and adducts, the degradation of which can be associated with oxidative stress. Thus, the glyoxalase system converts the levels of toxic a-oxoaldehydes to nontoxic R-2-hydroxy acids. The physiological substrate of the glyoxalase system, MG, could be formed non-enzymatically from dihydroxyacetone phosphate and glyceraldehyde–3 phosphate. When fructose is consumed as the sole source of carbohydrate, there is increased flux through the glycolytic pathway and increased formation of intermediates of glycolysis and an increased production of oxoaldehydes could be expected upon fructose feeding.
Plant phenolics are multifunctional antioxidants and they might act at one or more steps in the oxidative stress cascade.9 Rosmarinic acid (a-O- caffeoyl –3, 4 dihydroxy phenyllactic acid, RA) is a diphenolic derivative of caffeic acid, found as a secondary metabolite in many species of herbs and spices such as rosemary, salvia, sweet basil and mint that belong to the families of Boraginaceae or Laminaceae. These plants are widely used as culinary herbs, especially in Mediterranean dishes and have long been used in traditional medicine in Southern Europe, Japan and India for the treatment of numerous maladies including diabetes.10 RA has been shown to be a potent inhibitor of superoxide (O2 •-) and nitric oxide synthases (NOS) and an effective protector against peroxinitrite-mediated tissue damage.11 Though we know much about the antioxidant activity of RA, there is a lack of information on the effect of RA in combating oxidative stress in the insulin resistance state. In view of all the above, the present study was carried out to investigate the effect of RA on whole body insulin sensitivity, protein glycation, glyoxalase system and oxidative events in liver of mice fed a high fructose diet. In addition, the effect of RA on the utilization of glucose in diaphragm in vitro was studied in the presence and absence of insulin and reported.