Effects of Palm Oil Supplementation on Lipid Peroxidation and Glutathione Peroxidase Activity in Cholesterol – Fed Rats
O Oluba, C Onyeneke, G Ojieh, G Eidangbe, R Orole
cardiovascular disease, cholesterol feeding, glutathione peroxidase, lipid peroxidation, palm oil, supplementation
O Oluba, C Onyeneke, G Ojieh, G Eidangbe, R Orole. Effects of Palm Oil Supplementation on Lipid Peroxidation and Glutathione Peroxidase Activity in Cholesterol – Fed Rats. The Internet Journal of Cardiovascular Research. 2008 Volume 6 Number 1.
The effects of palm oil supplementation (5%) to a cholesterol-based (5%) diet on lipid peroxidation, and on glutathione peroxidase activity in rat's liver were examined. The rate of lipid peroxidation in the liver was determined in vitro by the thiobarbituric acid (TBA) test, and by conjugated diene measurement. Palm oil supplementation significantly reduced (p<0.05) the rate of lipid peroxidation in the liver of rats fed 5% cholesterol, compared to the rate observed in the liver of rats fed 5% cholesterol diet without palm oil. On the other hand glutathione peroxidase activity was significantly increased (p<0.05) in the liver of rats fed palm oil supplemented diet compared with those fed cholesterol diet without palm oil. The study suggests that palm oil feeding could be a veritable nutritional tool in the prevention of lipid-induced oxidative damage as usually observed in atheromatous plaque.
Cardiovascular disease (CVD), is the leading cause of death in the world. Elevated concentrations of serum total cholesterol (TC), and LDL-cholesterol (LDL-C), have proved to be among the risk factors in the development of CVD (1). Dietary fats play an important role in influencing blood lipid concentrations, thrombotic tendency and thus the onset of CVD (2). It is generally believed that diets high in cholesterol increase serum TC and LDL-C and in return increases the risk of cardiovascular disease.
Cholesterol feeding has often been used to elevate serum cholesterol levels in studying the etiology of hypercholesterolemic-related metabolic disturbances such as atherosclerosis. The metabolic alterations associated with cholesterol feeding have received increasing attention in recent years. Cholesterol feeding has recently been observed to increase the activity of some enzymes involved in lipid metabolism. These enzymes include triglyceride lipase (TGL), lipoprotein lipase (LPL), and lecithin: cholesterol acyl transferase (LCAT) which together plays a crucial role in the metabolism of HDL-cholesterol (3). Cholesterol inclusion in the diet has also been reported to decrease the circulating concentration of insulin and plasma reduced glutathione levels (4).
It is conceivable that these metabolic alterations may play significant roles in the development of hypercholsterolemic-related metabolic disturbances. Feeding of cholesterol to rats results in a rapid hepatic infiltration of lipids rich in triglycerides and cholesterol. Much of this accumulated cholesterol are usually in the esterified form (5) and thus highly susceptible to peroxidation. The elevated levels of liver cholesterol and cholesteryl esters presumably could increase the susceptibility of the tissue to lipid peroxidation unless proper amounts of antioxidant are present in the tissues.
Palm oil represents the second largest volume of vegetable oil produced in the world. It is highly saturated and contains nearly 50% palmitic acid. Thus, Keys et al. (6) considered palm oil a hypercholesterolemic oil. But this extrapolation of the Keys-Anderson equation about palm oil was not based on actual experimental studies. Studies with animals and humans have indicated that palm oil is quite different from other hypercholesterolemic fats such as lard or coconut oil (7,8,9). Thus well-controlled studies are required to investigate the effects of palm oil and its relation to cardiovascular disease.
Although palm oil is the major vegetable oil consumed in Nigeria, information about the relation of palm oil to health is limited. A few papers have shown that palm oil could maintain the normal growth of rats and cause a more significant reduction of serum cholesterol in rats compared with soybean oil (10). Therefore, it is very important to observe the effect palm oil inclusion as a supplement to a cholesterol-based diet on lipid peroxidation and on the activity of glutathione peroxidase, a physiologically important lipid peroxide-decomposing enzyme, in rats.
Materials and Methods
The remaining liver was then centrifuged under cold (40C) at 105,000 X g for 45 minutes. The supernatant fraction was then carefully removed for enzyme assays. Glutathione peroxidase (EC 18.104.22.168) activity was determined according to the method of Little et al. (14) with cumene hydroperoxide as substrate.
The results obtained in this study are presented in Table 2. Despite the differences in feed composition of the different diets fed the respective groups no significant difference (p>0.05) was observed in the body weight among rats in the different groups. Similarly, the average daily food consumption was not significantly different between the groups. Liver size was significantly higher (p>0.05) in the group fed cholesterol diet only and cholesterol + palm oil group compared with the control group and palm oil diet only group. The highest liver size value was recorded in the group fed cholesterol diet only.
Liver homogenates from rats fed cholesterol diet only and those fed cholesterol + palm oil diet had significantly higher (p<0.05) rate of lipid peroxidation based both on TBA tests and on conjugated diene measurements than samples from rats fed palm oil diet only and control diet. The rate of lipid peroxidation was observed to be highest in the liver samples from rats fed cholesterol diet without palm oil.
Liver glutathione peroxidase activity was significantly lower (p<0.05) in both rats fed cholesterol diet only and cholesterol + palm oil diet compared with the control rats and rats fed palm oil only diet. The enzyme activity was observed to be significantly lower (p<0.05) in rats fed cholesterol only diet compared with those fed cholesterol + palm oil diet.
In the present study, the concentration of TBA-reacting compounds, and the level of conjugated diene in the incubation of liver homogenates were assumed to represent the peroxidative activity of the livers. The observed effects of palm oil as a supplement to a cholesterol based-diet on lipid peroxidation, and on glutathione peroxidase activities in rat's liver were based on results expressed on a per gram liver basis.
The results generated in this study showed that the inclusion of palm oil as a supplement to a cholesterol-based diet markedly reduced lipid peroxidation in the liver, as opposed to the high rate of lipid peroxidation in the livers of rats fed cholesterol diet only (Table 2). These results suggest that factors related to cholesterol feeding such as accumulation of cholesterol or other lipids can influence the requirement of antioxidant in the liver. Anelevated liver cholesterol content has been demonstrated to be associated with increase rate of liver cholesterol degradation. Because lipid peroxidation has been suggested as a normal process in cholesterol catabolism (15,16), the increased rate of cholesterol degradation as a result of increased dietary intake may increase the rate of hepatic lipid peroxidation.
The potential ability of palm oil in being able to suppress the attendant rate of lipid peroxidation observed in this study is of interest since the pathogenesis of many diseases including CVD can involve free radical-mediated lipid peroxidation in biological membranes. Palm oil with a rich content of monounsaturated fatty acid and antioxidant vitamin (vitamin E), has been demonstrated to reduce oxidative stress-induced hypertension in normal rats (17). In a related study conducted by Tosaki et al.(18) , the level of oxidatively modified protein during reperfusion in hearts of rats fed palm oil supplemented diet was significantly reduced compared with that of control rats fed without palm oil. Tosaki et al.(18) , equally observed that the burst in generation of oxygen free radicals in the course of reperfusion was fully prevented in the hearts from palm oil-fed animals. Furthermore, Serbinova et al.(19) , reported that palm oil vitamin E (consisting of a mixture of 45% tocopherols and 55% tocotrienols) completely suppressed LDH leakage from ischaemic hearts, prevented decrease in adenosine triphosphate (ATP) and creatine phosphate levels, and also inhibited the formation of endogenous lipid peroxidation products.
Another interesting revelation from our results is that palm oil supplementation was observed to significantly elevate the activity of glutathione peroxidase in cholesterol-fed rats. We observed, in accord with other data (20) a pronounced decrease in the activity of glutathione peroxide in rats fed cholesterol diet only. However, palm oil was able to produce a significant increase in the enzyme activity despite the 5% cholesterol content of the diet. Glutathione peroxidase is an enzyme involved in the detoxification of lipid peroxides (20). A reduction in the activity of this enzyme would decrease the protection of the tissue against lipid peroxidation. Cholesterol accumulation is known to occur in the aorta or other arteries of hypercholesterolemic animal or human subjects leading to the formation of lipid-rich plaque and atheroma (21). Since the aorta and other arteries are known to contain a very low level of glutathione peroxidase (22), the elevated rate of lipid peroxidation in this tissue would seem to be especially undesirable and may be one of the mechanisms responsible for the development of atherosclerosis in this tissue.
In conclusion, the protective role of palm oil against cholesterol-induced lipid peroxidation in the liver, and in replenishing hepatic level of glutathione peroxidase as revealed in this study is more than mere academic interest as palm oil could be a veritable nutrition tool in the fight against CVD.