THE EFFECT OF DIETARY VITAMIN-E SUPPLY AND A MODERATELY OXIDIZED OIL ON ACTIVITIES OF HEPATIC LIPOGENIC ENZYMES IN RATS

Diets rich in polyunsaturated fatty acids (PUFA) are well known to sup press hepatic lipogenic enzymes compared to fat-free diets or diets ri ch in saturated fatty acids. However, the mechanism underlying suppres sion of lipogenic enzymes is not quite clear. The present study was un dertaken to investigate whether lipid peroxidation products are involv ed in suppression of lipogenic enzymes. Therefore, an experiment with growing male rats assigned to six groups over a period of 40 d was car ried out. Rats received semisynthetic diets containing 9.5% coconut oi l and 0.5% fresh soybean oil (coconut oil diet, peroxide value 5.1 meq O-2/kg oil), 10% fresh soybean oil (fresh soybean oil diet, peroxide value 9.5 meq O-2/kg oil), or 10% thermally treated soybean oil (oxidi zed soybean oil diet, peroxide value 74 meq O-2/kg oil). To modify the antioxidant state of the rats, we varied the vitamin E supply (11 and 511 mg alpha-tocopherol equivalents per kg of diet) according to a bi -factorial design. Food intake and body weight gain were not influence d by dietary fat and vitamin E supply. Activities of hepatic lipogenic enzymes were markedly influenced by the dietary fat. Feeding either f resh or oxidized soybean oil diets markedly reduced activities of fatt y acid synthase, (FAS), acetyl CoA-carboxylase, (AcCX), glucose-6-phos phate dehydrogenase, (G6PDH), 6-phosphogluconate dehydrogenase, and AT P citrate lyase (ACL) relative to feeding the coconut oil diet. Moreov er, feeding oxidized soybean oil slightly, but significantly, lowered activities of FAS, AcCX, and ACL compared to feeding fresh soybean oil . Activities of hepatic lipogenic enzymes were reflected by concentrat ions of triglycerides in liver and plasma. Rats fed the coconut oil di et had markedly higher triglyceride concentrations in liver and plasma than rats consuming fresh or oxidized soybean oil diets, and rats fed oxidized soybean oil had lower concentrations than rats fed fresh soy bean oil. The vitamin E supply of the rats markedly influenced concent rations of thiobarbituric acid-reactive substances in liver, but it di d not influence activities of hepatic lipogenic enzymes. Because the v itamin E supply had no effect, and ingestion of an oxidized oil had on ly a minor effect, on activities of hepatic lipogenic enzymes, it is s trongly suggested that neither exogenous nor endogenous lipid peroxida tion products play a significant role in the suppression of hepatic li pogenic enzymes by diets rich in PUFA. Therefore, we assumed that diet ary PUFA themselves are involved in regulation of hepatic lipogenic en zymes. Nevertheless, the study shows that ingestion of oxidized oils, regardless of the vitamin E supply, also affects hepatic lipogenesis, and hence influences triglyceride levels in liver and plasma.