The fatty acid compositions of the lipids and the lipid peroxide concentrations and rates of lipid peroxidation were determined in suspensions of liver endoplasmic reticulum isolated from rats fed on synthetic diets in which the fatty acid composition had been varied but the remaining constituents (protein, carbohydrate, vitamins and minerals) kept constant. Stock diet and synthetic diets containing no fat, 10% corn oil, herring oil, coconut oil or lard were used. The fatty acid composition of the liver endoplasmic reticulum lipid was markedly dependent on the fatty acid composition of the dietary lipid. Feeding a herring-oil diet caused incorporation of 8.7% eicosapentaenoic acid (C(20:5)) and 17% docosahexaenoic acid (C(22:6)), but only 5.1% linoleic acid (C(18:2)) and 6.4% arachidonic acid (C(20:4)), feeding a corn-oil diet caused incorporation of 25.1% C(18:2), 17.8% C(20:4) and 2.5% C(22:6) fatty acids, and feeding a lard diet caused incorporation of 10.3% C(18:2), 13.5% C(20:4) and 4.3% C(22:6) fatty acids into the liver endoplasmic-reticulum lipids. Phenobarbitone injection (100mg/kg) decreased the incorporation of C(20:4) and C(22:6) fatty acids into the liver endoplasmic reticulum of rats fed on a lard, corn-oil or herring-oil diet. Microsomal lipid peroxide concentrations and rates of peroxidation in the presence of ascorbate depended on the nature and quantity of the polyunsaturated fatty acids in the diet. The lipid peroxide content was 1.82+/-0.30nmol of malonaldehyde/mg of protein and the rate of peroxidation was 0.60+/-0.08nmol of malonaldehyde/min per mg of protein after feeding a fat-free diet, and the values were increased to 20.80nmol of malonaldehyde/mg of protein and 3.73nmol of malonaldehyde/min per mg of protein after feeding a 10% herring-oil diet in which polyunsaturated fatty acids formed 24% of the total fatty acids. Addition of alpha-tocopherol to the diets (120mg/kg of diet) caused a very large decrease in the lipid peroxide concentration and rate of lipid peroxidation in the endoplasmic reticulum, but addition of the synthetic anti-oxidant 2,6-di-t-butyl-4-methylphenol to the diet (100mg/kg of diet) was ineffective. Treatment of the animals with phenobarbitone (1mg/ml of drinking water) caused a sharp fall in the rate of lipid peroxidation. It is concluded that the polyunsaturated fatty acid composition of the diet regulates the fatty acid composition of the liver endoplasmic reticulum, and this in turn is an important factor controlling the rate and extent of lipid peroxidation in vitro and possibly in vivo.
I . The dependence of the rate of oxidative demethylation in the liver endoplasmic reticulum on the fatty acid composition of the endoplasmic reticulum has been studied by varying the lipid content of the diet. 3. Destruction of the polyunsaturated fatty acids in herring oil by irradiation with 400 krad caused incorporation of a smaller quantity of w 3 unsaturated acids into the endoplasmic reticulum and decreased the rate of oxidative demethylation (4.83 nmol formaldehyde/min per mg protein). 2.4. The inductive effects of phenobarbitone on oxidative demethylation were partially dependent on changes in the fatty acid composition of the endoplasmic reticulum. Phenobarbitone (100 mg/kg) increased the percentage of C,,:, from 25.1 to 29.4% in rats given a maize-oil diet, increased the percentage of Czo:5 from 8.7 to 10.3 % in rats given a herring-oil diet and decreased the percentage of arachidonic acid (20:4) and C, , : , in rats given a lard, maize-oil, herring-oil or irradiated-herring-oil diet. 5.Intraperitoned a-tocopherol (50 mg/kg) incre2sed the percentage of Ceo., from I 1 . 1 to 13.1 yo in rats given a lard diet and from 5.9 to 7.3 % in rats given a herring-oil diet.6. It is concluded that dietary Cla:l is an important factor in the regulation of the rate of oxidative deniethylation in the liver endoplasmic reticulum but this may be replaced effectively by dietary C, , : , w3 and C, , : , w 3 acids. Oxidative demethylation is regulated by changes in the fatty acid composition of the membranes of the liver endoplasmic reticulum.The endoplasmic reticulum in the liver is responsible for the oxidative metabolism of a wide variety of drugs and carcinogens and membrane integrity is essential for oxidative metabolism (Lu et al. 1969; Strobe1 et nl. 1970).The stability and permeability of membranes in the endoplasmic reticulum depends on the phospholipids and these are markedly influenced by the nature, extent of unsaturation and arrangement of their constituent fatty acids (Lucy, 1972;Chapman, 1973). Wills (1971) has shown that lipid peroxidation, which causes fragmentation of polyunsaturated fatty acids of membrane phospholipids, leads to a marked decrease in the rate of oxidative demethylation,The conformation of the fatty acyl chains of membrane phospholipids in the liver may be important in holding the cytochrome P450 complex in an active conformation in the endoplasmic reticulum membrane or in forming a fluid environment which facilitates electron transfer from NADPH to cytochrome P450 (Ingelman-Sundberg & Gustafsson, I 975 ;
The effects of irradiation doses of 200-1000 krad on the fatty acid compositions of saturated and unsaturated natural food fats have been studied. Lard, coconut oil, corn oil, methyl linoleate and herring oil have been analysed before and after irradiation for lipid peroxide content and fatty acid composition. The effects of storage under varied conditions after irradiation have also been investigated. Irradiation doses of 200-1000 krad had little effect on the fatty acid compositions of saturated fats (lard and coconut oil) or of fats with a high antioxidant content (corn oil) but caused destruction of 98 per cent of the highly unsaturated acids (18: 4,20 :5,22 : 6) and 46 per cent of the diene acids (18:2) in herring oil. The destruction of the polyunsaturated fatty acids increased with increasing storage temperature and storage time. The destruction of polyunsaturated fatty acids is accompanied by an increase in lipid peroxide formation. It is considered that changes in fatty acid composition in natural foods after irradiation are important in consideration of the use of irradiation for food preservation.
The effect of dietary lipid content on the fatty acid composition of the liver endoplasmic reticulum and the effect of the fatty acid composition on the oxidative metabolism of Benzo (a) Pyrene (BP) in the liver endoplasmic reticulum were studied in rats. The rate of oxidative metabolism of BP was found to be markedly dependent on the percentage of polyunsaturated fatty acids incorporated into the membrane of the endoplasmic reticulum. Diets containing 10% lard or 10% corn oil caused the incorporation of 10.3% and 25.1% linoleicacid (C 18:2 ) and oxidation rates of 68.3 and 113.8 nmol 3-hydroxybenzo (a) pyrene/30 min/g liver, respectively. Diets containing 10% herring oil caused incorporation of only 5.1 % C 18:2 but also 27.2% ω3 unsaturated fatty acids, including 8.7% eicosapentaenoic acid (C 20:5 ) and 17.0% docosahexaenoic acid (C 22:6 ) and caused a high oxidation rate (145.8 nmol) 3-hydroxybenzo (a) pyrene/30 min/g liver.Phenobarbitone (100 mg/kg) and 20-methylcholanthrene (20 mg/kg) strongly enhanced the oxidation rate of BP in groups of rats fed a herring oil diet and induction was much greater in the group fed an irradiated diet than in the group fed an untreated herring oil diet. Irradiation with 400 krad caused destruction of the major proportion of the polyunsaturated fatty acids in herring oil, incorporation of a smaller quantity of ω3 unsaturated acids into the endoplasmic reticulum, and a decrease in the rate of oxidation to 122.5 nmol 3-hydroxybenzo (a) pyrene/30 min/g liver.The antioxidant 2,6-di-tert-butyl-4 methylphenol, BHT, (0.01 %) added to irradiated herring oil diets strongly reduced the inductive effect of phenobarbitone. This effect may therefore be caused by the presence of lipid peroxides in the irradiated diet.It is concluded that dietary C 18:2 is an important factor in the regulation of the rate of oxidative metabolism of BP in the liver endoplasmic reticulum, but C 18:2 may be effectively replaced by dietary C 20:5 and C 22:6 . Oxidation of BP is regulated by changes in the fatty acid composition of the membranes of the liver endoplasmic reticulum.
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