Peroxidation of lipids in tissue homogenates as related to vitamin E

The effect of mercury and vitamin E on tissue glutathione peroxidase activity and thiobarbituric acid values

The effects of methyl mercuric chloride (CH3 HgCl) and mercuric chloride (HgCl2) on red blood cell (RBC), plasma, and liver glutathione peroxidase (GSH-Px) activities were determined in Japanese quail fed a semipurified diet with 1 ppm added selenium (Se). The RBC GSH-Px activity was significantly increased by 5 ppm mercury (Hg) as CH3 HgCl. Plasma GSH-Px activity was significantly depressed only in th 20 ppm Hg group. Liver GSH-Px activity was not affected. The RBC GSH-Px activity was significantly lower at all levels of HgCl2 compared to the Ch3 HgCl-treated birds. Plasma and liver GSH-Px activity of the HgCl2 -treated birds were not significantly different from control values. In a second experiment, GSH-Px activity and estimates of peroxidation via the thiobarbituric acid (TBA) test were determined on quail fed diets containing .07 ppm added Se to which either 10 ppm Hg as CH3 HgCl or 150 ppm Hg as HgCl2 were added with or without the supplementation of vitamin E. Plasma and kidney GSH-Px activities were unaffected by CH3 HgCl, whereas RBC GSH-Px was significantly elevated. Kidney GSH-Px was significantly depressed by HgCl2-Vitamin E had no effect on GSH-Px activities. Vitamin E protected the liver, kidney, and brain from peroxidation; however, peroxidation was not induced by either of the mercurials.

Influence of vitamin E and nitrogen dioxide on lipid peroxidation in rat lung and liver microsomes

Rat lung and liver microsomes were used to examine the effects of dietary vitamin E deficiency on membrane lipid peroxidation. Microsomes from vitamin-E-deficient rats displayed increased lipid peroxidation in comparison to microsomes from vitamin-E-supplemented controls. The extent of lipid peroxidation, as determined by measurement of thiobarbituric acid reacting materials, was enhanced by addition of reduced iron and ascorbate (or NADPH). Rats fed a vitamin-E-supplemented diet and exposed to 3 ppm NO2 for 7 days did not exhibit increases in microsomal lipid peroxidation compared to air-breathing controls. However, increase were found in microsomes prepared from rats fed a vitamin-E-deficient diet and exposed to NO2. Lung microsomes from vitamin-E-fed rats contained almost 10 times as much vitamin E as liver microsomes when expressed in terms of polyunsaturated fatty acid content. The extent of lipid peroxidation was, in turn, considerably less in lung than in liver microsomes. Lipid peroxidation in lung microsomes from vitamin-E-deficient rats comparable to liver microsomes from vitamin-E-supplemented rats as was the content of vitamin E in these respective microsomal samples. A combination of vitamin E deficiency and NO2 exposure resulted in the greatest increases in lung and liver microsomal lipid peroxidation with the largest relative increases occurring in lung microsomes. An inverse relationship was found between the extent of lipid peroxidation and vitamin E content. Most of the peroxidation in lung microsomes appeared to proceed nonenzymatically whereas peroxidation in liver was largely enzymatic. Vitamin E appears to be assimilated by the lung during oxidant inhalation, but with dietary vitamin E deprivation, the margin for protection in lung may be less than in liver.

Relative susceptibility of microsomes from lung, heart, liver, kidney, brain and testes to lipid peroxidation: correlation with vitamin E content

Rates of in vitro lipid peroxidation of microsomes and homogenates were found to vary widely among different tissues and species. In rats and rabbits, lung microsomes peroxidized at a 25- to 50-fold lower rate than liver, kidney, testes and brain microsomes. Heart microsomes peroxidized at a rate slightly greater than, but most similar to, lung microsomes. Comparison of tissue homogenates also revealed the unique resistance of lung and heart to lipid peroxidation. The ratio of vitamin E to peroxidizable polyunsaturated fatty acids in lung and heart microsomes was several-fold higher than in microsomes from the other tissues studied, which accounted for the relative resistance of lung and heart to lipid peroxidation. Liposomes of extracted rat lung microsomal lipid were also resistant to peroxidation and the amount of vitamin E contained in the lung lipid extract was sufficient to confer the same degree of resistance when incorporated into an equivalent amount of rat liver lipid. Higher rates of peroxidation in mouse lung microsomes relative to rabbit, rat and human lung microsomes were similarly correlated with a lower ratio of vitamin E to peroxidizable fatty acids in mouse lung microsomes. These data provide strong support for the role of vitamin E as the major cellular antioxidant, especially in the highly oxygenated tissues of heart and lung, and demonstrate the utility of the microsomal system in characterizing tissue differences in susceptibility to peroxidative membrane decomposition.

Ethanol-induced hepatotoxicity; experimental observations on the role of lipid peroxidation

Hepatic lipid peroxidation in vivo or in vitro as measured by UV absorption spectra of microsomal lipids or by production of TBA-reacting substances by whole liver homogenates, was studied after acute or during prolonged administration of ethanol. No evidence of peroxidative derangement of liver microsomal lipids in vivo was detected in either experimental situation, while the production of TBA-reacting substances by pooled liver homogenates incubated in vitro appeared slightly increased. Treatment with reduced glutathione (GSH and 2-mercaptopropionylglycine (2-MPG) was able to reduce fatty liver in acute and prolonged ethanol dosing, as well as the production of TBA-reacting compounds. Similar effects were obtained with 3-amino-1,2,4-triazole which was assayed only in acute experiments. By contrast, hepatic triglyceride accumulation induced by a single intoxicating dose of ethanol was not affected by preventive treatment with pyrazole which seemed to act as a pro-oxidant agent as far as the production of TBA-reacting substances is concerned. The role of lipid peroxidation as a pathogenic mechanism for acute and chronic ethanol-induced hepatotoxicity is discussed in relation to the action of anti-oxidant compounds which are active in preventing liver injury. It is concluded that lipid peroxidation is unlikely to be an important mechanism in alcohol hepatotoxicity.

Rat liver alpha-tocopherol binding protein

1. The properties of rat liver cytoplasmic alpha-tocopherol binding protein have been studied. 2. The binding protein sedimented in the 3 S region of sucrose density gradients, and gel filtration indicated an approximate molecular weight of 30 500. 3. Of the tissues examined by the present assay, binding was detectable only in the liver. 4. Optimal binding was achieved by incubation at 26 degrees C for 4 h and was independent of pH between 7.4 and 9.0. 5. Pronase completely abolished binding. The binding protein was, however, almost completely resistant to trypsin, and unaffected by RNAase, DNAase, triacylglycerol lipase, and phospholipase C. 6. A variety of tocopherol analogues and other lipid-soluble compounds were tested for their ability to compete for binding. Only alpha-tocopherol and to a lesser extent alpha-tocotrienol and gamma-tocopherol exhibited competition. alpha-Tocopherol acetate, alpha-tocopherol quinone and 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid had no effect on binding. 7. Tocopherol binding was reversible, and the tocopherol was not metabolized during incubation.

Oxidative damage of retinal rod outer segment membranes and the role of vitamin E

Highly purified bovine rod outer segment membranes show loss of structural integrity under an air atmosphere. Obvious ultrastructural changes are preceded by increases in absorbance below 400 nm. These changes are inhibited by Ar or N2 atmospheres and appear to be due primarily to oxidative damage to the polyunsaturated fatty acids of the membrane lipids. Loss of polyunsaturated fatty acids, formation of malonaldehyde and fluorescent products characteristic of lipid oxidation accompany the spectral alterations. The elevated ultraviolet absorbance can largely be removed from the membranes by gentle extraction of the lipids using phospholipase C and hexane without changing the visible absorbance of rhodopsin. We have found a large seasonal variation in the endogenous level of alpha-tocopherol (vitamin E) in the bovine rod outer segment preparations. For much of the year we find that the rod outer segment membranes contain higher levels of alpha-tocopherol than have been previously reported in biological membranes. Rod outer segments which are low in endogenous tocopherol can be protected from oxygen damage by adding exogenous tocopherol. The rod outer segments are extremely susceptible to oxygen damage due to the unusually high content of polyunsaturated fatty acids in the membrane lipids. The presence of tocopherol inhibits oxygen damage but does not eliminate it. The tocopherol in the rod outer segments is consumed in air, thus complete protection from peroxidation in vitro requires an inert atmosphere as well as high levels of tocopherol. This work suggests that extensive precautions against oxidative degradation should also be employed in studies of other membrane systems where important deleterious effects of oxygen may be less obvious.

Effect of ascorbic acid on ACTH-induced cyclic AMP formation and steroidogenesis in isolated adrenal cells of vitamin E-deficient rats

Isolated adrenal cells from Vitamin E-deficient and control rats were prepared by a trypsin digestion method. Cyclic adenosine 3',5'-monophosphate (cyclic AMP) formation was studied in response to adrenocorticotropin (ACTH) in the presence and absence of ascorbate by measuring the conversion of prelabeled adenosine 5'-triphosphate [14C]ATP to cyclic [14C]AMP. Ascorbate (0.5 mM) inhibited ACTH-induced cyclic [14C]AMP formation in adrenal cells isolated from Vitamin E-deficient rats but had no effect in the control cells. The inhibitory effect of ascorbate on ACTH-induced cyclic AMP formation in Vitamin E-deficient rats decreased as the concentration of ACTH increased. In Vitamin E-deficient rats ascorbate inhibited ACTH-induced cyclic [14C]AMP formation after 30 min of incubation. There was no further significant accumulation of cyclic [14C]AMP at 60 min or 120 min although in the absence of ascorbate cyclic [14C]AMP continued to be formed. The in vitro addition of alpha-tocopherol reduced the inhibition of ACTH-induced cyclic [14C]AMP formation by ascorbate in Vitamin E-deficient rats. These studies suggest that alpha-tocopherol and ascorbate may affect ACTH-induced cyclic AMP formation through interaction with the membrane-bound enzyme adenylate cyclase.

Ozone and vitamin E

Vitamin E deficiency in rats is associated with a greater susceptibility to lethal levels of ozone. Exposure of rats to sublethal ozone concentrations produces an accelerated decline in serum vitamin E levels. These findings are consistent with the possibility that lipid peroxidation is a mechanism of ozone toxicity.

Dietary fat composition and tocopherol requirement: II. Nutritional status of heated and unheated vegetable oils of different ratios of unsaturated fatty acids and vitamin E

In studies conducted on male and female rats and involving evaluation of growth, reproductive and lactation performances and of lipid peroxidation, no evidence could be found for the need for added vitamin E (a‐tocopherol) over and above that naturally present as tocopherols in the vegetable oils investigated. These oils are in common usage in industry, i.e., liquid nonhydrogenated cottonseed oil, a lightly hydrogenated cottonseed oil and a hydrogenated soybean oil shortening. The ratio of polyunsaturates to total tocopherol in the test oils varied from 640:1 to 9:1. Even those oils obtained from a commercial frying operation after a steady state had been attained contained sufficient vitamin E to meet dietary requirements. Results of in vitro peroxide hemolysis tests conducted on the red blood cells of the test animals did not correlate well with biological performance.

Studies of the possible transference of flavonol antioxidants from the diet to the tissue lipids of rats

1. The oxidative stability of lipids in the tissues (erythrocytes, liver and depot fat) from vitamin E-deficient rats was not improved by feeding these animals, for a period of 39–55 days before killing, on a vitamin E-free diet richly supplemented with the flavonoid antioxidants quercetin (3,5,7,3',4'-pentahydroxyflavone) and flavone (3,7,8,2',5'-pentahydroxy-6-tert.-butyl flavone).

2. Addition of α-tocopherol (in the form of α-tocophery1 acetate) to the diet relieved, as expected, the investigated symptoms of vitamin E deficiency and greatly improved the stability of the tissue lipids.

3. Quercetin and flavone, when added in vitro to liver homogenate or to extracted adipose tissue, strongly retarded oxidation. It was concluded that those flavonols either were not absorbed by the animals or were destroyed in the body.