Alfonso Valenzuela

The biological significance of malondialdehyde determination in the assessment of tissue oxidative stress

Although lipid peroxidation of biological samples may be assessed by different chemical and physical methods, those based on the measurement of malondialdehyde formed from the breakdown of endoperoxides during the last stages of the oxidation of a polyunsaturated fatty acid, appear as the most widely used. Among the various methods to evaluate malondialdehyde, which include direct spectrophotometry or high pressure liquid chromatography, the reaction with thiobarbituric acid to form a colored adduct appears as a more rapid, inexpensive and sensitive technique. This method, however, is subject to some interferences which, if not considered, may lead to erroneous results. The present review emphasizes the significance of malondialdehyde measurement in biological samples, the chemical conditions of its reaction with thiobarbituric acid and the different procedures to isolate and determine the malondialdehyde-thiobarbituric acid adduct.

Alcohol ingestion, liver glutathione and lipoperoxidation: Metabolic interrelations and pathological implications☆

Data reviewed here indicate that acute and chronic ethanol ingestion induce a decrease in the concentration of GSH and an increase in lipoperoxidation in the liver both in experimental animals and in man, changes that are closely interrelated GSH depletion is suggested to be due to an oxidation in the liver tissue and to a translocation into the extrahepatic medium as free glutathione and/or as conjugates with ethanol-derived acetaldehyde. As a result, the hepatic GSH/GSSG ratio is drastically reduced. Lipoperoxidation seems to be related to the metabolism of ethanol and acetaldehyde by secondary pathways that are known to generate oxygen-related free radicals. Being lipoperoxidation a process associated with cell damage and death, its stimulation by ethanol ingestion could play a role in the production of alcoholic liver damage in man. The involvement of several contributory factors in the development of a high lipoperoxidative index in the liver in this situation is discussed.

Marine oils: the health benefits of n-3 fatty acids

The first indication that fats may be essential for healthy growing animals was proposed by Aron in 1918.1 He proposed that butter, in addition to its contribution to calories, had a specific nutritive value possibly related to its content of certain lipid components. The concept that some fatty acids may be necessary for the proper growth and development of animals and possibly humans was demonstrated in the 1930s by Burr and Burr.2 Yet essential fatty acids (EFAs) were considered of marginal importance in human nutrition until the 1960s, when signs of clinical deficiency became apparent in infants fed skim-milk–based formula and in those given lipid-free parenteral nutrition. Over the past two decades, the essentiality of n-3 fatty acids for neurodevelopment and vascular health has been demonstrated. The essentiality of n-6 and n-3 fatty acids for humans is best explained by the inability of animal tissue to introduce double bonds in positions n-1 to n-9.

Effect of acute ethanol intoxication on the content of reduced glutathione of the liver in relation to its lipoperoxidative capacity in the rat

Glutathione is considered to be the most abundant and important intracellular sulfhydryl compound [ 1.21. Both reduced (GSH) and oxidized (GSSG) glutathione are related to several structural and func- tional processes of the cell, and are involved in the protective mechanisms against the deleterious effects of several agents and/or their metabolites [2]. The later function of glutathione has been proposed to be accomplished by either the formation of excretable conjugates [3] or by its participation in the metabolism of peroxides arising from the enhancement of lipo- peroxidative processes [4]. Lipoperoxidation, the oxidative alteration of poly- unsaturated fatty acids that seems to be of importance in the production of liver injury by some hepato- toxins [5,6], has been shown to be increased in the liver following acute [7-l 0] and chronic [8,1 l-141 alcohol ingestion. However, this finding has not been confirmed in the acute model [ 15 171. This report describes the influences of the sex, nutritional status, dosage and the period of intoxication of animals given alcohol acutely on the content of GSH of the liver in relation to its lipoperoxidative capacity. The effect of alcohol on the activity of the enzymes of peroxide metabolism, the other main contributors to the maintenance of the antioxygenic capacity of the hepatocyte [1,4,18], is dealt with in [19]. 2.

Ingestion of high doses of fish oil increases the susceptibility of cellular membranes to the induction of oxidative stress

Feeding rats with 4 g/kg body weight of sardine oil during 7 or 14 days increases the content of eicosapentaenoic acid and docosahexanoic acid in the erythrocyte and hepatic microsomal membranes by 2 to 6%. These membranes show increased susceptibility to the induction of oxidative stress, expressed as lipid peroxidation, when they are exposed to Fe2+-ascorbate and to NADPH-Fe3+-ADP, respectively. The results indicate that in order to prevent the increased susceptibility to lipid peroxidation, supplementation with larger amounts of antioxidants may be needed than those required to stabilize the oil.

Validation of the rancimat test for the assessment of the relative stability of fish oils

The induction periods for the peroxidation of various fish oils at 55–90°C were studied by the Rancimat test. The natural logarithms of the induction periods varied linearly with respect to temperature, with a mean coefficient of −7.5×10−2°C−1, which was significantly different from that reported for vegetable oils. The activation energy for the formation of volatile acids had a mean value of 38.9 kJ/mol and was independent of the fish oil source. Peroxide formation under Rancimat test conditions followed first-order kinetics. The same kinetics were followed under Schaal Oven test conditions (forced-air oven, 60°C). On the basis of the results obtained, the Rancimat test appears to be useful in determining the relative stabilities of fish oils without the change in peroxide decomposition kinetics that may occur at elevated temperatures.

Trans fatty acid isomers in human health and in the food industry

Trans fatty acids are unsaturated fatty acids with at least one double bond in the trans configuration. These fatty acids occur naturally in dairy and other natural fats and in some plants. However, industrial hydrogenation of vegetable or marine oils is largely the main source of trans fatty acids in our diet. The metabolic effect of trans isomers are today a matter of controversy generating diverse extreme positions in light of biochemical, nutritional, and epidemiological studies. Trans fatty acids also have been implicated in the etiology of various metabolic and functional disorders, but the main concern about its health effects arose because the structural similarity of these isomers to saturated fatty acids, the lack of specific metabolic functions, and its competition with essential fatty acids. The ingestion of trans fatty acids increases low density lipoprotein (LDL) to a degree similar to that of saturated fats, but it also reduces high density lipoproteins (HDL), therefore trans isomers are considered more atherogenic than saturated fatty acids. Trans isomers increase lipoprotein(a), a non-dietary-related risk of atherogenesis, to levels higher than the corresponding chain-length saturated fatty acid. There is little evidence that trans fatty acids are related to cancer risk at any of the major cancer sites. Considerable improvement has been obtained with respect to the metabolic effect of trans fatty acids due the development of analytical procedures to evaluate the different isomers in both biological and food samples. The oleochemical food industries have developed several strategies to reduce the trans content of hydrogenated oils, and now margarine and other hydrogenated-derived products containing low trans or virtually zero trans are available and can be obtained in the retail market. The present review provides an outline of the present status of trans fatty acids including origin, analytical procedures, estimated ingestion, metabolic effects, efforts to reduce trans isomers in our diet, and considerations for future prospects on trans isomers.

Cholesterol oxidation: Health hazard and the role of antioxidants in prevention

Cholesterol is a molecule with a double bond in its structure and is therefore susceptible to oxidation leading to the formation of oxysterols. These oxidation products are found in many commonly-consumed foods and are formed during their manufacture and/or processing. Concern about oxysterols consumption arises from the potential cytotoxic, mutagenic, atherogenic, and possibly carcinogenic effects of some oxysterols. Eggs and egg-derived products are the main dietary sources of oxysterols. Thermally-processed milk and milk-derived products are another source of oxysterols in our diet. Foods fried in vegetable/animal oil, such as meats and French-fried potatoes, are major sources of oxysterols in the Western diet. Efforts to prevent or to reduce cholesterol oxidation are directed to the use of antioxidants of either synthetic or natural origin. Antioxidants are not only able to inhibit triglyceride oxidation, some of them can also inhibit cholesterol oxidation. Among synthetic antioxidants 2,6-ditertiarybutyl-4-methylphenol (BHT), and tertiary butylhydroquinone (TBHQ) can efficiently inhibit the thermal-induced oxidation of cholesterol. Some natural antioxidants, such as alpha- and gamma-tocopherol, rosemary oleoresin extract, and the flavonoid quercetin, show strong inhibitory action against cholesterol oxidation.

Flavonoids as stabilizers of fish oil: An alternative to synthetic antioxidants

The antioxidant activities against fish oil oxidation of six commercially available flavonoids and of five flavonoids purified from two Chilean native plants were compared to those ofdl-α-tocopherol and of two synthetic antioxidants, butylated hydroxytoluene and butylated hydroxyanisole. Among the commercial flavonoids, catechin, morin and quercetin showed a higher activity when fish oil oxidation (either spontaneous or Fe2+-induced) was assessed from the formation of peroxides or thiobarbituric acid-reactive substances. Among the native flavonoids, the 5,3′,4′-trihydroxy-7-methoxy flavanone (designated as Pt-2) showed the highest antioxidant activity. Mixtures of quercetin or of Pt-2 withdl-α-tocopherol produced better inhibitory effects when compared to that of each substance assayed by itself. Also, when Pt-2 and quercetin were assayed in combination (0.3 g/kg oil and 0.7 g/kg oil, respectively), a synergistic antioxidant effect was observed. Results indicate that several flavonoids could be used as natural antioxidants as a means to replace those synthetic antioxidants, the use of which has been questioned.

Hepatic and biliary levels of glutathione and lipid peroxides following iron overload in the rat: Effect of simultaneous ethanol administration

The administration of 125 mg of iron/kg (iron-dextran-Imferon) to fed rats was followed by an increase in the non-hem iron content in plasma and liver over a period of 22 hr, reaching a peak value after 6 hr. Plasma and hepatic iron levels were not modified by ethanol ingestion (5 g/kg). Iron and ethanol treatments enhanced liver lipid peroxidation (malondialdehyde (MDA) formation) by 70 and 35%, respectively, at 6 hr. Since the hepatic MDA formation increased by 92% after the joint iron-ethanol treatment, an additive effect in lipid peroxidation was suggested to occur in this condition. Both iron and ethanol treatments increased biliary levels and release of MDA, in the absence of changes in bile flow. These parameters were further enhanced by the joint iron-ethanol exposure, in that hepatic MDA levels and biliary MDA release were significantly correlated (r = 0.86; p less than 0.05). Plasma MDA levels also increased after iron, ethanol, and iron-ethanol treatments, but they did not reflect the changes in MDA levels in liver. Iron exposure resulted in 26 to 33% decreases in hepatic GSH content at the 6-hr treatment, associated with the peak effect on lipid peroxidation. In this situation, glutathione disulfide (GSSG) levels in liver were not changed, but its biliary release increased by 76%. Hepatic reduced glutathione (GSH) levels were recovered by 18 hr and increased by 23% after 22 hr of iron ingestion. Acute ethanol intake diminished liver GSH content by 30% and enhanced that of GSSG by 73%, thus eliciting a net decrease of 20% in total GSH equivalents (GSH + 2GSSG). Biliary release of total GSH was reduced in this condition. The combined administration of iron and ethanol further influenced the decrease in hepatic GSH and the increase in GSSG levels elicited by the separate treatments, but no alterations in the biliary content and release of total GSH were observed in this situation. These data indicate that iron exposure accentuates the changes in lipid peroxidation and in the glutathione status of the liver cell induced by acute ethanol intoxication.