The properties of 22 synthetic peptides containing histidine, which were designed on the basis of the antioxidative peptide (Leu-Leu-Pro-His-His) derived from proteolytic digests of a soybean protein, were examined with regard to their antioxidative activity against the peroxidation of linoleic acid and the scavenging effects on active oxygen and free radical species. The antioxidative activities of these peptides in an emulsion oxidation system using 2,2'-azobis(2-amidinopropane) dihydrochloride as a radical initiator correlated well within an aqueous system. Although the histidine-containing peptides had a quenching activity on singlet oxygen, they did not show antioxidative activity in an 2,2'-azobis(2,4-dimethylvaleronitrile)-induced oxidation system or scavenging effects on 1,1-diphenyl-2-picrylhydrazyl radical and superoxide. The metal-ion chelating activities and the hydrophobicities of these peptides showed no direct correlation with their antioxidative activities. Leu-Leu-Pro-His-His was modified with a hydroxyl radical in an aqueous ethanol system during the peroxidation of linoleic acid.

Docosahexaenoic acid [DHA, 22:6(n-3)], a major component of membrane phospholipids in brain and retina, is profoundly susceptible to oxidative stress in vitro. The extent of this peroxidation in organs when DHA is ingested in mammals, however, is not well elucidated. We investigated the effect of dietary DHA-containing oils (DHA 7.0-7.1 mol/100 mol total fatty acids), in the form of triacylglycerols (TG), ethyl esters (EE) and phospholipids (PL), on tissue lipid metabolism and lipid peroxidation in rats. Groups of Sprague-Dawley rats were fed semipurified diets containing 15 g/100 g test oils and were compared with those fed 80% palm oil and 20% soybean oil as the control (unsupplemented group) for 3 wk. The DHA oil diets markedly increased (P: < 0.05) the levels of DHA in the plasma, liver and kidney, 1.5-1.9, 2.5-3.8 and 2.2-2.5 times the control values, respectively, whereas there was a concomitant reduction (P: < 0.05) in arachidonic acid. All forms of DHA oil caused lower TG concentrations in plasma (P: < 0.05) and liver (P: < 0.05), but had no effect in kidney. The DHA oil-fed rats had greater phospholipid hydroperoxide accumulations in plasma (191-192% of control rats), liver (170-230%) and kidney (250-340%), whereas the alpha-tocopherol level was reduced concomitantly (21-73% of control rats). Consistent with these results, rats fed DHA-containing oils had more thiobarbituric reactive substances in these organs than the controls. Thus, high incorporation of (n-3) fatty acids (mainly DHA) into plasma and tissue lipids due to DHA-containing oil ingestion may undesirably affect tissues by enhancing susceptibility of membranes to lipid peroxidation and by disrupting the antioxidant system.

A modified method without using benzene for quantitative determination of total carbonyl compounds in heated and frying fats and oils has been developed. The analysis is done by the reaction of 2,4-dinitrophenylhydrazine (2,4-DNP) with aldehydes and ketones in 2-propanol. The optimal wavelength to determine the total carbonyl compounds is 420 nm where the 2,4-dinitrophenylhydrazone (2,4-DNPH) derivatives from saturated and unsaturated aldehydes and ketones had the same molar absorption coefficient. The modified method for the estimation of total carbonyl compounds in heated and frying oils using 2-propanol instead of benzene as a solvent had a good correlation with the conventional method.

The extent of oxidation of ethyl esters of eicosapentaenoic (EPA) and docosahexaenoic acids (DHA) was compared quantitatively with that of ethyl linoleate (Lo) and ethyl linolenate (Ln) by oxygen uptake and formation of conjugated diene, hydroperoxide and secondary oxidation products. EPA and DHA esters were oxidized rapidly even at 5 C in the dark after an induction period of 3–4 days, while the induction periods of Ln and Lo esters were 20 days and more than 60 days, respectively. Oxygen uptake of EPA and DHA esters after the induction period was 5.2 and 8.5 times faster than that of ethyl Ln, respectively. Hydroperoxides of EPA and DHA esters are much less stable than those of ethyl Lo. The peroxide value is not necessarily a good indication of oxidation in these polyenoic acids because a considerable amount of secondary products is formed at the early stage of oxidation. Polymers were found to be major secondary products in these polyenoic esters.

α-Tocopherol (Toc) is the tocopherol isoform with the highest vitamin E activity. Since soybean seeds contain a low percentage of α-Toc, increasing the α-Toc content to improve vitamin E activity is a breeding objective. Little is known about the genetic variability of the tocopherol content and composition in soybeans. In the present study, we analyzed the tocopherol composition of 1,109 germplasm accessions of cultivated and wild soybean by high performance liquid chromatography (HPLC) and identified three varieties, Dobrogeance, Dobruoza 14 Pancevo and Keszthelyi Aproszemu Sarga with a high α-Toc content. Dobrogeance showed a similar Toc composition to that of Dobruoza 14 Pancevo with the α-Toc content being four to seven times higher than that of standard varieties and almost the same as that of total Toc. Keszthelyi Aproszemu Sarga showed a markedly increased content of α-Toc, but a significantly lower total Toc content.

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