Manometric studies of the effect of certain amino acids on oxidation (measured as oxygen consumption) of linoleic acid, as well as the methyl esters of linoleic acid and linolenic acid dispersed in water or phosphate buffers at pH 7 to pH 5 have shown that
The amino acids tested (except cysteine) have a potential antioxidative effect.
The antioxidative capacity of different amino acids may be rather different (it is especially pronounced in the case of histidine and tryptophane).
Under suitable conditions extremely low amino acid concentrations may have rather strong effect.
The antioxidative efficiency is less pronounced than in earlier experiments with linoleate at pH>7, and decreases with decreasing pH.
There may be a tendency towards a prooxidative inversion with relatively high amino acid concentrations, or at low pH.
The antioxidative effect is enhanced and a prooxidative effect may be lowered or inverted into an antioxidative one by an addition of phosphate, or an emulsifier like Tween.
A strong inhibitory effect is obtained by combined addition of phosphate and emulsifier like Tween, together with the amino acid.
The antioxidative tendency was stronger in the case of methyl linoleate than with linoleic acid, and was also stronger with methyl linoleate than with methyl linolenate.
The TBA reactivities of several aldehydes, most of them known as ordinary products of lipid autoxidation, have been investigated systematically. Gas liquid chromatography‐purified alkanals, 2‐alkenals and 2,4‐alkadienals were reacted with TBA in water solution. The formation of pigments with maximum absorbance at 450 and 530 nm was measured at optimum time‐temperature conditions‐different for readings at 450 and 530 nm‐ and values for absorbance per mole aldehyde were calculated. These values show that on reaction with TBA all studied aldehydes build a yellow 450 nm pigment, while only 2,4‐alkadienals and, to a lesser extent, 2‐alkenals produce the red 530 nm pigment. Consequently both pigments are measures of aldehydic products of lipid autoxidation: In the case of predominant unsaturated aldehyde formation, determination of the pigment with maximum absorbance at 530 nm is preferable. However, if alkanals are predominant, the determination of the yellow pigment at 450 nm is more apporpriate, as it grants higher sensitivity.
Oxidation was measured by oxygen consump‐tion in a Warburg apparatus, modified to main‐tain constant partial oxygen pressure by auto‐matic electrolytic generation of oxygen with automatic recording of the oxygen consumed.
The decrease in rate of oxygen consumption on the lowering of partial oxygen pressure at at‐mospheric pressure was found to depend on a) the varying influence of the nonoxygen‐dependent and the oxygen‐dependent reactions of the prop‐agation which may vary with the conditions such as the reactivity of the substrate, the tem‐perature, and the pH value but which is not affected by light irradiation; b) the varying rate‐limiting effect of slow oxygen diffusion, depending on the ratio between the rate of oxidation and the rate of oxygen diffusion.
Earlier reported kinetic studies on the dependence of lipid oxidation on oxygen pressure in emulsions were continued by studying this relationship in the presence of antioxidants. The substances tested represented two types of antioxidants, phenolic inhibitors (α‐tocopherol, BHA, PG) and amino acid‐retarders (glycine, alanine, histidine, tryptophane). The inhibiting effect of the first mentioned group, i.e., the formation of an induction period was, in general, not dependent on oxygen pressure, while the retardation caused by amino acids was stronger at low oxygen pressure than in air. The effect of lowering oxygen pressure was practically the same, when phenolic inhibitors were added as without such addition. It was, however, enhanced by the addition of amino acid‐retarders. When representatives of these two types of antioxidants were added in combination, their synergistic effect was considerably enhanced at low oxygen pressure.
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