Reactions of proteins with oxidizing lipids. 2. Influence on protein quality and on the bioavailability of lysine, methionine, cyst(e)ine and tryptophan as measured in rat assays.

The consequences of reactions between protein and oxidizing lipids on the nutritional quality of food proteins have been investigated using a whey protein-methyl linolenate-water model system. In rat assays, significant reductions were observed in protein efficiency ratio, net protein ratio, net protein utilization, biological value and true nitrogen digestibility, especially when the reaction had taken place at high moisture content, high temperature and in the presence of excess oxygen. The losses of bioavailable lysine and tryptophan as measured by rat assays followed a similar pattern. The chemical value of each amino acid multiplied by the true N digestibility closely resembled the rat assay value. In general, the reaction products of lysine and tryptophan formed during lipid oxidation were biologically unavailable. The bioavailabilities of methionine and of 'methionine plus cyst(e)ine' were determined in separate assays. Cyst(e)ine was calculated as 'methionine plus cyst(e)ine' minus methionine. In whey protein which had reacted with oxidizing methyl linolenate, the bioavailable methionine content was not significantly reduced even though 82% of the methionine residues were present as methionine sulphoxide. In hydrogen peroxide-treated casein in which all methionine residues were oxidized to the sulphoxide, methionine sulphoxide was found to be 96% as utilizable as a methionine source to the rat. Free methionine sulphoxide was 87% utilizable. Cyst(e)ine appeared to be as sensitive as lysine to reactions with lipid oxidation products. In whey protein which had reacted with oxidizing methyl linolenate, the bioavailabilities of cyst(e)ine, lysine, tryptophan and methionine were reduced by 28, 24, 11 and 8% respectively and true N digestibility by 9%. These results are discussed in relation to food products.