A chemical approach to the nutritional availability of methionine in food proteins.

The background to earlier experiments that were designed to evaluate the nutritional availability of peptide-bound methionine sulphoxide is described. After mild acid hydrogen peroxide treatment of casein, NPU had fallen from 71 to 58. A gas-chromatographic analysis for intact methionine residues that has been developed since then has shown that in the oxidised casein all methionine residues had been oxidised to the sulphoxide, whereas in several foods only moderate levels of sulphoxide were found. The methionine potency of sulphoxide residues is considered to be less a function of digestion and absorption than of the body's capacity to reduce it to methionine. Evidence is given that hydrogen peroxide also modified cystine residues in casein to be rendered nutritionally unavailable. On a molar basis, half the sulphur amino acids in most foods is furnished by cystine which therefore plays a more important role in protein nutrition than is generally recognised.