[Use of small peptides to study the loss of available methionine, lysine and tryptophan during technological...

Peptides such as glycyl-L-methionyl-glycine, glycyl-L-lysine, L-lysyl-glycine and glycine-L-tryptophyl-glycine are used to study side-chain reactivity of three essential amino acids during food processing. The treatment of glycyl-L-methionyl-glycine with sodium hypochlorite resulted in the two following types of reaction: 1. Methionyl residues are oxidised to the corresponding sulfoxide at sodium hypochlorite concentrations up to 0,1 p. 100 w/v; 2. Oxidation of methionine residues to methionine sulfone and deamination reactions also occur for sodium hypochlorite concentrations over 0,2 p. 100 w/v. Sodium hypochlorite treatment of glycyl-L-lysine and L-lysine-glycine causes, probably by deamination of the epsilon-NH2 groups, a loss of lysine of 20 and 30 p. 100, for sodium hypochlorite concentrations of 0,1 and 0,2 p. 100 w/v, respectively. Treatment of glycyl-L-tryptophyl-glycine with hydrogen peroxide (0,05 M) modifies tryptophan residues in such a way that it cannot be retrieved after hydrolysis of the tripeptide with methane sulfonic acid and subsequent chromatographic analysis; six new unidentified components appear on the chromatogram. Glycyl-L-methionyl-glycine was incubated with food constituents or additives such as reducing oses, acrolein, p-benzoquinone, methyl iodide, or dichloro I, I ethylene. Thin layer chromatography and RMN show that carbonyl compounds or quinones do not react with the thioether group. Alkylating agents sometimes used in food processing only gave traces of sulfonium compounds. Thus, it appears that the only reaction liable to render methionine residues unavailable in foods would be its oxidation to methionine sulfone.