Methionine Peptides as Potential Food Supplements: Efficacy and Susceptibility to Maillard Browning

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Abstract
Three experiments were conducted with young chicks fed purified diets to ascertain the bioefficacy of six different methionylmethionine (Met-Met) dipeptides, differing only in isomeric configuration, an oligopeptide (n = 8) of L-methionine, and also N-glycyl-L-methionine. DL/LD-Met-Met (50% D-Met-L-Met and 50% L-Met-D-Met), D-Met-L-Met, L-Met-D-Met, L-Met-L-Met, and N-Gly-L-Met were utilized as well as L-Met based on weight gain per millimole of sulfur consumed from each experimental compound. DD/LL-Met-Met (50% D-Met-D-Met and 50% L-Met-L-Met), D-Met-D-Met and L-oligo-Met were utilized less efficiently for chick growth. When the Met compounds were pretreated to facilitate Maillard browning (incubated at 37.6°C with dextrose at pH 9.5), all Met sources (except N-acetyl-L-Met, which was used as a positive control) exhibited reduced efficacy relative to an equivalent dietary addition of unreacted L-Met. The Met-Met peptides which when unreacted had been equivalent in efficacy to L-Met were, however, superior to L-Met when each compound, including L-Met, had undergone browning. N-Gly-L-Met was very reactive when heat-treated with dextrose, producing an almost black color after the 14-day incubation period, and its efficacy after browning was reduced by an even greater extent than was the case for L-Met.