Methionine peptides as potential food supplements: efficacy and susceptibility to Maillard browning

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 degrees 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.