Peptide-bound methionine can be a source of methionine for the synthesis of secreted proteins by mammary tissue explants from lactating mice.

Mammary tissue explants from d 10-11 of lactating CD-1 mice were used to study the ability of methionine-containing di- to octapeptides to substitute for free methionine for the synthesis of secreted proteins. Explants were incubated in a medium containing 3H-leucine and either L-methionine or one of 23 methionine-containing peptides. The ability of methionine substrates to promote incorporation of 3H-leucine into secreted proteins was quantified. Mammary tissue explants were able to utilize methionine from all peptides tested. All of the peptides were at least as effective as free methionine in promoting 3H-leucine incorporation into secreted proteins. Most di- and tripeptides promoted 15-76% greater (P < 0.05) 3H-leucine incorporation than did free methionine. There was a negative correlation (r = -0.89, P < 0.01) between the rate of 3 H-leucine incorporation promoted by peptides and the number of amino acid residues in the peptides. The incorporation of 3H-leucine promoted by some dipeptides was reduced (P < 0.05) in the presence of a 200-fold higher concentration of glycylsarcosine or carnosine. The results indicate that peptide-bound methionine can serve as a source of methionine for the synthesis of secreted proteins by lactating mammary tissue. Mediated transport of some peptides is probably involved in peptide utilization.