Methionine- and lysine-containing di- to octapeptides were evaluated for their ability to serve as methionine and lysine sources respectively for the synthesis of secreted proteins. Mammary tissue explants from lactating (10 to 11 d) CD-1 mice and cultured bovine mammary epithelial cells (MAC-T) were used as experimental models. Explants and cultured cells were incubated at 37°C in a humidified atmosphere of 90% air/10% CO₂ and 95% air/5% CO₂, respectively, for 1 to 24 h in Dulbecco's modified Eagle's medium containing hormones, ³H-leucine, and methionine or lysine substrate in either free or peptide-bound form. The ability of methionine and lysine substrates to promote incorporation of ³H-leucine into secreted proteins was quantified. Mouse mammary explants were able to utilize methionine and lysine from all peptides tested except the lysyl octapeptide. All the methionyl peptides were at least as effective as free methionine in promoting ³H-leucine incorporation into secreted proteins. Most methionyl di- and tri-peptides promoted 15 to 76% greater (P < .05) ³H-leucine incorporation than did free methionine. A negative correlation (r = -.89, P < .01) was detected between the rate of ³H-leucine incorporation and the number of amino acid residues in the peptides. The incorporation of ³H-leucine promoted by some methiony] dipeptides was reduced (P < .05) in the presence of a 200-fold higher concentration of glycylsarcosine or carnosine. Incorporation of ³H-leucine promoted by lysyl peptides ranged from 91 to 117% of the incorporation promoted by free lysine. MAC-T cells were also able to utilize methionine from all di- and tri-peptides studied. The ability of the peptides to promote ³H-leucine incorporation varied with experimental conditions. For cells allowed to grow/differentiate for 3 or 8 d, incorporation of ³H-leucine promoted by peptides ranged from 67 to 85% and 86 to 110% of the incorporation promoted by free methionine, respectively. The effect of extracellular matrix on the utilization of peptide-bound methionine by MAC-T cells was also examined. Generally, there was no difference in ³H-leucine incorporation/DNA promoted by methionyl dipeptides in MAC-T cells cultured on matrigel, collagen, laminin, or fibronectin coated or uncoated plates. These results suggest that peptides can serve as sources of amino acids for the synthesis of secreted proteins by both lactating mammary explants and cultured mammary epithelial cells. Mouse mammary explants appear to have a greater ability to utilize peptide-bound methionine than to utilize peptide-bound lysine. Mediated transport of some methiony] peptides may be involved in the peptide utilization by the mammary explants. More extensive utilization of peptides by MAC-T cells following a longer (3 vs 8 d) growth/differentiation period may indicate that some maturation process(s) in the cells was necessary for the most effective utilization of peptides. / Ph. D.