Twenty two methionine-containing di- to octa-peptides were evaluated for their ability to serve as methionine sources to support protein accretion and cell proliferation in C₂C₁₂ myogenic, MAC-T mammary epithelial and ovine myogenic satellite cells. Factors in serum that may be involved in regulating peptide utilization was investigated using MAC-T cells. Growth of MAC-T cells was studied in the presence of methionine-containing dipeptides with 6% desalted adult animal serum from chickens, horses, humans, pigs or rabbits. Serumal peptidase activities on the twenty two methionine-containing peptides were examined in cell-free, methionine-free Dulbecco’s modified Eagle’s medium supplemented with 6% fetal bovine serum. The cell cultures were incubated for 72 h at 37°C in a humidified environment of 90% air : 10% CO₂ for C₂C₁₂ and ovine satellite cells or 95% air : 5% CO₂ for MAC-T cells. The basal medium contained methionine-free Dulbecco’s modified Eagle’s medium supplemented with 6% desalted animal serum or one of the following serumal factors: .4% bovine serum lipids, 1% chemically defined lipid concentrate, bovine insulin (1 ug/mL), or 3% low protein serum replacement (LPSR-1). Treatment media tested included basal medium or basal media supplemented with L-methionine or one of the methionine-containing peptides. Cell cultures incubated with the basal media for 72 h were characterized by decreased cell number and decreased protein content compared with initial cultures. All the methionine-containing peptides (with the exception of glycylmethionine and prolylmethionine for C₂C₁₂ cells), regardles of chain length, were able to support protein accretion with responses ranging from 29 to 123% of that of free L-methionine. The DNA contents of ovine satellite cell cultures indicated that cell proliferation occurred in the presence of all the methionine-containing peptides with responses ranging from 45 to 144% of the L-methionine response. Bovine insulin and lipids were not effective in promoting peptide utilization by MAC-T cells. However, the LPSR-1 facilitated the utilization of methionine-containing peptides in C₂C₁₂ and MAC-T cells. In the cell-free, methionine-free Dulbecco’s modified Eagle’s medium, peptidases could release all the methionine residues from the tetra- to octapeptides during 24 h of incubation and 42 to 70% of the methionine residues from the di- and tripeptides tested. The results demonstrated that cultured animal cells possess the ability to utilize methionine-containing peptides as methionine sources for protein accretion and cell proliferation, but serumal peptidases are at least partially responsible for the observed responses. / Ph. D.