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The effect of free amino acid supply on the contribution of peptide-bound phenylalanine and tyrosine to casein synthesis in late lactation goats

The mammary gland of lactating ruminants (Guinard & Rulquin 1994) does not appear to extract sufficient quantities of free amino acids (AA) to account for their output as milk protein. Based upon application of a precursor (blood or plasma free AA):product (casein) labelling technique (Backwell et al. 1996) in goats, results suggest that blood peptides or proteins taken up by the gland probably account for this deficit. However, the deficiency appears to be alleviated when supplemental protein or AA are infused (Guinard & Rulquin 1994), suggesting that uptake of peptide bound AA is reduced while that of the free AA is increased. The objective of the current study was to corroborate these findings, thus the precursonproduct labelling technique was applied in lactating goats to determine whether arterial free phenylalanine supply affects the contribution of peptide bound phenylalanine and tyrosine to casein synthesis and compare the results to the net uptake (NU) method.