Vascular sources of phenylalanine, tyrosine, lysine, and methionine for casein synthesis in lactating goats

The contribution to casein biosynthesis of peptides derived from blood was examined in late lactation goats (254 to 295 d in milk). Ratios of mammary uptake of free amino acids (AA) in blood to output of AA in milk protein and ratios of the enrichments of Phe, Tyr, Met, and Lys at isotopic plateau in secreted milk casein to the free AA in arterial and mammary vein blood were monitored during the last 5 h of a 30-h continuous i.v. infusion of [1-13C]Phe, [2H4]Tyr, [5-13CH3]Met, and [2-15N]Lys on two occasions: before (control) and on d 6 of an i.v. infusion of Phe (6 g/d). During the control, uptakes of free Phe and Met were less than their output in milk. This result was comparable with the labeling kinetic results, suggesting that vascular peptides contributed 5 to 11% of Phe and 8 to 18% of Met. Free Tyr and Lys uptakes during the control were sufficient for milk output; however, the labeling kinetics indicated that 13 to 25% of the Tyr and 4 to 13% of the Lys were derived from peptides. Infusion of Phe increased the uptake of free AA but reduced the contribution of peptides toward Phe (0 to 3%) and Tyr (8 to 14%) supply for casein synthesis. Whole body hydroxylation of Phe to Tyr increased from 10 to 18% with the infusion of Phe; within the mammary gland, this conversion was lower (3 to 5%). Results suggest that the mammary utilization of peptides containing Phe and Tyr appears to depend on the supply of free AA in blood.