High rates of mammary tissue protein turnover in lactating goats are energetically costly

The high energetic demands and metabolism of amino acids (AA) within the lactating mammary gland have been ascribed to the requirements for milk component synthesis and tissue maintenance. Our objective in this work was to assess rates of protein synthesis from several AA so that the energetic costs of tissue maintenance could be better reflected. Lactating goats (n = 4) were given staggered infusions of 5 labeled forms of phenylalanine (Phe) initiated at 30, 12, 9, 6, and 3 h before goats were killed. [5-¹³CH₃] Methionine (Met), [1-¹³C] leucine, and [1-¹³C] valine were also infused for 30 h, during which time, the glands were milked hourly and arteriovenous flux measurements were performed the last 6 h. A dynamic, compartmental model capable of simulating fluxes of AA through extracellular and intracellular free, slow and fast turnover tissue-bound, and milk protein pools was developed and fitted to the observed data. The udder removed 81% of the Phe present in plasma using 31% for milk protein synthesis and releasing 66% back into plasma. Transamination accounted for 40% of Phe flux in the mammary and transmethylation accounted for a portion of mammary Met flux. Mammary tissue protein synthesis was >300% the value of milk protein synthesis with fractional protein synthesis rates >130%/d. Assuming 4 mol of ATP/mol of peptide bond formed, we estimate that ∼;50% of ATP generated by the lactating mammary glands is used for synthesis of tissue (nonmilk) protein.