Current concepts of amino acid and protein metabolism in the mammary gland of the lactating ruminant

Milk protein responses to protein nutrition are typically poor and, in part, may be due to the low efficiency ( ∼25 to 30%) of converting dietary N into milk. Posthepatic availability of amino acids (AA) is not limited, yet only ∼30 % is converted into milk. The poor capture of AA by the mammary gland may relate to the imbalanced and uncoordinated timing of nutrient delivery to the gland. The infusion of essen-tial AA improves the efficiency of utilization (0.31); however, further catabolism of AA within the mam-mary gland suggests that AA transport is not a major limitation. These losses may serve ancillary or func-tional roles, but mammary oxidation of some AA oc-curs only when AA extraction exceeds the stoichio-metric requirements for milk protein synthesis. Intracellular substrate supply may be more limiting than is the appartus for protein synthesis. Studies utilizing isotope labeling and conducted in vitro and in vivo now suggest that circulating peptides and proteins can serve as sources of perhaps all AA for casein synthesis, but the source of these remains elusive. Constitutive protein and casein turnover con-tribute significantly (42 to 72%) to mammary protein synthesis. All AA are extensively channeled through an intermediary protein pool or pools that have rapid turnover rates. The AA are then incorporated into casein, which appears to be fixed in association with protein turnover. The mammary gland is a major controller of its metabolism, and the mechanisms of AA extraction and conversion into milk protein are linked to secretion events. Blood flow may be a key point of regulation whereby mechanisms sense and respond to nutrient supply and balance to the gland via alterations in hemodynamics. ( Key words: amino acid, mammary gland, casein, lactation) Abbreviation key: BCAA = branched-chain AA, EAA = essential AA, EPV = external pudic vein, GIT = gastrointestinal tract, GSH = glutathione, MG = mammary gland, NEAA = nonessential AA