Amino Acid Metabolism in the Mammary Gland

Proteins are composed of amino acids. Amino acid utilization by mammary tissue is affected by a) blood concentrations of amino acid, b) mechanisms of amino acid uptake into mammary cells, and c) intracellular metabolism of the amino acid. Each of these levels of control is further influenced by a range of factors. Blood amino acid concentrations are affected by nutrition, but also by physiological state of the animal and other factors. Multiple uptake mechanisms may exist in tissues for each amino acid. And, intracellular metabolism of amino acid involves several competing biochemical pathways. Uptake and intracellular metabolism of amino acids by the mammary gland are more complex than simply incorporation into milk proteins. For example, nonessential amino acid uptake varies considerably from hour to hour, generally showing a deficit in uptake relative to their amounts in milk protein. Some essential amino acids are taken up stoichiometrically (1 for 1) (lys, met, phe, tyr, trp), while others (branched chain amino acids and arginine) are taken up by the gland in amounts greater than may be accounted for in milk protein. Some of the latter amino acids are thought to be used by the mammary tissue as a potential energy source and as a source of carbon and nitrogen for synthesis of nonessential amino acids.

In terms of intracellular metabolism of amino acids, once inside the cell, AA's may:

1. undergo mRNA-directed polymerization to form milk proteins.
2. enter into metabolic reactions yielding carbon dioxide, urea, polyamines, and nonessential AA's (NEAA's)
3. be retained in cells as part of structural proteins and enzymes
4. pass unchanged into milk, blood or lymph

Routes 3 and 4 are minor in the mammary gland relative to the tremendous synthesis and secretion of milk proteins.

Extraction of essential and nonessential amino acids by the mammary gland of goats :

*arterio-vensous difference

For more on amino acid metabolism in the mammary gland, see Mepham, T. B. 1982. Amino acid utilization by lactating mammary gland. J. Dairy Sci. 65:287; Baumrucker CR 1985 Amino acid transport systems in bovine mammary tissue. J. Dairy Sci. 68:2436; Shennan DB, Millar ID, Calvert DT 1997 Mammary-tissue amino acid transport systems. Proc Nutr Soc 56:177; Shennan DB Mammary gland membrane transport systems. J. Mammary Gland Biology Neoplasia 3:247-258.