Modeling mammary amino acid metabolism

Milk pricing schemes place economic importance on milk components. Most current nutrient requirement models do not predict milk component yields accurately. Deaggregation of energy and protein terms in those models may improve prediction accuracy. Descriptions of energy metabolism by the major postabsorptive tissues have progressed over the last 20 years. More recent efforts have been directed at representing amino acid metabolism. Mammary amino acid metabolism appears to be a function of amino acid supply and regulatory elements. Regulation of uptake and blood flow occurs and is represented in some models. Intracellular metabolism of amino acids and possibly energy are determinants of removal. Both the rate of amino acid oxidation and use for protein synthesis appear to be functions of intracellular concentrations. Experimental observations suggest that representation of protein synthesis as a linear function of the first-limiting amino acid is inadequate. A multi-substrate Michaelis-Menten equation form is more consistent with experimental observations and appears to yield better predictions as compared to the single-limiting model. Consideration of energy supply as a driver of milk protein synthesis also appears to be warranted. Additional knowledge of the substrate response surface for protein synthesis and how it is regulated is needed. (C) 2001 Elsevier Science B.V. All rights reserved.