Mathematical integration of protein metabolism in growing lambs

A mathematical integration of whole-body protein synthesis and degradation based on protein metabolism in 10 individual tissues in growing lambs is described. The tissues represented are adipose, central nervous system (CNS), gastrointestinal tract (GIT), heart, kidney, liver, muscle, pancreatic and salivary glands (PSG), reticuloendothelial system (RES) and skin, together with a blood pool of amino acids. The fluxes represented in the equations are generally assumed to follow simple or modified mass-action kinetics. The fractional rates for protein synthesis in each tissue were calculated from published values for lambs, where these were available, or alternatively derived using assumptions based on data obtained with rats. The initial protein content of each tissue and the fractional degradation rate were assigned values calculated using data from a slaughter experiment. The model was used to examine whole-body protein synthesis at different rates of growth and to compare the relative contributions of each tissue. The GIT (25-26%) and skin (23-26%) had the highest contributions to total protein synthesis, followed by muscle (21-26%), liver (13-14%), RES (6-7%) and PSG (3-6%), while adipose, CNS, heart and kidney together contributed less than 5%. These values agree reasonably well with experimental values, and thus the model can be used to examine the effect of different growth rates on protein metabolism and its associated energy costs.