AMINO-ACID UTILIZATION AND ISOTOPE DISCRIMINATION OF AMINO NITROGEN IN NITROGEN-METABOLISM OF RAT-LIVER IN-VIVO

Urea and plasma protein differ in natural N-15 abundance up to 100 par ts per thousand. The origin of this difference is the branched nitroge n metabolism in the liver. One main branch is the protein synthesis pa thway, the other the urea synthesis pathway. By this branching N-15 Of precursor amino acids is depleted in urea while it is enriched in pro tein. With the N-15 abundance of precursor amino acids, which may be t aken from jejunum tissue, utilization of amino acids in liver metaboli sm can be calculated from isotope discrimination in either pathway. Th is was investigated by feeding different proteins to rats. When feedin g high quality protein (whey protein) utilization of amino acids in li ver metabolism at requirement intake was better than at zero protein i ntake (> 85% vs. 70%). From this we conclude that the pattern of amino acids available from the metabolic pool at zero protein intake is cha racterized by an imbalance. This endogenous imbalance can be complemen ted by exogenous dietary amino acids set that nitrogen excretion may e ven be smaller than the so-called ''obligatory'' losses of intakes not exceeding requirement. Thus, the quality of dietary protein is reflec ted not only by N balance. It also may be quantified by analysis of is otope discrimination in nitrogen metabolism of the liver. In addition, the quality of amino acid pattern available from the metabolic pool i s indicated by this method.