Measurement of the synthesis of liver-produced plasma proteins with special reference to their regulation by dietary protein and amino acid supply

About 300 g of protein are synthesized by the body each day.The plasma proteins account only for about 30 g, of which albumin represents about 12 g.Yet, a great part of our knowledge of adaptation to protein or amino acid deprivation is based on studies of albumin metabolism and relatively few studies relate to the minor plasma proteins or specific tissue proteins.This is not surprising when one contrasts the comparative ease of studies on albumin metabolism with the technical problems of isolation and measurement of metabolic changes in other proteins.Clearly, adaptive changes in albumin metabohm cannot be regarded as representative of all tissue proteins, nor even of all plasma proteins.But the effect on albumin synthesis of alterations in amino acid supply is of interest, not only because of the importance of this protein in maintaining plasma oncotic pressure and in providing a transport mechanism for many substrates, but because the rate of its synthesis reflects one of the fundamental functions of the liver-albumin represents about a quarter of all the protein produced by this organ.Furthermore, the liver plays a vital role in regulating total body amino acid metabolism through its modulation of the postdigestive surge of amino acids into the portal blood-stream ; adaptive changes in this buffering action of the liverperhaps reflected in the rate of plasma protein synthesiscould materially alter the amount of amino acids available for metabolic purposes throughout the body. Methods of measuring synthesis rates of liver-produced plasma proteinsHepatic protein production has been studied by many different techniques, and it is desirable to examine these critically before considering the response to alterations in dietary protein or amino acid supply.For this purpose synthesis of albumin only will be considered, but most of the techniques may also be applied to the study of other liver-produced proteins. In vivo Indirect or inferential.Injection of albumin molecules labelled with radioactive iodine allows determination of a daily fractional catabolic rate for the protein; by relating this to the intravascular albumin pool an absolute figure can be derived for