The Lack of Selective Reincorporation into Tissue Protein of the Amino Acids Derived from the Catabolism of Serum Protein

Homologous S(35)-labeled albumin, gamma globulin, and alpha-beta globulin were transfused into rabbits and the specific activities of the electrophoretic fractions of the sera of the recipients were determined at various time intervals up to 12 days after injection. Detectable reincorporation into a fraction other than that transfused was found only in the gamma globulin fraction after albumin injection. This activity rose between 2 and 12 days and reached a level of 2 to 3 per cent of the extrapolated zero time activity of the albumin fraction. When homologous serum protein doubly labeled with I(131) and S(35) was transfused into mice, marked drops in the ratios of I(131) to S(35) in the serum and tissue proteins were observed between 1 and 48 hours after injection. On the basis of a determination of the absolute and relative amounts of I(131) and S(35) found in the various tissue and serum proteins, the amount of reincorporation of S(35) into each protein was calculated. The relative amounts of reincorporation of S(35) among the various tissues were remarkably similar to the relative amounts of incorporation of S(35) after the injection of labeled free amino acids. It is concluded that serum protein does not form a major direct source of amino acids to the tissues but feeds them indirectly through the extracellular pool.