The metabolism of serum proteins. 3. Kinetics of serum protein metabolism during growth.

In most studies of the turnover rates’ of the serum proteins, the incorporation into, or more usually, the loss of radioactivity from, the protein fractions are measured. Such data are interpreted as an index of the turnover rate, assuming that the animals are in a “steady state” with respect to serum proteins. The interpretation of the loss of a radioactive protein from the vascular system in animals gaining or losing weight is more complex. It has been suggested that turnover of protein does not occur in rapidly growing microorganisms (1, 2) and in some animal tissues (3,4). However, other evidence (5, 6) has been reported showing that turnover of nucleic acids and protein does occur in liver. It has been established (7) that a serum protein labeled with S35 can be successfully employed as a tracer for the determination of the turnover rate of the protein. Previous work in this laboratory (8) has shown that the turnover rate of albumin, but not of the various globulins, is increased when rats are maintained on a high protein diet and decreased when they are fed a low protein diet. The suggestion was made (8) that serum albumin can serve as a primary source of amino acids for the synthesis of other body proteins, either directly, without complete hydrolysis, or by contributing to the tissue amino acid pool. Yuile et al. (9) recently reported a slower turnover of both albumin and globulins when dogs were protein depleted, and a short half-life when the dogs were maintained on high protein diets. I f this hypothesis is valid, the turnover rate of serum albumin would be expected to increase during growth when the synthesis of tissue protein is rapid, whereas the turnover rates of the various globulins would be unchanged. In this study, the rates of loss of radioactivity from an injected labeled serum protein fraction has been followed twice in the same animals, once during the growth period, and again when the animals were adults and presumably in a steady state. There was a more rapid decrease in the specific activity of albumin during growth than in the adult animal. The decrease in the specific activity of y-globulin was essentially the same during growth and maturity. By approximating the increase in the size of the albumin and y-globulin pools during growth, it is possible to calculate the turnover rates of albumin and y-globulin in growing rats. These rates were compared with the turnover rates of albumin and y-globulin in the same animals at maturity.