Introduction
Serum proteins incorporate amino acids very rapidly, and it seems justifiable to conclude that this process takes place within cells somewhere in the body, whether by synthesis de novo or by some sort of exchange in the cell between serum proteins and amino acids. Implicit in the latter is the assumption that circulating serum proteins continuously return to cells of origin. In recent years it has been found that serum proteins do indeed pass from the blood stream into cells of many of the body tissues 1,4,7,9,10,16,17,21,26,28,31,32,34,39 Also, recent evidence indicates that serum proteins are utilized metabolically by cells. Both albumins and globulins appear to be involved in these cellular relationships.39-41On the other hand, proteins of many kinds are elaborated by body tissues and appear in various extracellular fluids. The serum proteins, composed of a number of different molecular species, represent at least a considerable fraction of