A serum albumin precursor in cytoplasmic particles.

The existence of an intermediate in the biosynthesis of serum albumin was suggested by a previous study of the kinetics of appearance of Cl4 in soluble serum albumin in liver slices (1). In the course of investigations on the nature of this intermediate, it has been found that treatment of cytoplasmic particles of chicken liver with deoxycholate or other agents will release non-dialyzable material which precipitates with antibodies against chicken serum albumin. Isotopic experiments in vitro and in vivo indicate that this intraparticulate (especially microsomal) “serum albumin” is the precursor of the soluble form. The previously observed lag in the appearance of Cl4 in serum albumin is found to represent not the time of synthesis of the protein molecule but the time required for the physiological release of albumin from its binding by particles.