In vivo immunosorption--a method for the examination of hormone heterogeneity at low plasma concentrations. Application to progastrin and its products

In order to investigate whether antibody desorption followed by chromatography is useful for the study of ligand heterogeneity in plasma, endogenous plasma gastrins were released from immunoglobulins by denaturation of antisera from 11 rabbits immunized against different fragments of human progastrin. The molecular nature of the in vivo immunosorbed plasma gastrins was characterized by gel chromatography monitored by a library of sequence-specific radioimmunoassays before and after enzyme cleavage. Subsequently, the plasma gastrins were compared with tissue gastrins in extracts of rabbit antral mucosa. The results show that carboxyamidated gastrins and their immediate glycine-extended precursors circulate in rabbit plasma. The carboxyamidated gastrins eluted as gastrin-34 and gastrin-17, both in sulfated and non-sulfated form. Correspondingly, glycine-extended gastrin-34 and gastrin-17 also occurred in plasma. The results confirm that the plasma of immunized animals contains substantial quantities of the corresponding endogenous ligands bound to specific antibodies. This in vivo immunosorption phenomenon can be used to study the molecular heterogeneity of hormones and their precursors, when they circulate in concentrations which are otherwise too low to permit examination of their molecular nature.