Immunochemical studies on macromolecular gastrins: evidence that "big big gastrins" are artifacts in blood and mucosa, but truly present in some large gastrinomas.

Whereas true gastrin macromolecules are present in some large gastrinomas, it is still unsettled whether macromolecular gastrin–earlier designated “big big gastrin” (BBG) – is truly present in blood and tissue. This question has now been evaluated by gel and affinity chromatography monitored radioimmunochemically. Gel chromatography on small Sephadex G-50 columns (10 by 500 mm) of serum from 8 normal fasting subjects confirmed that apparent gastrin immunoreactivity is eluted in the void volume. However, in contrast to the established serum gastrins (components I to IV) the apparent serum BBG was not removed by affinity chromatography. The amount of apparent BBG was related to the amount of proteins present in the void volume fractions and to the ability of serum proteins to interfere with the binding affinity of three different antisera used. Gel chromatography on Sephadex G-50 columns of antral, duodenal, and jejunal extracts revealed gastrin in the void volume only when large amounts of extracts were applied to the columns. By refiltration of these void volume fractions the apparent BBG disappeared and gastrin immunoreactivity was now eluted corresponding to components I, II (gastrin-34 or “big” gastrin), and III (gastrin-17 or “little” gastrin). In contrast to these findings an extract of a large gastrinoma, previously used to substantiate the nature of BBG, contained a number of gastrin macromolecules with apparent molecular masses ranging from 30,000 to 100,000 daltons. These gastrins, defined as BBG by their elution in the void volume of Sephadex G-50 columns, were removed by affinity chromatography using antibodies specific for the carboxyl terminal sequence of gastrin-17 from buffer or serum solutions. Elution in urea gradient on Sephadex G-50 columns of extracts from further three gastrinomas disclosed BBG in still one tumor. The results indicate that there are no detectable true macromolecular gastrins in serum and non-neoplastic gastrointestinal mucosa. The apparent BBG is rather attributable to interference in the gastrin assay by normal serum proteins or to weak noncovalent protein-binding of smaller gastrins. In contrast, true gastrin macromolecules consisting of gastrin-17 covalently coupled in its amino-terminus to proteins of varying length are present in detectable amounts in some large gastrinomas.