Immunochemical studies on macromolecular gastrins: evidence that "big big gastrins" are artifacts in blood and mucosa, but truly present in some large gastrinomas
Rehfeld, J.F.; Schwartz, T.W.; Stadil, F.
Gastroenterology 73(3): 469-477
1977
ISSN/ISBN: 0016-5085 PMID: 892344 Document Number: 120327
Whereas true gastrin macromolecules are present in some large human gastrinomas, it is still unsettled whether macromolecular gastrin earlier designated big big gastrin (BBG) is truly present in blood and tissue. This question was now 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. In contrast to the established serum gastrins (components I-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 the ability of serum proteins to interfere with the binding affinity of 3 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 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, 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 a further 3 gastrinomas disclosed BBG in 1 tumor. The results indicate that there are no detectable true macromolecular gastrins in serum and non-neoplastic gastrointestinal mucosa. The apparent BBG is attributable to interference in the gastrin assay by normal serum proteins or weak noncovalent protein-binding of smaller gastrins. 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.