Three components of gastrin in human serum Gel filtration studies on the molecular size of immunoreactive serum gastrin

Abstract

Sera obtained from 6 normal subjects during a protein-rich meal and sera from 16 subjects with hypergastrinaemia (pernicious anaemia) were subjected to gel filtration on Sephadex G-50 fine columns. The concentration of gastrin in the effluent was determined by a sensitive radioimmunoassay. Three components of gastrin were observed. (1) Component I with a Stokes radius of 2.27 ± 0.08 nm ( mean ± S. D.) was eluted immediately prior to proinsulin. 18.4 ± 7.6% ( mean ± S. D.) of the total immunoreactive gastrin was found in this component, and it was present in 19 of 22 sera. (2) Component II with a Stokes radius of 1.52 ± 0.04 nm was eluted corresponding to the “basic gastrin” of Yalow and Berson. It comprised 72.6 ± 11.5% of the immunoreactive gastrin. In one serum this second component could not be detected. (3) Component III was measured in the effluent eluted after insulin. It comprised on average 9.1% of the immunoreactivity, and was detectable in only half of the sera. The elution pattern of this component was less well defined, and did not allow calculation of a molecular size. Gel filtration of serum in human plasma at 37°C, incubation with 8 M urea, and incubation with 0.1 M dithiothreitol did not alter the distribution of the components. Incubation with trypsin converted the two big components to a heptadecapeptide-like gastrin component with a Stokes radius of 0.81 ± 0.05 nm. In calibration studies synthetic human heptadecapeptide gastrin was found to have a radius of 0.88 ± 0.08 nm. No relationship between cholecystokinin and the gastrin components could be demonstrated using three gastrin antisera with different specificity against cholecystokinin. The findings demonstrate that a significant fraction of immunoreactive gastrin in serum is present as a component with a molecular size greater than that of the two components described so far. It is suggested, that the poorly defined small component (III) represents decomposition fragments of the larger components (I and II).