Presence of Glucagon Immunoreactivity in the Globulin Fraction of Human Plasma (“Big Plasma Glucagon”)

Abstract

Filtration of whole human plasma on Bio Gel P-30 yielded two major glucagon-immunoreactive fractions. One fraction eluted like true glucagon (in the 3500 MW region); the other fraction appeared with the proteins and has been provisionally termed “big plasma glucagon” (BPG). In a series of ten experiments, BPG represented 29.7–100% of the total glucagon immunoreactivity in the basal sample, and 13.2–56.6% after alpha-cell stimulation with arginine. Plasma chromatography on Sephadex G-100 showed BPG to elute within the globulin zone. Cellulose acetate electrophoresis of eluates containing BPG showed it to extend from the γ to the β1 globulin zone. Isolated BPG conserved its elution characteristics as determined with Unger 30K and two less specific antisera. Treatment of BPG with 8M urea–1M acetic acid did not alter its chromatographic pattern. The incubation of BPG with trypsin originated a glucagon-immunoreactive peak that appeared in a MW zone near the glucagon-1125 marker. A glucagon-immunoreactive fraction with an elution volume similar to that of BPG was also demonstrated upon gel filtration of acid-alcohol extracts of dog pancreas and of isolated mouse pancreatic islets. Although the identity of BPG has not been established, it could represent glucagon bound to a protein, or a polypeptide containing immunological glucagon determinants within its structure. The demonstration of a similar component in pancreas extracts suggests that it might be a secretory product of the alpha-cell. At any rate, the presence of BPG in circulating blood behooves a reevaluation of glucagon radioimmunoassay.