Immunologic characterization of plasma glucagon components in a patient with malignant glucagonoma

Abstract

Plasma immunoreactive glucagon (IRG) components were analyzed by gel filtration on either a Bio-Gel P-30 or a Sephadex G-150 column (1.0 × 68 cm) in a 47-year-old male with biopsy-proven malignant glucagonoma. Plasma samples were obtained before and after 20 courses of streptozotocin treatment as well as after administration of a somatostatin-derivative (SRIF-D, 0.38 mg, subcutaneous), regular insulin (0.2 U/kg, intravenous), and secretin (2 U/kg, intravenous). The fractions from the columns were assayed for IRG by simultaneous radioimmunoassay with C-terminal (Unger 30 K) and N-terminal (OAL 196) antibodies to glucagon. Four IRG components were observed. The largest had a molecular weight of approximately 150,000 daltons and cross-reacted much more strongly with the N-terminal antibody than with the C-terminal. The second IRG component appeared to be about 9000 daltons and cross-reacted more strongly with the N-terminal antibody. The third and major IRG component comprised 51.8% to 88.1% of the total IRG as measured with C-terminal antibody, corresponded in molecular weight to synthetic 3500 dalton glucagon, and reacted roughly equally with each of the two antibodies. The fourth IRG component cross-reacted only with N-terminal antibody and appeared to be smaller than 3500 daltons. The plasma IRG level decreased from 8829 pg/mL to 1421 pg/mL (averages of five consecutive determinations) after 20 courses of treatment with streptozotocin with significant clinical improvement. A marked (74%) but transient decrease in plasma IRG was observed after the SRIF-D injection, whereas secretion and insulin caused increases in plasma IRG level of 53% and 22%, respectively. The increases or decreases in total plasma IRG were associated with parallel increases or decreases mainly in the third and fourth IRG components, with somewhat less remarkable changes in the second; the first IRG component remained essentially unchanged. These results suggest that the largest IRG component, or so-called “big” plasma glucagon, was immunologically but not biosynthetically related to glucagon. The remaining three IRG components appeared to be biosynthetically glucagon-related because of the parallel changes after both suppression and stimulation. The smallest IRG may possibly represent the N-terminal fragment of the 3500 dalton glucagon, in view of its molecular size and selective cross-reactivity with the N-terminal antibody.