Differential immunoreactivity of plasma glucagon components in man: studies with different glucagon antibodies

To evaluate the relationship between glucagon antibody antigenic determinants and selective reactivity with plasma void volume (Vo) and lower molecular weight immunoreactive glucagon (IRG) components, we studied plasma IRG levels and molecular profiles in normal subjects and patients with disturbances in plasma glucagon levels using three glucagon antibodies, 30K and P7 raised against the whole peptide and antibody X4 raised against the C-terminal tryptic fragment of glucagon. In normal subjects and pancreatectomized patients, plasma IRG levels were 2- to 3-fold higher with the C-terminus-directed antibody X4 than with either 30K or P7, but in glucagonoma and uremic patients, this discrepancy was smaller. Gel filtration analysis revealed that these antibodies reacted identically with 3500 mol wt IRG and 9000 mol wt IRG in normal, glucagonoma, pancreatectomized, and uremic plasma. Relative immunoreactivity of Vo IRG was approximately 5:2:1 with antibodies X4, 30K, and P7, respectively. In two subjects with unexplained hyperglucagonemia, recovery of IRG was entirely in the Vo, with antiserum X4 reacting one third as well as 30K and P7 not reacting at all. Furthermore, this material did not dilute out in parallel to glucagon standard. These data indicate differential immunoreactivity of the high molecular weight circulating IRG component, with a series of three glucagon antibodies reacting similarly with all other plasma IRG fractions, and suggest that Vo IRG material in plasma is predominantly the result of an immunologically cross-reacting peptide sequence in a plasma protein. The selective immunoreactivity of this component with different antibodies has important implications for the glucagon RIA and may have some bearing on other immunoassays as well.