Heterogeneity of peptide hormones: its relevance in clinical radioimmunoassay

Radioimmunoassay of extracted glucagon compared with three non-extraction assays

Radioimmunoassay of glucagon was performed with three different antisera, i.e. E7, 30K and 4305, all directed against the carboxyl-terminal region of glucagon and thus avoiding co-determination of glucagon-like polypeptides from the gut. Plasma samples from five healthy people subjected to various A-cell stimulation and suppression tests were used and immunoreactive glucagon assessed with the three antisera. Aliquots from all plasma samples were also extracted with acetone and glucagon re-assessed with antiserum E7. Even though all four baseline glucagon concentrations obtained were different, the glucagon profiles were comparable after superimposing the baselines. The differences in baseline concentrations of immunoreactive glucagon seem due to the interference of "big plasma glucagon", a still unidentified factor in the E7 and 30K assays that can be precipitated by acetone. Since acetone extraction yielded the lowest baselines without altering the glucagon profiles, it is suggested that the baseline glucagon concentrations of acetone-extracted plasma reflect the physiological level of the biologically active hormone. Using antiserum E7, our own antiserum, the normal range of glucagon values in acetone-extracted plasma samples from 22 healthy, fasting people of both sexes was 42 +/- 16 ng/l (mean +/- 2 S.D.). These values agree well with those obtained by other assay techniques.