Effect of N-terminal iodination on the biological, immunological and receptor binding properties of secretion. A role for the alpha-amino group of histidine in stabilizing hormone-receptor interactions.

Both radiotrace labeled and high specific activity 125I-labeled derivatives of secretin were prepared by direction iodination of the histidyl residue with chloramine T [( 125I]secretin) and by conjugation of a preiodinated Bolton-Hunter group (iodo-3-(4-hydroxyphenyl)propionate) to the free alpha-amino group at the N-terminus [( 125I]BH-secretin). Following purification, the biological, immunological and receptor binding properties of both secretin derivatives were compared. [125I]secretin and [125I]BH-secretin were equally effective in a sensitive radioimmunoassay that detected secretin and secretin (5-27) but not CCK-8, VIP and glucagon. Although both derivatives retained 60% of the biological potency of secretin as measured by cAMP accumulation in pancreatic acinar cells, only the directly iodinated peptide [( 125I]secretin) could be used to characterize specific binding sites on rat pancreatic membranes. The N-terminal blocked derivative [( 125I]BH-secretin) in contrast dissociated rapidly from pancreatic membranes reflecting an unstable hormone-receptor complex. These results suggest that a free alpha-amino group at the N-terminus may be essential for an optimal interaction of secretin with its pancreatic receptor.