Effect of N‐terminal iodination on the biological, immunological and receptor binding properties of secretin.

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Abstract
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 α‐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 α‐amino group at the N‐terminus may be essential for an optimal interaction of secretin with its pancreatic receptor.