Selective labeling of alpha- or epsilon-amino groups in peptides by the Bolton-Hunter reagent

Incorporation of N-succinimidyl-3(4-hydroxyphenyl) propionate (Bolton-Hunter reagent) or its 125I-labeled derivative into peptides can be selectively directed towards either alpha- or epsilon-amine functions by modifying the pH of the reaction. Acylation of alpha-amino groups is favored at pH 6.5 whereas epsilon-amino groups react more readily at pH 8.5. We have taken advantage of this result to prepare two new 125I-labeled analogues of substance P and neurotensin that bind selectively and reversibly to their respective receptors. The method described here is of general interest and can be used to incorporate various reporter groups into peptide structures.