Stoichiometric labeling of peptides by iodination on tyrosyl or histidyl residues.

Radioiodination with 125I or 131I is a favored technique for labeling biologically active peptides or proteins because of high specific radioactivities and convenience in counting gamma-emissions. Previous studies used trace labeling, in which fewer than 1% of the molecules are iodinated. We describe procedures for obtaining stoichiometrically iodinated and therefore chemically homogeneous peptides with specific activities exceeding 10(7) cpm/micrograms (approximately 10 Ci/mmol). By analyzing the pH dependence of iodination on tyrosyl and histidyl residues, we show that the method described can be applied to many short peptides and optimized for labeling on tyrosine and/or histidine. The power of reverse-phase HPLC is exploited to resolve multiple products substituted with different molar equivalents of iodine from each other and from unlabeled peptide. Specific radioactivity ratios can be used to identify the products, as confirmed by Edman sequence analysis under conditions that separate iodinated tyrosine and histidine derivatives from all other amino acids. We also show that the biological activities of iodinated and uniodinated peptides can differ by several orders of magnitude in a T cell assay and demonstrate the usefulness of stoichiometric labeling to overcome ambiguities inherent in studying biological activities with trace-labeled peptides.