Acetylation is certainly an easy and cheaper way to 3H label your peptide as suggested by Ruth. If you have already synthesized the peptide you can also trace label it with high specific radioactive acetic anhydride (3H). It will put label on amino terminus and other free amino groups at lysines and some tyr also. The tyrosine labeling can be easily reversed by dialysis in 0.5M hydroxylamine. The peptide will maintain its activity. For reference you may check BBRC vol205, pages 1688-1695.
Good luck
Satya Yadav
Molecular Biotechnology Core
The Lerner research Institute
The Cleveland Clinic Foundation
Cleveland, Ohio 44143