Tritiated peptides. 12. Synthesis and biological activity of [4-3H-Phe8]substance P

Substance P has been prepared 3H labeled at Phe8 by catalytic deiodination of a protected precursor. Synthesis of the precursor was by solid-phase methodology on polydimethylacrylamide resin and by condensation in solution of fragments covering sequences 1-4, 5-7, and 8-11. Free peptide made by each route analyzed satisfactorily and had the same chromatographic characteristics as unlabeled substance P. It was indistinguishable from the latter by radioimmunoassay when N and C terminally directed antisera were used and in the ability to cause contractions of isolated guinea pig ileum. Specific radioactivity was 23 Ci/mmol.