Tritiated photoactivatable analogs of the native human thrombin receptor (PAR-1) agonist peptide, SFLLRN-NH2

Six photoactivatable analogs of the human thrombin receptor activating pept ide (TRAP), SFLLRN-NH2, were synthesized by substituting the photoactive am ino acid, p-benzoylphenylalanine (Bpa), into each position of the peptide s equence. Platelet aggregation assays indicated that the peptides with Bpa s ubstitutions at positions 3 to 6 retained agonist activity. These peptides were prepared in tritiated form as potential thrombin receptor photoaffinit y labels. The [H-3]Bpa-containing analogs were constructed by resynthesizin g the peptides with the amino acid, 4-benzoyl-2 ' ,5 ' -dibromophenylalanin e (Br(2)Bpa), and subjecting the purified peptides to Pd-catalyzed tritiode bromination. The radiochemical yields for the reductive tritiation were <2% for peptides with [H-3]Bpa in the third and fourth positions, and between 7 and 16% for the peptides with substitutions at the fifth and sixth positi ons. The low yields were due to over-reduction of the Bpa carbonyl group an d nonspecific degradation during reductive tritiation. This report describe s the first use of Br(2)Bpa for the preparation of tritiated photoactivatab le peptides.