Preparation of intrinsically-labelled kinins

As part of a program to prepare bradykinin (H-Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg-OH) labelled at high specific radioactivities, we have synthesized three analogs for dehalogenation in tritium gas: [4-Br-Phe5]-bradykinin (BK), [4-Br-Phe8]-BK and [4-Br-Phe5,8]-BK. The analogs were synthesized by the Merrifield solid-phase method and were purified by molecular sieve and partition chromatography. The analogs themselves possess biological activity (as assayed for effects on mean arterial blood pressure and isolated rat uterus). [4-Br-Phe8]-BK was 1.5 to 3 times as active as bradykinin. [4-Br-Phe5,8]-BK was approx. 22% as active as BK and [4-Br-Phe5]-BK was approx. 18% as active. [4-Br-Phe5]-BK was submitted to catalytic dehalogenation with 10% Pd/C and 5% Rh/CaCO3 in H2O and DMF (1:1) plus 10 Ci of 3H2. [4-3H-Phe5]-BK was obtained at 6.7 Ci/mmole in an overall yield of 15%. [4-3H-Phe8]-BK was prepared similarly to yield an intrinsically-labelled peptide with a specific radioactivity of 21 Ci/mmole.