BUFOKININ - A SUBSTANCE-P RELATED PEPTIDE FROM THE GUT OF THE TOAD, BUFO-MARINUS WITH HIGH BINDING-AFFINITY BUT LOW SELECTIVITY FOR MAMMALIAN TACHYKININ RECEPTORS

A tachykinin peptide, termed bufokinin, was isolated in pure form from an extract of the intestine of the toad, Bufo marinus, and its primar y structure was established as: Lys-Pro-Arg-Pro-Asp-Gln-Phe-Tyr-Gly-Le u-Met.NH2. This sequence was confirmed by chemical synthesis and shows four amino acid substitutions (Arg(1) --> Lys,Lys(3) --> Arg,Gln(5) - -> Asp and Phe(8) --> Tyr) compared with substance P. Binding paramete rs for synthetic bufokinin and mammalian tachykinins were compared usi ng receptor-selective radioligands and crude membranes from rat tissue s enriched in the NK-1 (submandibular gland), NK-2 (stomach fundus) an d NK-3 (brain) receptors, In terms of inhibiting the binding of the se lective radioligands, bufokinin (K-d = 0.3 nM) was 1.8-fold more poten t than substance P at the rat NK-1 site, but it was only 2-fold less p otent (K-d = 2.8 nM) than neurokinin A at the NK-2 site and only 2-fol d less potent (K-d = 48 nM) than neurokinin B at the NK-3 site. Thus, bufokinin shows relatively high affinity but lack of selectivity for a ll three tachykinin binding sites in rat tissues. (C) Munksgaard 1998.