A tachykinin peptide, termed bufokinin, was isolated in pure form from an extract of the intestine of the toad, Bufo marinus, and its primary structure was established as: Lys-Pro-Arg-Pro-Asp-Gln-Phe-Tyr-Gly-Leu-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 parameters for synthetic bufokinin and mammalian tachykinins were compared using receptor-selective radioligands and crude membranes from rat tissues enriched in the NK-1 (submandibular gland), NK-2 (stomach fundus) and NK-3 (brain) receptors, In terms of inhibiting the binding of the selective radioligands, bufokinin (K-d = 0.3 nM) was 1.8-fold more potent than substance P at the rat NK-1 site, but it was only 2-fold less potent (K-d = 2.8 nM) than neurokinin A at the NK-2 site and only 2-fold 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 all three tachykinin binding sites in rat tissues. (C) Munksgaard 1998.
