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 primary structure was established as: Lys-Pro-Arg-Pro-Asp-Gln-Phe-Tyr-Gly-Leu-Met. N₂. This sequence was confirmed by chemical synthesis and shows four amino acid substitutions (Arg¹ → Lys,Lys³ → Arg,Gln⁵ → Asp and Phe⁸ → 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.