MIT(1), a black mamba toxin with a new and highly potent activity on intestinal contraction.

Mamba intestinal toxin (MIT(1)) isolated from Dendroaspis polylepis venom is a 81 amino acid polypeptide cross-linked by five disulphide bridges. MIT(1) has a very potent action on guinea-pig intestinal contractility. MIT(1) (1 nM) potently contracts longitudinal ileal muscle and distal colon, and this contraction is equivalent to that of 40 mM K(+). Conversely MIT(1) relaxes proximal colon again as potently as 40 mM K(+). The MIT(1)-induced effects are antagonised by tetrodotoxin (1 microM) in proximal and distal colon but not in longitudinal ileum. The MIT(1)-induced relaxation of the proximal colon is reversibly inhibited by the NO synthase inhibitor L-NAME (200 microM). (125)I-labelled MIT(1) binds with a very high affinity to both ileum and brain membranes (K(d)=1.3 pM and 0.9 pM, and B(max)=30 fmol/mg and 26 fmol/mg, respectively). MIT(1) is a very highly selective toxin for a receptor present both in the CNS and in the smooth muscle and which might be an as yet unidentified K(+) channel.