Kinetic evidence for different mechanisms of interaction of black mamba toxins MT alpha and MT beta with muscarinic receptors

By studying the influence of two toxins from the black mamba Dendroaspis po lylepis on the kinetics of [H-3]-N-methylscopolamine binding to muscarinic acetylcholine receptors from rat cerebral cortex, it was revealed that thes e toxins, MT alpha and MT beta, interact with the receptors via kinetically distinct mechanisms. MT beta bound to receptors in a one-step, readily rev ersible process with the dissociation constant K-d = 5.3 mu M. The binding mechanism of MT alpha was more complex, involving at least two consecutive steps. A fast receptor-toxin complex formation (K-T = 3.8 mu M) was followe d by a slow process of isomerisation of this complex (k(i) = 1.8 x 10(-2) s (-1), half-time 39 s). A similar two-step interaction mechanism has been es tablished for a related toxin, MT2 from the green mamba D. angusticeps (K-T = 1.4 mu M, k(i) = 8.3 x 10(-4) s(-1), half-time 840 s). The slow isomeris ation process delays the effect of MT alpha and MT2, but increases their ap parent potency compared to toxins unable to induce the isomerisation proces s. (C) 2000 Elsevier Science Ltd. All rights reserved.