Kinetic evidence for different mechanisms of interaction of black mamba toxins MTα and MTβ with muscarinic receptors

Abstract

By studying the influence of two toxins from the black mamba Dendroaspis polylepis on the kinetics of [ 3H]- N-methylscopolamine binding to muscarinic acetylcholine receptors from rat cerebral cortex, it was revealed that these toxins, MTα and MTβ, interact with the receptors via kinetically distinct mechanisms. MTβ bound to receptors in a one-step, readily reversible process with the dissociation constant K d=5.3 μM. The binding mechanism of MTα was more complex, involving at least two consecutive steps. A fast receptor–toxin complex formation ( K T=3.8 μM) was followed by a slow process of isomerisation of this complex ( k i=1.8×10 −2 s −1, half-time 39 s). A similar two-step interaction mechanism has been established for a related toxin, MT2 from the green mamba D. angusticeps ( K T=1.4 μM, k i=8.3×10 −4 s −1, half-time 840 s). The slow isomerisation process delays the effect of MTα and MT2, but increases their apparent potency compared to toxins unable to induce the isomerisation process.