Changes to biological activity following acetylation of dendrotoxin I from Dendroaspis polylepis (black mamba)

The potassium channel blocker dendrotoxin I was acetylated with acetic anhydride. Mono-acetyl derivatives of all seven lysine residues (N-terminus blocked) and a di-derivative were isolated by chromatography on the cation-exchanger Bio-Rex 70 and reversed-phase high-performance liquid chromatography. The derivative acetyl-Lys 29 and the di-derivative of Tyr 24 and Lys 28 had more than 1000 times lower affinity than the native toxin as determined by inhibition of the 125I-dendrotoxin binding to synaptosomal membranes from rat brain. Lys 29 is part of the triplet Lys-Lys-Lys (28-30) which also occurs in the homologous alpha-dendrotoxin where the triplet is not in the functional site, as shown by site-directed mutagenesis. Acetylation of Lys 29 may have produced large structural perturbations that inactivated the toxin. Acetylation of Lys 28 alone had little effect, but the toxin became almost inactive when both Lys 28 and Tyr 24 were modified. Ten experiments were conducted under similar conditions, but a derivative of Tyr 24 was obtained only three times. In these cases the toxin apparently had a different structure, with Tyr 24 accessible to the reagent. This may depend on freeze-drying, which can alter the structure of proteins. The third derivative with low activity was acetyl-Lys 5, with affinity decreased 20-fold. Lys 5 has a protruding side-chain that does not interact with any other group in the toxin molecule. Therefore, Lys 5 is probably part of the functional site for dendrotoxin's binding to the voltage-dependent K+ channels.