Novel pharmacological properties of transient potassium currents in central neurons revealed by N-bromoacetamide and other chemical modifiers

The chemical modifiers N-bromoacetamide (NBA), N-bromosuccinimide (NBS), and chloramine-T (ChT) are commonly used to remove fast inactivation of sodium currents and transient potassium currents (I(A)). In the present study, I examined the effects of these chemical modifiers as well as of others on I(A) in neurons dissociated from several brain regions. External application of NBA irreversibly inhibited I(A), with higher NBA concentrations increasing the rate of inhibition. The current kinetics, however, were not altered by external NBA at any concentration. I(A) was also inhibited by internal NBA but only at high concentrations, and the rate of inhibition was much slower. The current kinetics were not altered by internal NBA at any concentration. NBA is a nonspecific chemical reagent that can modify a protein at several target amino acids. The NBA-induced irreversible inhibition of I(A) was reproduced by external diethylpyrocarbonate, a reagent that specifically modifies histidine residues, and by ChT and cyanogen bromide, reagents that modify methionine residues through distinct mechanisms. However, NBS, a reagent that cleaves the peptide bond at tryptophan residues, had no effect on I(A), nor did chemical modifiers specific for cysteine and tyrosine residues. Taken together, these results suggest that the conserved, functionally important methionine and/or histidine residues are the likely targets for NBA modifications. These novel pharmacological properties are in sharp contrast to those known previously, despite their similarity in both kinetics and 4-AP sensitivities. Therefore, the pharmacological treatments presented in the present study should be useful for characterizing other I(A).