LAUSR

&NA; Nux vomica has been effectively used in Traditional Chinese Medicine. The processing of Nux vomica is necessary to reduce toxicity before it can be used in clinical practice. However, the mechanism for processing detoxification is unclear. hERG channels have been subjected to a routine test for compound cardiac toxicity in the drug development process. Therefore, we examined the effects and mechanisms of strychnine and brucine, two main ingredients of Nux vomica, and their N‐oxides on hERG channels. Strychnine and brucine exhibited concentration‐dependent inhibition of hERG channels with IC50 values of 25.9 &mgr;M and 44.18 &mgr;M, respectively. However, their nitrogen oxidative derivatives produced by processing of Nux vomica, strychnine N‐oxide and brucine N‐oxide, lost their activity on hERG channels. Compared to their parent compounds, only an oxygen atom was introduced in the nitrogen oxidative isoforms to compensate for the N+ − charge, suggesting that the protonated nitrogen is the key group for strychnine and brucine binding to hERG channel. Alanine‐mutagenesis identified Y652 is the most important residue for strychnine and brucine binding to hERG channel. Y652A mutation increased the IC50 for strychnine and brucine by 21.64‐fold and 29.78‐fold that of WT IhERG, respectively. Docking simulations suggested that the protonated nitrogen of strychnine and brucine formed a cation–&pgr; interaction with the aromatic ring of Y652. This study suggests that introduction of an oxygen to compensate for the N+ − charge could be a useful strategy for reducing hERG potency and increasing the safety margin of alkaloid‐type compounds in drug development. HighlightsStrychnine and brucine exhibited concentration‐dependent inhibition of hERG channel.Strychnine N‐oxide and brucine N‐oxide lost their activity on hERG channels.Protonated nitrogen is the key group for strychnine and brucine binding to hERG channel.Y652 is the most important residue for strychnine and brucine binding to hERG channel.