Point mutations at N434 in D1-S6 of μ1 Na⁺ channels modulate binding affinity and stereoselectivity of local anesthetic enantiomers

Voltage-gated Na⁺ channels are the primary targets of local anesthetics (LAs). Amino acid residues in domain 4, transmembrane segment 6 (D4-S6) form part of the LA binding site. LAs inhibit binding of the neurotoxin batrachotoxin (BTX). Parts of the BTX binding site are located in D1-S6 and D4-S6. The affinity of BTX-resistant Na⁺ channels mutated in D1-S6 (μ1- N434K, μ1-N437K) toward several LAs is significantly decreased. We have studied how residue μ1-N434 influences LA binding. By using site-directed mutagenesis, we created mutations at μ1-N434 that vary the hydrophobicity, aromaticity, polarity, and charge and investigated their influence on state- dependent binding and stereoselectivity of bupivacaine. Wild-type and mutant channels were transiently expressed in human embryonic kidney 293t cells and investigated under whole-cell voltage-clamp. For resting channels, bupivacaine enantiomers showed a higher potency in all mutant channels compared with wild-type channels. These changes were not well correlated with the physical properties of the substituted residues. Stereoselectivity was small and almost unchanged. In inactivated channels, the potency of bupivacaine was increased in mutations containing a quadrupole of an aromatic group (μ1-N434F, μ1-N434W, μ1-N434Y), a polar group μ1-N434C), or a negative charge (μ1-N434D) and was decreased in a mutation containing a positive charge (μ1-N434K). In mutation μ1-N434R, containing the positively charged arginine, the potency of S(-)-bupivacaine was selectively decreased, resulting in a stereoselectivity (stereopotency ratio) of 3. Similar results were observed with cocaine but not with RAC 109 enantiomers. We propose that in inactivated channels, residue μ1-N434 interacts directly with the positively charged moiety of LAs and that D1-S6 and D4-S6 form a domain- interface site for binding of BTX and LAs in close proximity.