Specificity for block by saxitoxin and divalent cations at a residue which determines sensitivity of sodium channel subtypes to guanidinium toxins.

摘要: bTyrosine 401 of the skeletal muscle isoform (mu 1) rat Na channel is an important determinant high affinity block by tetrodotoxin (TTX) and saxitoxin (STX) in Na-channel isoforms. In mammalian heart channels, this residue substituted cysteine, which results low for TTX/STX enhanced sensitivity to Zn2+ Cd2+. study, we investigated molecular basis channels STX divalent cations measuring inhibition macroscopic Na+ current a series point mutations at Tyr401 mu 1 expressed Xenopus oocytes. Substitution Gly, Ala, Ser, Cys, Asp, His, Trp, Phe produced functional currents without major perturbation gating or ionic selectivity. High neosaxitoxin (NEO) with Ki values range 2.6-18 nM required Tyr, Phe, suggestive interaction between aromatic ring guanidinium group toxin. The Cys mutation resulted 7- 23-fold enhancement dissociation rate NEO, respectively, corresponding rapid toxin rates cardiac channels. (Ki = 8-23 microM) three residues commonly found coordinate directly metalloproteins. For mutant also (rh1) L6 cell line, conductance reached plateau 85-90% inhibition, suggesting presence substate current. Asp displayed Ca2+ 0.3 mM vs approximately 40 wild type), but was incomplete, saturating 69% inhibition. contrast, Cd2+ completely blocked line. These imply that magnitude depends on particular position species cation. His exhibited H+ pKa 7.5 imidazole group. Our findings provide further evidence located within outer vestibule external single-filing region permeant ions.