Functional groups of the Na+ channel: role of carboxyl and histidyl groups.

Two titratable groups, with effect on Na+ transport and with apparent acid dissociation constants (pKaS) of 4.2 and 6.7, were found in the apical membrane of toad urinary bladder and are tentatively identified as a carboxyl and an imidazole. N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), a reagent selective for carboxyl residues, inhibits Na+ transport in the urinary bladder of toads. The underlying chemical reaction whereby EEDQ produces inhibition through potential modification of carboxyl residues was studied. The inhibitory action of EEDQ on Na+ transport was dependent on pH of reaction media and availability of nucleophile, indicating that formation of a covalent acyl-nucleophile bond is probably involved in the irreversible inhibition of Na+ transport. The kinetics of the inhibition showed a stoichiometry of formation of one acyl-nucleophile bond per closure of one Na+ transport site, presumably the Na+ channel. The nucleophile that appears to be involved in the formation of the acyl-nucleophile bond was tentatively identified as having an apparent pKa of 6.7. Amiloride and two analogues of amiloride added to the mucosal Ringer solution (but not serosal amiloride) protected against inhibition of Na+ transport by EEDQ--a finding consistent with the hypothesis that the EEDQ-activated carboxyl group undergoes reaction with a nucleophile at or near the site of specific binding of amiloride onto the apical membrane, most likely at the Na+ channel. Our findings led us to postulate that amiloride must interact with at least two sites on the Na+ channel in order to block the channel. One of the two sites appears to be an ionic interaction between the anionic carboxyl group at the Na+ channel and the cationic guanidinium group of amiloride.