Sucrase-isomaltase is an adenosine 3′,5′-cyclic monophosphate-dependent epithelial chloride channel

Abstract

We previously isolated a monoclonal antibody against a Necturus gallbladder epitope that blocks native adenosine 3',5'-cyclic monophosphate (cAMP)-dependent chloride channels in intestine, gallbladder, urinary bladder, and airway epithelia in various animals. Using this antibody, we purified a 200-kilodalton protein that, when reconstituted in lipid bilayers, forms 9-pS chloride channels that are blocked by the antibody. Amino acid sequencing of the purified protein showed strong homology to rabbit sucrase-isomaltase, an abundant intestinal enzyme. Western blot analysis of the in vitro-translated sucrase-isomaltase was indistinguishable from that of the protein used in the lipid bilayer studies. Expression of this protein in Chinese hamster ovary cells and in Xenopus laevis oocytes yielded cAMP-dependent chloride currents that in the latter system were blocked by the antibody. Because the monoclonal antibody blocks cAMP-dependent currents in epithelia as well as those produced both by the reconstituted and by the heterologously expressed protein, sucrase-isomaltase is a cAMP-dependent epithelial chloride channel. Thus an enzyme that can also function as an ion channel has been described for the first time.