Sucrase-isomaltase is an adenosine 3',5'-cyclic monophosphate-dependent epithelial chloride channel.
OBJECTIVE
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.


METHODS
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.


RESULTS
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.


CONCLUSIONS
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.
Finn
A L
AL
Department of Medicine, University of North Carolina School of Medicine, Chapel Hill, North Carolina, USA. afinn@med.unc.edu
Kuzhikandathil
E V
EV
Oxford
G S
GS
Itoh-Lindstrom
Y
Y
D000595:Amino Acid Sequence; D000818:Animals; D000911:Antibodies, Monoclonal; D001426:Bacterial Proteins; D016466:CHO Cells; D018118:Chloride Channels / Q000235:genetics* / Q000276:immunology / Q000378:metabolism*; D003106:Colon / Q000166:cytology / Q000201:enzymology; D006224:Cricetinae; D000242:Cyclic AMP / Q000378:metabolism*; D015971:Gene Expression Regulation, Enzymologic; D007413:Intestinal Mucosa / Q000166:cytology / Q000201:enzymology*; D008969:Molecular Sequence Data; D009337:Necturus; D009865:Oocytes / Q000502:physiology; D013394:Sucrase-Isomaltase Complex / Q000235:genetics* / Q000276:immunology / Q000378:metabolism*; D014162:Transfection; D014982:Xenopus laevis; D000520:alpha-Glucosidases / Q000235:genetics* / Q000276:immunology / Q000378:metabolism*
D000911:Antibodies, Monoclonal; D001426:Bacterial Proteins; D018118:Chloride Channels; D000242:Cyclic AMP; D013394:Sucrase-Isomaltase Complex; C112179:sucrase-isomaltase-maltase; D000520:alpha-Glucosidases