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First published online January 25, 2005
Journal of Experimental Biology 208, 487-496 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01398

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A distinct carbonic anhydrase in the mucus of the colon of humans and other mammals

Tanja Kleinke¹, Siegfried Wagner², Harald John³, David Hewett-Emmett⁴, Seppo Parkkila⁵, Wolf-Georg Forssmann³ and Gerolf Gros^*

¹ Zentrum Physiologie, Medizinische Hochschule Hannover, D-30625 Hannover, Germany
² Department of Gastroenterology, Hepatology and Endocrinology, Medizinische Hochschule Hannover, D-30625 Hannover, Germany
³ IPF PharmaCeuticals GmbH, Feodor-Lynen-Strasse 31, D-30 625 Hannover, Germany
⁴ Human Genetics Center, University of Texas School of Public Health at Houston, Texas, USA
⁵ Institute of Medical Technology, University of Tampere and Tampere University Hospital, FIN-33014 Tampere, Finland

^* Author for correspondence (e-mail: gros.gerolf{at}mh-hannover.de)

Accepted 15 November 2004

We have collected gastrointestinal, mainly colonic, mucus from humans, guinea pigs, rats, and normal and carbonic anhydrase II (CAII)-deficient mice. In the mucus of all species, substantial CA activity was present. Using antibodies against human CA isoforms we found that the human mucus CA differs from cytosolic CAI and CAII, membrane-bound CAIV, and the secreted CAVI of saliva. The high sensitivity of mucus CA to acetazolamide rules out its identity with cytosolic CAIII. Partial sequences obtained from the purified human mucus CA show similarity, but not identity, with human CAI, and clear differences from the other known CAs. Additional evidence concerning the CA isoform present in mucus was obtained for the mucus CA of other species and was derived from: (1) the mucus of CAII-deficient mice, whose high CA activity confirms that it is not CAII that is responsible; (2) the inhibitory effect of iodide, which shows that mucus CA from mice, guinea pig and humans does not have the high anion sensitivity of CAI; (3) the inactivating effect of 0.2% SDS on guinea pig, mouse and human mucus CA, ruling out the SDS-resistant CAIV; and (4) the partitioning of guinea-pig mucus CA into the water phase in Triton X114 phase separation experiments, which also argues against its identity with membrane-bound CAs, such as CAIV. A comparison of colonic mucus CA activity in normal and germ-free rats indicates that the mucus CA is not of bacterial origin but is produced by the gastrointestinal tissues. We conclude (from its immunoreactivity, from iodide inhibition and from partial amino acid sequences) that mucus CA of human origin probably represents an isozyme, which is specific for mucus and is not identical with the known CA isozymes. The results obtained for mucus CA of other species collectively point in the same direction.

Key words: human, guinea pig, rat, mouse, gastrointestinal mucus, carbonic anhydrase, colon, stomach

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