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First published online May 5, 2005
Journal of Experimental Biology 208, 1951-1961 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01551

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Cytoplasmic carbonic anhydrase isozymes in rainbow trout Oncorhynchus mykiss: comparative physiology and molecular evolution

A. J. Esbaugh^1,*, S. F. Perry², M. Bayaa², T. Georgalis², J. Nickerson², B. L. Tufts¹ and K. M. Gilmour^3,

¹ Department of Biology, Queen's University, Kingston, ON, Canada K7L 3N6
² Department of Biology, University of Ottawa, Ottawa, ON, Canada K1N 6N5
³ Department of Biology, Carleton University, Ottawa, ON, Canada K1S 5B6

^* Author for correspondence (e-mail: esbaugha{at}biology.queensu.ca)

Accepted 21 February 2005

It is well established that the gills of teleost fish contain substantial levels of cytoplasmic carbonic anhydrase (CA), but it is unclear which CA isozyme(s) might be responsible for this activity. The objective of the current study was to determine if branchial CA activity in rainbow trout was the result of a general cytoplasmic CA isozyme, with kinetic properties, tissue distribution and physiological functions distinct from those of the red blood cell (rbc)-specific CA isozyme. Isolation and sequencing of a second trout cytoplasmic CA yielded a 780 bp coding region that was 76% identical with the trout rbc CA (TCAb), although the active sites differed by only 1 amino acid. Interestingly, phylogenetic analyses did not group these two isozymes closely together, suggesting that more fish species may have multiple cytoplasmic CA isozymes. In contrast to TCAb, the second cytoplasmic CA isozyme had a wide tissue distribution with high expression in the gills and brain, and lower expression in many tissues, including the red blood cells. Thus, unlike TCAb, the second isozyme lacks tissue specificity and may be expressed in the cytoplasm of all cells. For this reason, it is referred to hereafter as TCAc (trout cytoplasmic CA). The inhibitor properties of both cytoplasmic isozymes were similar (K_i acetazolamide 1.21±0.18 nmol l^-1 and 1.34±0.10 nmol l^-1 for TCAc and TCAb, respectively). However, the turnover of TCAb was over three times greater than that of TCAc (30.3±5.83 vs 8.90±1.95 e⁴ s^-1, respectively), indicating that the rbc-specific CA isoform was significantly faster than the general cytoplasmic isoform. Induction of anaemia revealed differential expression of the two isozymes in the red blood cell; whereas TCAc mRNA expression was unaffected, TCAb mRNA expression was significantly increased by 30- to 60-fold in anaemic trout.

Key words: carbonic anhydrase, red blood cell, gill, isozyme, evolution, anaemia

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