Changes in Ouabain-Sensitive Adenosine Triphosphatase Activity in Gills of Coho Salmon (Oncorhynchus kisutch) During Parr–Smolt Transformation

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Abstract
The effects of incubation temperature, pH, sodium, potassium, and ATP concentration, and ouabain on the activity of Na+–K+-activated ATPase of the gills of seawater-adapted juvenile coho salmon (Oncorhynchus kisutch) were determined. The temperature and pH optima were 40 C and 7.4, respectively. The apparent Km for ATP at equimolar Mg++ concentration was 0.2 mM at Na+ and K+ concentrations of 100 and 20 mM, respectively. Maximal enzyme activity for Na+ concentration of 10.50 and 100 mM occurred at K+ concentrations of 12.5, 15.0, and 20.0 mM, respectively. The Ki for ouabain was 2 × 10−6 M and 7 × 10−6 for K+ concentrations of 10 and 20 mM, respectively.A large portion (up to 60%) of the ouabain-sensitive ATPase activity in freshwater fish was activated by sodium ions in the absence of potassium ions (Na+-activation). Exposure to sea water resulted in a large increase in total ouabain-sensitive activity and a sharp decrease in the proportion of sodium activation. These changes occurred within 14 days after transfer to full strength sea water.On a seasonal basis, total ouabain-sensitive enzyme activity in juvenile freshwater coho was low (less than 5 μmol Pi/mg N per h) to the end of November, increased to a peak (over 125 μmol Pi/mg N per h) in mid-January, and subsequently declined by late February. A slow, steady rise in activity occurred during the smoking period of March and April and the relative contribution of sodium ions to the total activity declined in this period.