Characterization of Na/K-ATPase in Macrobrachium rosenbergii and the effects of changing salinity on enzymatic activity

A ouabain-sensitive Na/K-ATPase kinetic assay system based on the hydrolysi s of ATP and the oxidation of NADH was adapted in order to characterize enz ymatic activity in gills and examine the effects of changing salinity in Ma crobrachium rosenbergii. Maximum inhibition by ouabain occurred at a concen tration of 1.4 mM, and the K-m of the reaction was 0.2 mM. In a first exper iment, animals were acclimated to freshwater, 1/3 seawater, 2/3 seawater an d full seawater for up to 1 week. Na/K-ATPase activity in front gills was 1 .62 +/- 0.19 mu mol ADP/mg protein per h in freshwater, and was seen to inc rease slightly in 1/3 seawater (1.88 +/- 0.19 mu mol ADP/mg protein per h) and 2/3 seawater (2.09 +/- 0.24 mu mol ADP/mg protein per h), decreasing sl ightly in full seawater (1.92 +/- 0.43 mu mol ADP/mg protein per h); howeve r, differences were not significant. Back gills showed slightly higher leve ls, and a similar pattern of Na/K-ATPase activity. In a second experiment, animals were acclimated to 1/3 seawater and 2/3 seawater, and then transfer red to freshwater. However, no changes in activity were seen, indicating th at exposure to dilute media did not effect enzymatic activity. Whereas Na/K -ATPase is important in osmoregulatory function in marine euryhaline crusta ceans, it may not play a significant role in adaptation in freshwater crust aceans that inhabit a more narrow range of salinities. (C) 2000 Elsevier Sc ience Inc. All rights reserved.