Gill V-ATPase activity in freshwater- versus salinity-acclimated crabs
Assays of gill microsomal ATPase activity, performed using up to 20 min pre-incubation with 0.2 to 1.5 μg alamethicin/μg protein, revealed sealed vesicles in the fractions from crabs at all salinity/time combinations. Activity assays were thus conducted using 1.0 μg alamethicin/μg protein without pre-incubation, a condition that resulted in maximum orthovanadate- and bafilomycin-insensitive ATPase activities, irrespective of the salinity/time combination.
The effect of pH on the gill V-ATPase activity of crabs held in fresh water or acclimated to 21‰ salinity for 10 days is shown in Fig. 1A. Activity was negligible at pH 6.5, increasing markedly up to pH 7.5 then abruptly decreasing at higher values to about 50% and 35% of maximum activity at pH 8.0 and 8.5, respectively, under both conditions. Gill V-ATPase activities in crabs at all other salinity/time combinations showed similar pH profiles, with the same optimal pH (7.5) (data not shown).
The stimulation by ATP and Mg2+ (each at saturating concentrations of the other) of V-ATPase activity in D. pagei maintained in fresh water or acclimated for 10 days to 21‰ salinity is given in Fig. 1B and C, respectively. Activity was stimulated by ATP ( Fig. 1B) following simple saturation curves, with maximum velocities of 26.5 ± 2.1 U mg− 1 and 8.4 ± 0.7 U mg− 1 in the freshwater- and 21‰-acclimated crabs, respectively. The apparent affinity for ATP in crabs from fresh water (KM = 4.2 ± 0.3 mmol L− 1) was 4.4-fold less than that for 21‰-acclimated crabs (KM = 0.96 ± 0.08 mmol L− 1). Magnesium ions ( Fig. 1C) stimulated the V-ATPase from freshwater crabs to V = 27.9 ± 2.5 U mg− 1 with KM = 0.92 ± 0.09 mmol L− 1; in the 21