The effect of bafilomycin A1 on the orthovanadate-insensitive gill microsomal ATPase activity is shown in Fig. 1D. Concentrations of around 1 μmol bafilomycin L− 1 completely inhibited V-ATPase activity in crabs held in fresh water or acclimated to 21‰ salinity for 10 days, with residual activities of 36.1 ± 0.4 U mg− 1 and 6.3 ± 0.5 U mg− 1, respectively, revealing lower activities of ATPases other than V- and P-ATPases in the salinity-acclimated crabs. The apparent dissociation constants (KI) for bafilomycin inhibition in freshwater- (57.1 ± 5.8 nmol L− 1) or salinity-acclimated (47.9 ± 4.6) crabs were not significantly different (inset to Fig. 1D).
The kinetic characterization of V-ATPase activity in crabs acclimated for 10 days to salinities between fresh water (< 0.5‰) and 21‰ showed a marked decrease in maximum velocity up to 15‰, sustained at 21‰ (Fig. 2A). In contrast, the apparent affinity of the enzyme for ATP (KM = 4.2 ± 0.3 mmol L− 1 in fresh water) increased about 3-fold up to 10‰, reaching KM = 1.3 ± 0.1 mmol L− 1, showing little variation at higher salinities ( Fig. 3A). Similarly, the apparent affinity for Mg2+ increased up to 10‰ salinity, with KM = 0.46 ± 0.04 mmol L− 1, about 2-fold less than in fresh water (KM = 0.92 ± 0.09 mmol L− 1); further salinity increase had little effect on the KM for Mg2+ ( Fig. 3B). Differently, the KI for bafilomycin A1 was fairly constant over the salinity range used ( Fig. 3C).