Salinity adaptation of HCO3−-dependent ATPase activity in the gills of blue crab (Callinectes sapidus)

A bicarbonate-dependent ATPase (EC 3.6.1.3) was found in microsomal preparations from blue crab gills. When the crabs were transferred to low salinity (200 mosmolal) from seawater (1000 mosmolal), the HCO₃^?-dependent ATPase increased in all gill pairs, reaching its new steady state in 2 weeks. The greatest increase occurred in the sixth and seventh gill pairs (approx. 2.5-fold). Maximal enzyme activity was observed at an Mg²⁺ concentration of 3 mM and an optimal pH of 7.8. The apparent K_a for HCO₃^? was found to be 8.9 mM. Kinetic analysis showed that low-salinity adaptation increased the V_max without altering the K_m for ATP. When the microsomes from high-salinity crab gills were treated with detergent or assayed at different temperatures, the total enzyme activity did not reach the activity levels seen after adaptation to low salinity. These results suggest that the alteration of HCO₃^?-ATPase activity may be due to synthesis, rather than modulation of membranes or of the existing enzyme activity.