A Kinetic Characterization of the Gill (Na, K)-ATPase from the Semi-terrestrial Mangrove Crab Cardisoma guanhumi Latreille, 1825 (Decapoda, Brachyura).

We provide a kinetic characterization of (Na, K)-ATPase activity in a posterior gill microsomal fraction from the semi-terrestrial mangrove crab Cardisoma guanhumi. Sucrose density gradient centrifugation reveals two distinct membrane fractions showing considerable (Na, K)-ATPase activity, but also containing other microsomal ATPases. The (Na, K)-ATPase, notably immuno-localized to the apical region of the epithelial pillar cells, and throughout the pillar cell bodies, has an M of around 110 kDa and hydrolyzes ATP with V  = 146.8 ± 6.3 nmol Pi min mg protein and K  = 0.05 ± 0.003 mmol L obeying Michaelis-Menten kinetics. While stimulation by Na (V  = 139.4 ± 6.9 nmol Pi min mg protein, K  = 4.50 ± 0.22 mmol L) also follows Michaelis-Menten kinetics, modulation of (Na, K)-ATPase activity by MgATP (V  = 136.8 ± 6.5 nmol Pi min mg protein, K  = 0.27 ± 0.04 mmol L), K (V  = 140.2 ± 7.0 nmol Pi min mg protein, K  = 0.17 ± 0.008 mmol L), and NH (V  = 149.1 ± 7.4 nmol Pi min mg protein, K  = 0.60 ± 0.03 mmol L) shows cooperative kinetics. Ouabain (K  = 52.0 ± 2.6 µmol L) and orthovanadate (K  = 1.0 ± 0.05 µmol L) inhibit total ATPase activity by around 75%. At low Mg concentrations, ATP is an allosteric modulator of the enzyme. This is the first study to provide a kinetic characterization of the gill (Na, K)-ATPase in C. guanhumi, and will be useful in better comprehending the biochemical underpinnings of osmoregulatory ability in a semi-terrestrial mangrove crab.