Synergistic stimulation by potassium and ammonium of K+-phosphatase activity in gill microsomes from the crab Callinectes ornatus acclimated to low salinity: Novel property of a primordial pump

We provide an extensive characterization of the modulation by p-nitrophenylphosphate, Mg2+, Na+, K+, Rb+, NH 4 + and pH of gill microsomal K+-phosphatase activity in the posterior gills of Callinectes ornatus acclimated to low salinity (21‰). The synergistic stimulation by K+ and NH 4 + of the K+-phosphatase activity is a novel finding, and may constitute a species-specific feature of K+/ NH 4 + interplay that regulates crustacean gill (Na+, K+)-ATPase activity. p-Nitrophenylphosphate was hydrolyzed at a maximum rate (V) of 69.2 ± 2.8 nmol Pi min−1 mg−1 with K0.5 = 2.3 ± 0.1 mmol L−1, obeying cooperative kinetics (nH = 1.7). Stimulation by Mg2+ (V = 70.1 ± 3.0 nmol Pi min−1 mg−1, K0.5 = 0.88 ± 0.04 mmol L−1), K+ (V = 69.6 ± 2.7 nmol Pi min−1 mg−1, K0.5 = 1.60 ± 0.07 mmol L−1) and NH 4 + (V = 90.8 ± 4.0 nmol Pi min−1 mg−1, K0.5 = 9.2 ± 0.3 mmol L−1) all displayed site-site interaction kinetics. In the presence of NH 4 + , enzyme affinity for K+ unexpectedly increased by 7-fold, while affinity for NH 4 + was 28-fold greater in the presence than absence of K+. Ouabain partially inhibited K+-phosphatase activity (KI = 320 ± 14.0 μmol L−1), more effectively when NH 4 + was present (KI = 240 ± 12.0 μmol L−1). We propose a model for the synergistic stimulation by K+ and NH 4 + of the K+-phosphatase activity of the (Na+, K+)-ATPase from C. ornatus posterior gill tissue.