RESUMEN
We provide an extensive characterization of the modulation by p-nitrophenylphosphate, Mg²âº, Naâº, K(+), Rbâº, NH(4)(+) and pH of gill microsomal Kâº-phosphatase activity in the posterior gills of Callinectes ornatus acclimated to low salinity (21). The synergistic stimulation by K⺠and NH(4)(+) of the Kâº-phosphatase activity is a novel finding, and may constitute a species-specific feature of K(+)/NH(4)(+) interplay that regulates crustacean gill (Naâº, Kâº)-ATPase activity. p-Nitrophenylphosphate was hydrolyzed at a maximum rate (V) of 69.2 ± 2.8nmolPimin⻹mg⻹ with K(0.5)=2.3 ± 0.1mmolL(-1), obeying cooperative kinetics (n(H)=1.7). Stimulation by Mg²âº (V=70.1 ± 3.0nmolPimin⻹mg⻹, K(0.5)=0.88 ± 0.04mmolL⻹), K⺠(V=69.6 ± 2.7nmolPimin⻹mg⻹, K(0.5)=1.60 ± 0.07mmolL⻹) and NH(4)(+) (V=90.8 ± 4.0nmolPimin⻹mg⻹, K(0.5)=9.2 ± 0.3mmol L⻹) all displayed site-site interaction kinetics. In the presence of NH(4)(+), enzyme affinity for K⺠unexpectedly increased by 7-fold, while affinity for NH(4)(+) was 28-fold greater in the presence than absence of Kâº. Ouabain partially inhibited Kâº-phosphatase activity (K(I)=320 ± 14.0µmolL⻹), more effectively when NH(4)(+) was present (K(I)=240 ± 12.0µmolL⻹). We propose a model for the synergistic stimulation by K⺠and NH(4)(+) of the Kâº-phosphatase activity of the (Naâº, Kâº)-ATPase from C. ornatus posterior gill tissue.