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 examine stimulation of crab gill K+-phosphatase by Na, K, Rb, NH4, Mg and pH. ► Synergistic stimulation of enzyme activity by K+ and NH4+ is a novel property. ► Enzyme affinity for K+ is increased 7-fold by NH4+. ► Enzyme affinity for NH4+ is increased 28-fold by K+. ► Rb+ substitutes for K+ in the catalytic cycle of K+-phosphatase activity.