Differential effects of cobalt ionsin vitroon gill (Na⁺, K⁺)-ATPase kinetics in the blue crabCallinectes danae(Decapoda, Brachyura)

Abstract

To evaluate the crustacean gill (Na+, K+)-ATPase as a molecular marker for toxic contamination by heavy metals of estuarine and coastal environments, we provide a comprehensive analysis of the effects of Co2+ in vitro on modulation of the K+-phosphatase activity of a gill (Na+, K+)-ATPase from the blue crab Callinectes danae. Using p-nitrophenyl phosphate as a substrate, Co2+ can act as both stimulator and inhibitor of K+-phosphatase activity. Without Mg2+, Co2+ stimulates K+-phosphatase activity similarly but with a 4.5-fold greater affinity than with Mg2+. With Mg2+, K+-phosphatase activity is almost completely inhibited by Co2+. Substitution of Mg2+ by Co2+ slightly increases enzyme affinity for K+ and NH4+. Independently of Mg2+, ouabain inhibition is unaffected by Co2+. Mg2+ displaces bound Co2+ from the Mg2+-binding site in a concentration dependent mechanism. However, at saturating Mg2+ concentrations, Co2+ does not displace Mg2+ from its binding site even at elevated concentrations. Saturation by Co2+ of the Mg2+ binding site does not affect pNPP recognition by the enzyme. Given that the interactions between heavy metal ions and enzymes are particularly complex, their toxic effects at the molecular level are poorly understood. Our findings elucidate partly the mechanism of action of Co2+ on a crustacean gill (Na+, K+)-ATPase.