Ca2+-stimulated ATPase activities in the gill of the eel: interactions of Mg2+ ions.

Enriched plasma membrane preparations of the branchial epithelium of freshwater-adapted eels were used to study adenosine triphosphatase (ATPase) activities insensitive to ouabain and responding to Ca2+ and Mg2+. Ca2+ induced ATP hydrolysis; two kinetics were observed in the presence or absence of chelators, one with a high-affinity site (0.3 microM) and one with a lower affinity site (10-20 microM). The high-affinity Ca2+ site or enzyme had a prerequisite for Mg2+ (endogenous Mg2+ being sufficient to satisfy the Mg2+ need) but was inhibited by exogenous Mg2+ (Ki0.5 less than 10 microM Mg2+). The low-affinity site or enzyme appears to have kinetic parameters comparable to those found for Mg2+-induced ATP hydrolysis. In the absence of Ca2+ ligands and with no exogenous Mg2+, the two Ca2+ sites or enzymes can be considered stimulated. The results are discussed in relation to the branchial ion environment and transport ion capacities.