Gill microsomal (Na +,K +)-ATPase from the blue crab Callinectes danae: Interactions at cationic sites

Euryhaline crustaceans tolerate exposure to a wide range of dilute media, using compensatory, ion regulatory mechanisms. However, data on molecular interactions occurring at cationic sites on the crustacean gill (Na +,K +)-ATPase, a key enzyme in this hyperosmoregulatory process, are unavailable. We report that Na + binding at the activating site leads to cooperative, heterotropic interactions that are insensitive to K +. The binding of K + ions to their high affinity sites displaces Na + ions from their sites. The increase in Na + ion concentrations increases heterotropic interactions with the K + ions, with no changes in K 0.5 for K + ion activation at the extracellular sites. Differently from mammalian (Na +,K +)-ATPases, that from C. danae exhibits additional NH 4 + ion binding sites that synergistically activate the enzyme at saturating concentrations of Na + and K + ions. NH 4 + binding is cooperative, and heterotropic NH 4 + ion interactions are insensitive to Na + ions, but Na + ions displace NH 4 + ions from their sites. NH 4 + ions also displace Na + ions from their sites. Mg 2+ ions modulate enzyme stimulation by NH 4 + ions, displacing NH 4 + ion from its sites. These interactions may modulate NH 4 + ion excretion and Na + ion uptake by the gill epithelium in euryhaline crustaceans that confront hyposmotic media.