title: Gill microsomal (Na+,K+)-ATPase from the blue crab Callinectes danae: Interactions at cationic sites.
abstract: 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 K0.5 for K+ ion activation at the extracellular sites. Differently from mammalian (Na+,K+)-ATPases, that from C. danae exhibits additional NH4+ ion binding sites that synergistically activate the enzyme at saturating concentrations of Na+ and K+ ions. NH4+ binding is cooperative, and heterotropic NH4+ ion interactions are insensitive to Na+ ions, but Na+ ions displace NH4+ ions from their sites. NH4+ ions also displace Na+ ions from their sites. Mg2+ ions modulate enzyme stimulation by NH4+ ions, displacing NH4+ ion from its sites. These interactions may modulate NH4+ ion excretion and Na+ ion uptake by the gill epithelium in euryhaline crustaceans that confront hyposmotic media.
authors.affiliation: Departamento de Química, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo, Avenida Bandeirantes 3900, Ribeirão Preto 14040-901, SP, Brazil.
authors.forename: D C
authors.initials: DC
authors.lastname: Masui
authors.forename: R P M
authors.initials: RP
authors.lastname: Furriel
authors.forename: E C C
authors.initials: EC
authors.lastname: Silva
authors.forename: F L M
authors.initials: FL
authors.lastname: Mantelatto
authors.forename: J C
authors.initials: JC
authors.lastname: McNamara
authors.forename: H
authors.initials: H
authors.lastname: Barrabin
authors.forename: H M
authors.initials: HM
authors.lastname: Scofano
authors.forename: C F L
authors.initials: CF
authors.lastname: Fontes
authors.forename: F A
authors.initials: FA
authors.lastname: Leone
mesh_terms: D000818:Animals; D001665:Binding Sites; D003386:Brachyura / Q000201:enzymology*; D002412:Cations / Q000378:metabolism; D004789:Enzyme Activation; D005880:Gills / Q000201:enzymology*; D017136:Ion Transport / Q000187:drug effects; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D008861:Microsomes / Q000201:enzymology; D008954:Models, Biological; D011188:Potassium / Q000378:metabolism; D000644:Quaternary Ammonium Compounds / Q000378:metabolism; D012964:Sodium / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
chemical_list: D002412:Cations; D000644:Quaternary Ammonium Compounds; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D008274:Magnesium; D011188:Potassium