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 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.
Masui
D C
DC
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.
Furriel
R P M
RP
Silva
E C C
EC
Mantelatto
F L M
FL
McNamara
J C
JC
Barrabin
H
H
Scofano
H M
HM
Fontes
C F L
CF
Leone
F A
FA
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*
D002412:Cations; D000644:Quaternary Ammonium Compounds; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D008274:Magnesium; D011188:Potassium