Masui D C, Furriel R P M, Silva E C C, Mantelatto F L M, McNamara J C, Barrabin H, Scofano H M, Fontes C F L, Leone F A
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.
Int J Biochem Cell Biol. 2005 Dec;37(12):2521-35. doi: 10.1016/j.biocel.2005.06.004. Epub 2005 Jul 14.
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.
广盐性甲壳类动物通过补偿性离子调节机制来耐受暴露于多种稀释介质中。然而，关于甲壳类动物鳃上阳离子位点（Na +，K +）-ATP酶（这种高渗调节过程中的关键酶）发生的分子相互作用的数据却尚未可得。我们报告称，在激活位点结合的Na +会导致协同的、异源性相互作用，且这种相互作用对K +不敏感。K +离子与其高亲和力位点的结合会将Na +离子从其位点上置换下来。Na +离子浓度的增加会增强与K +离子的异源性相互作用，而细胞外位点上K +离子激活的K0.5没有变化。与哺乳动物的（Na +，K +）-ATP酶不同，达纳隐虾（C. danae）的该酶具有额外的NH4 +离子结合位点，在Na +和K +离子饱和浓度下能协同激活该酶。NH4 +的结合具有协同性，且异源性NH4 +离子相互作用对Na +离子不敏感，但Na +离子会将NH4 +离子从其位点上置换下来。NH4 +离子也会将Na +离子从其位点上置换下来。Mg2 +离子调节NH4 +离子对酶的刺激作用，将NH4 +离子从其位点上置换下来。这些相互作用可能会调节广盐性甲壳类动物鳃上皮细胞在面对低渗介质时对NH4 +离子的排泄和对Na +离子的摄取。
