Modulation of gill Na+,K+-ATPase activity by ammonium ions: Putative coupling of nitrogen excretion and ion uptake in the freshwater shrimp Macrobrachium olfersii.
The effect of NH4+ ions on (Na+,K+)-ATPase hydrolytic activity was examined in a gill microsomal fraction from M. olfersii. In the absence of NH4+ ions, K+ ions stimulated ATP hydrolysis, exhibiting cooperative kinetics (nH=0.8), to a maximal specific activity of V=556.1+/-22.2 nmol.min(-1).mg(-1) with K(0.5)=2.4+/-0.1 mmol.L(-1). No further stimulation by K+ ions was observed in the presence of 50 mmol.L(-1) NH4+ ions. ATP hydrolysis was also stimulated by NH4+ ions obeying Michaelian kinetics to a maximal specific activity of V=744.8+/-22.3 nmol.min(-1).mg(-1) and KM=8.4+/-0.2 mmol.L(-1). In the presence of 10 mmol.L(-1) K+ ions, ATP hydrolysis was synergistically stimulated by NH4+ ions to V=689.8+/-13.8 nmol.min(-1).mg(-1) and K(0.5)=6.6+/-0.1 mmol.L(-1), suggesting that NH4+ ions bind to different sites than K+ ions. PNPP hydrolysis was also stimulated cooperatively by K+ or NH4+ ions to maximal values of V= 235.5+/-11.8 nmol.min(-1).mg(-1) and V=234.8+/-7.0 nmol.min(-1).mg(-1), respectively. In contrast to ATP hydrolysis, K(+)-phosphatase activity was not synergistically stimulated by NH4+ and K+ ions. These data suggest that at high NH4+ ion concentrations, the (Na+, K+)-ATPase exposes a new site; the subsequent binding of NH4+ ions stimulates ATP hydrolysis to rates higher than those for K+ ions alone. This is the first demonstration that (Na+, K+)-ATPase activity in a freshwater shrimp gill is modulated by ammonium ions, independently of K+ ions, an effect that may constitute a fine-tuning mechanism of physiological relevance to osmoregulatory and excretory processes in palaemonid shrimps.
Furriel
Rosa Prazeres Melo
RP
Departamento de Química, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo, Ribeirão Preto 14040-901, SP, Brazil.
Masui
Douglas Chodi
DC
McNamara
John Campbell
JC
Leone
Francisco Assis
FA
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001938:Brazil; D005618:Fresh Water; D005880:Gills / Q000378:metabolism*; D006868:Hydrolysis / Q000187:drug effects; D007700:Kinetics; D010042:Ouabain / Q000494:pharmacology; D033541:Palaemonidae / Q000378:metabolism*; D010744:Phosphoric Monoester Hydrolases / Q000378:metabolism; D000644:Quaternary Ammonium Compounds / Q000494:pharmacology*; D000254:Sodium-Potassium-Exchanging ATPase / Q000187:drug effects* / Q000378:metabolism
D000644:Quaternary Ammonium Compounds; D010042:Ouabain; D000255:Adenosine Triphosphate; D010744:Phosphoric Monoester Hydrolases; D000254:Sodium-Potassium-Exchanging ATPase