title: Ca2+-stimulated, Mg2+-dependent ATPase activity in neutrophil plasma membrane vesicles. Coupling to Ca2+ transport.
abstract: Low concentrations of free Ca2+ stimulated the hydrolysis of ATP by plasma membrane vesicles purified from guinea pig neutrophils and incubated in 100 mM HEPES/triethanolamine, pH 7.25. In the absence of exogenous magnesium, apparent values obtained were 320 nM (EC50 for free Ca2+), 17.7 nmol of Pi/mg X min (Vmax), and 26 microM (Km for total ATP). Studies using trans- 1,2-diaminocyclohexane- N,N,N',N',-tetraacetic acid as a chelator showed this activity was dependent on 13 microM magnesium, endogenous to the medium plus membranes. Without added Mg2+, Ca2+ stimulated the hydrolysis of several other nucleotides: ATP congruent to GTP congruent to CTP congruent to ITP greater than UTP, but Ca2+-stimulated ATPase was not coupled to uptake of Ca2+, even in the presence of 5 mM oxalate. When 1 mM MgCl2 was added, the vesicles demonstrated oxalate and ATP-dependent calcium uptake at approximately 8 nmol of Ca2+/mg X min (based on total membrane protein). Ca2+ uptake increased to a maximum of approximately 17-20 nmol of Ca2+/mg X min when KCl replaced HEPES/triethanolamine in the buffer. In the presence of both KCl and MgCl2, Ca2+ stimulated the hydrolysis of ATP selectively over other nucleotides. Apparent values obtained for the Ca2+-stimulated ATPase were 440 nM (EC50 for free Ca2+), 17.5 nmol Pi/mg X min (Vmax) and 100 microM (Km for total ATP). Similar values were found for Ca2+ uptake which was coupled efficiently to Ca2+-stimulated ATPase with a molar ratio of 2.1 +/- 0.1. Exogenous calmodulin had no effect on the Vmax or EC50 for free Ca2+ of the Ca2+-stimulated ATPase, either in the presence or absence of added Mg2+, with or without an ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N',-tetraacetic acid pretreatment of the vesicles. The data demonstrate that calcium stimulates ATP hydrolysis by neutrophil plasma membranes that is coupled optimally to transport of Ca2+ in the presence of concentrations of K+ and Mg2+ that appear to mimic intracellular levels.
mesh_terms: D000818:Animals; D001693:Biological Transport, Active; D017301:Ca(2+) Mg(2+)-ATPase; D002118:Calcium / Q000097:blood*; D000252:Calcium-Transporting ATPases / Q000097:blood*; D002462:Cell Membrane / Q000201:enzymology; D006168:Guinea Pigs; D006863:Hydrogen-Ion Concentration; D009504:Neutrophils / Q000201:enzymology*; D009711:Nucleotides / Q000378:metabolism; D013449:Sulfonamides / Q000494:pharmacology; D014268:Trifluoperazine / Q000494:pharmacology
chemical_list: D009711:Nucleotides; D013449:Sulfonamides; D014268:Trifluoperazine; C017967:W 7; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D002118:Calcium