Some properties of calcium-activated adenosine triphosphatase from the hermatypic coralGalaxea fascicularis
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Samples of the hermatypic coralGalaxea fascicularis were collected between April 1987 and April 1990 from coral reefs off Singapore (103 °45′E; 1 °13′N). Ca2+-activated adenosine triphosphatase (ATPase) activity was detected in the plasma-membrane-enriched heavy microsomal fraction ofG. fascicularis. The high affinity component hadKm andVmax values of 0.0021 mM and 0.050 µmol Pi mg−1 protein min−1, respectively; corresponding values for the low affinity component were 0.15 mM and 0.85 µmol mg−1 protein min−1. The activity of the high affinity component was inhibited 80 and 50%, respectively, by the anticalmodulin drugs calmidazolium and chlorpromazine. The low affinity component of the Ca2+-ATPase may represent activities of alkaline phosphatase, Ca2+-ATPase from membranes of mitochondria and endoplasmic reticulum, or calmodulin-dissociated plasma membrane Ca2+-ATPase resulting from the removal of Ca2+ by EDTA during the isolation process. The high affinity Ca2+-ATPase is probably the enzyme responsible for Ca2+ extrusion from the cells ofG. fascicularis. The high and low affinity components of this Ca2+-ATPase could use ATP and ADP as substrates. Maximum activities of both components were registered at pH 7 and at 45°C. Ruthenium red, a specific inhibitor of Ca2+-ATPase, inhibited the activities of the high and low affinity Ca2+-ATPase by 100 and 60%, respectively. Inhibition of the activities of both components was also observed with sulphydryl reagents (PCMB and mersalyl). However, DCMU, diamox, dinitrophenol, iodoacetate, fluoride, cyanide, ouabain, oligomycin B and L-phenylalanine had no effect on the enzyme activities.
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