Enzymatic Properties of Two Calcium-stimulated ATPases in the Neutral and Acidic pH Regions Found in the Membrane Fraction of Human Milk

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Two calcium-stimulated ATPases at optimal pH values of 5.0 and 7.0,which are designated as acid and neutral Ca^<2+>-ATPase, respectively, were found in the membrane fraction of human milk, and their enzymatic properties were studied. For maximal activity, neutral Ca^<2+>-ATPase required 0.45mM Ca ion, while acid Ca^<2+>-ATPase required 207mM Ca ion. Neutral Ca^<2+>-ATPase activity was not enhanced by adding the Mg ion at more than 0.1 mM. Among the nucleotides, neutral Ca^<2+>-ATPase showed a higher substrate specificity to GTP, CTP, ITP, and UTP than to ATP, while ATP was the best substrate for acid Ca^<2+>-ATPases. Neutral and acid Ca^<2+>-ATPases had apparent K_m values of 0.361 and 0.192 mM, and V_<max> of 186 and 178 μmol ATP hydroyzed/mg of protein per min, respectively. Both Ca^<2+>-ATPases were potently inhibited by fluoride, lanthanide, vanadate, and p-chloromercuribenzoate, and inactivated by EDTA, EGTA, and CDTA, but were unaffected by N-ethylmaleimide, NaN_3,ouabain, oxidized glutathione, or oligomycin, and were inactivated by heating at 60℃ for 10 min. These enzmes were concentrated in the membrane fraction of the cream and skim milk membrane.
社団法人日本農芸化学会の論文
1992-06-23