Ca2+‐ or Mg2+‐Dependent Enzymic ATP Hydrolysis Associated with the Microsomal Fraction of Frog Sciatic Nerves

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Abstract
The microsomal fraction of frog sciatic nerves contains Ca2+- or Mg2+-dependent hydrolytic activity toward different nucleoside di- and triphosphates. In the presence of Ca2+ substrate, specificity was in the order CTP > UTP > GTP > ATP. When Mg2+ was used, the triphosphates were approximately equally good substrates. ATP hydrolytic activity was very similar with Ca2+ or Mg2+ as the cofactor; Ca2+ was the more potent activator of hydrolysis of the other triphosphates tested. The preparation showed some activity toward the nucleoside diphosphates but none toward the monophosphates or p-nitrophenylphosphate. ATP hydrolysis was optimal at 18-24.degree. C in the presence of 1 mM-Ca2+ or 1 mM-Mg2+. Ca2+- and Mg2+-ATP hydrolysis displayed pH maxima around 8.0-8.5 and 7.4-8.0, respectively. Vmax values for Ca2+- and Mg2+-ATP hydrolysis were similar: approximately 12 .mu.mol Pi/h per mg protein with a Km value of approximately 0.05 mM. ATP hydrolysis activity was inhibited by NaF but unaffected by ouabain, vanadate, cytochalasin B and various drugs [sodium azide, oligomycin, dinitrophenol] known to influence ATPase activity of mitochondria. Zn2+ stimulated the ATP hydrolysis activity at low concentrations (10-6 to 10-5 M) and inhibited it at higher concentrations. These observations may account for stimulation and inhibition of axonal transport in frog sciatic nerves exposed to similar concentrations of Zn2+.