Unique Ca2+-activated ATPase in the nervous ganglia of Phyllocaulis soleiformis (Mollusca)

Nucleotide-metabolizing enzymes play important roles in the regulation of intracellular and extracellular nucleotide levels. We studied ATPase activity in the nervous ganglia of Phyllocaulis soleiformis, a terrestrial slug. The ATPase was divalent cation-dependent, with a maximal rate for ATP hydrolysis at pH 6.0 and 7.2 in the presence of Ca²⁺ (5 mM). Mg²⁺-ATPase activity was only 26% of the activity observed in the presence of Ca²⁺ (5 mM). ZnCl₂ (10 mM) produced a significant inhibition of 70%. Ca²⁺-ATPase activity was insensitive to the classical ATPase inhibitors ouabain, N-ethylmaleimide, orthovanadate and sodium azide. Levamisole, an inhibitor of alkaline phosphatase, was ineffective. Among nucleotides, ATP was the best substrate. The apparent K_{m (ATP)} for Ca²⁺-ATPase was 348±84 μM ATP and the V_max was 829±114 nmol Pi min⁻¹ mg⁻¹ protein. The P. soleiformis ganglial ATPase does not appear to fit clearly into any of the previously described types of Ca²⁺-ATPases.