Ca-stimulated Mg dependent ATPase activity in a plasma membrane enriched fraction of bovine corneal epithelium.

In a subcellular plasma membrane enriched fraction of bovine corneal epithelium, Ca2+ stimulated Mg2+ dependent ATPase activity was characterized. This membrane fraction was more than 5-fold and 4-fold enriched with 5'-nucleotidase and alkaline phosphatase activities, respectively, relative to the 100,000 X g pellet. With 250 microM ATP, maximum stimulation of a high affinity form of Ca2+ stimulated Mg2+ dependent ATPase activity was obtained with 1.7 microM free Ca2+. This activation required no exogenously added Mg2+ and was unaffected by either 0.1 mM ouabain, 3 microM ruthenium red, 20 mM sodium azide or 0.2 microgram/ml oligomycin. Exogenous calmodulin (6 microM) elicited a 53% increase in this activity which was completely inhibited by 300 microM trifluoperazine (TFP). These effects of calmodulin and TFP are consistent with the notion of a plasma membrane origin for this activity and also suggest that this activity could be a basis for the regulation of intracellular Ca2+ activity in the submicromolar range.