[Ca²⁺+Mg²⁺]-dependent ATPase activity in rat pineal gland

[Ca²⁺+Mg²⁺]-dependent ATPase activity in the pineal gland of the rat was examined. The enzyme possesses an apparent K_m (Ca²⁺) of 0.23 μM, and moderately high affinity for Mg²⁺ and ATP (K_m = 53.2 μM and K_m = 86.4 μM, respectively). The ATPase activity is sensitive to low concentrations (I₅₀ approximately 1 μM) of vanadate, which specifically inhibits Ca²⁺-ATPase in the plasma membranes of the erythrocyte, cardiomyocytes and synapses. The calmodulin antagonist trifluoperazine reduced significantly Ca²⁺-stimulated, Mg²⁺-dependent ATP hydrolysis. The [Ca²⁺+Mg²⁺]-dependent ATPase in rat pineal gland exhibits very high affinity for Ca²⁺, is highly vanadate sensitive and appears to require calmodulin. The enzyme is similar to the Ca²⁺-ATPase of the erythrocyte, cardiomyocytes and synaptic plasma membranes. These new findings may help to elucidate the mechanisms of intracellular calcium homeostasis and the effect of the enzyme on the synthesis of melatonin in the pineal gland.