Abstract
1. The ATPase activity of insect mitochondria has been investigated. A comparison was made to determine the distribution and nature of such activity in other isolated fractions of the house fly, Musca domestica L. 2. The ATPase in insect mitochondria is specific in that orthophosphate can be cleaved only from ATP. The Michaelis-Menten constant K(8) = 2.78 x 10(-3)M and V(max.) = 76 micrograms P min.(-1) mg.(-1) dry weight. 3. Mg(++) and Mn(++) activate this enzymatic reaction in mitochondria, but Ca(++) does not. The extent of activation is 60 per cent with the optimal concentration 6 x 10(-4)M. Experiments with combinations of Mg(++) and Mn(++) show that either ion can replace the other and that the effects are additive, depending solely on the final concentration of the combination. Concentrations of Mg, Mn, or Ca ions higher than 6 x 10(-3)M inhibit the enzyme. 4. Fluoride does not inhibit the ATPase of insect mitochondria, whereas azide and chloromercuribenzoate do. The per cent inhibition depends on the concentration of inhibitor. 5. Finely dispersed mitochondrial particles have much greater ATPase activity than intact mitochondria. The possible relationship of this observation to latent ATPase is considered. 6. A magnesium-activated adenylate kinase is present in these mitochondria. The liberated orthophosphate, derived from ADP, is the result of the activity of adenylate kinase followed by the specific ATPase. 7. ATP can be dephosphorylated by enzymes found in the muscle fibrils, and in a "soluble" fraction, as well as in mitochondria. The fibrillar ATPase is Ca(++)-activated. The "soluble" fraction, however, like the mitochondria, is Mg(++)-activated. The "soluble" ATP dephosphorylation mechanism is distinguished from the mitochondrial ATPase in that it is inhibited by fluoride. 8. The "soluble" fraction also contains a magnesium-activated inorganic pyrophosphatase. Fluoride completely inhibits this enzymatic reaction. 9. The possible mechanism of ATP dephosphorylation in the "soluble" fraction is discussed.