Comparison of the rat microsomal Mg-ATPase of various tissues

Abstract

The microsomal Mg-ATPase from various rat tissues was compared. After fractionating the microsomal vesicles by sucrose gradient centrifugation, the highest specific activity of the Mg-ATPase was found in the low-density vesicles which contained plasma membrane. A large fraction (25–90%) of the microsomal Ca-independent Mg-ATPase found in each tissue had the following properties: (1) the K m for ATP was 0.2 mM; (2) the rate of ATP hydrolysis by the Mg-ATPase was nonlinear due to an ATP-stimulated inactivation of the enzyme; (3) wheat germ agglutinin, concanavalin A, glutaraldehyde, and antiserum prevented inactivation induced by ATP or AdoPP[NH]P; (4) detergents at relatively low detergent:protein ratios increased the rate of inactivation with little change in the initial rate of ATP hydrolysis; (5) the Mg-ATPase was inactivated by irradiation in the presence of 8-azido ATP. (6) in addition to ATP, the Mg-ATPase was able to hydrolyze CTP, GTP, UTP, ITP, and GTP but was unable to hydrolyze any of the 10 nonnucleotide phosphocompounds which were tested; (7) the bivalent cation requirement of the Mg-ATPase could be provided by Mg 2+, Ca 2+, Mn 2+, Zn 2+, or Co 2+ but the enzyme was inactive in the presence of Cu 2+, Sr 2+, Ba 2+, or Be 2+; (8) the Mg-ATPase activity was not altered by ionophores or inhibitors of the Na,K-ATPase, the Ca,Mg-ATPase or the mitochondrial F 1ATPase. These data suggest that a major portion of the microsomal, basal Mg-ATPase activity is due to one unique enzyme found in most if not all tissues.