Kinetic attributes of rat liver microsomal adenosine 5’ triphosphate phosphohydrolase (ATPase)

Abstract

The kinetic properties of the rat liver microsomal ATPase, with respect to Na⁺, K⁺ and ATP requirements were examined. Presence of Na⁺ and K⁺ or both hardly caused any stimulation of the enzyme activity. The Km values for Na⁺ and K⁺were substantially low (0.32 and 0.05 mM, respectively), compared to those reported for the Na⁺, K⁺ ATPases from different tissues. Substrate kinetics studies revealed that in the absence of Na⁺ and K⁺, ATP is an activator of the enzyme. The enzyme displayed increased activity with increase in the energy of activation in the absence of Na⁺ and K⁺. The activity was partially inhibited by ouabain only in the presence of Na⁺ and K⁺. The results suggest that the liver microsomal enzyme is not a Na⁺, K⁺ ATPase, but has requirement of monovalent cations for the regulation of its activity. Also, the β3 subunit of the enzyme has a K_m lowering effect.