THE INHIBITION OF MITOCHONDRIAL ADENOSINE TRIPHOSPHATASE. THE EFFECTS OF ALKALI-METAL IONS

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Abstract
The effects of alkali-metal ions on the adenosine-triphosphatase activity of a membrane suspension from deoxycholate-disrupted rat-liver mitochondria were studied. The alkali-metal ions Na+, K+, Li+. Rb+, Cs+ and NH4+ inhibited the adenosine triphosphatase to about the same extent With Na+ or K+ ions, significant inhibition was observed at a concentration of 10 m[image], and the inhibition increased progressively with increasing concentrations. Although various bi-valent cations, Mg2+, Mn2+, Ca2+, Fe2+ Cu2+, Zn2+, Co2+ and Cd2+, activated the adenosine triphosphatase to a variable extent, the inhibition by alkali-metal ions was similar. The enzyme hydrolysed ATP, ITP, CTP, UTP and GTP at different rates, but again there were only small differences in the inhibition by alkali-metal ions. The activation of adenosine triphosphatase by 0.1 m[image]-2,4-dinitrophenol was diminished in the presence of an alkali-metal ion. With the same alkali-metal ion and in a tris-acetate buffer, pH 7.4, the anions inhibited the adenosine triphosphatase in the decreasing order: NO3-; Cl-; SO42-. These differences were decreased when the cation was incubated in a tris buffer containing the same anion as the alkali-metal salt. The inhibition was reversible and preincubation of the adenosine triphosphatase with the alkali-metal ion did not affect the inhibition. With Mg2+ ion as the activating cation, the pH optimum of the adenosine triphosphatase was about 8. This was shifted to about 9 with Ca2+ ion. Inhibition of the adenosine triphosphatase by an alkali-metal ion was similar over the pH range 5-10. The results are discussed in relation to the similarities and differences in the properties of myosin adenosine triphosphatase, mitochondrial adenosine triphosphatase and the "Na+ ion-plus-K+ ion-activated" or "glycoside-sensitive" adenosine triphosphatase of several membrane preparations.