Mitochondria from Vigna sinensis (L.) Savi cv. Pitiuba contain the polyamines spermine, spermidine, and putrescine. The membrane-bound F1-ATPase from mitochondria of Vigna sinensis is activated by these polyamines at physiological concentrations. The effect of polyamines on the membrane-bound of F1-ATPase is dependent on the concentrations of Na+, K+, MgATP, and Mg2+. Excess Na+ or K+ prevents the activation of the membrane-bound F1-ATPase by spermine and spermidine, but not by putrescine. The most pronounced effects were observed at low MgATP concentrations in the absence of Na+ and K+. At [MgATP] = 0.08 mM, spermine activation of the membrane-bound F1-ATPase was 130%. The membrane-bound F1-ATPase is slightly activated by Mg2+ at lower concentrations and strongly inhibited by Mg2+ at higher concentrations. Activation as well as inhibition is dependent on the substrate MgATP concentration. Although there is competition between Mg2+ and MgATP, the binding sites for these two ligands are different (pseudocompetitive inhibition). The inhibition of the membrane-bound F1-ATPase can be reversed by polyamines. There is evidence that the binding sites for Mg2+ and polyamines are identical. The F1-ATPase detached from the membrane is neither activated by polyamines nor inhibited by Mg2+. Therefore, the binding sites for Mg2+ and polyamines seem to be localized on the membrane.
Mitochondria from the bean Vigna unguiculata contain an ATPase system that is strongly activated by K+ and Na+ in the presence of low concentrations of substrate. Optimum activation by physiological concentrations of K+ is achieved in the presence of 0.1 mM MgATP. At the substrate optimum of the total ATPase, which is 2.3 mM, the enzyme is only poorly activated by K+. The K+-activated component of the total F1-ATPase behaves like an individual enzyme with kinetic properties different to the total F1-ATPase. On the basis of enzyme kinetic measurements, we determined the mechanism of activation by K+ as follows:[Formula: see text]K+ is not required for the binding of the substrate MgATP to the enzyme but for the splitting of the enzyme–substrate complex into the reaction products.
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