Diadenosine tetraphosphate activates cytosol 5'-nucleotidase.

The rate of hydrolysis of IMP (0.5 mM) by cytosol 5'-nucleotidase from Artemia embryos was increased up to 7-fold by concentrations of around 10 microM diadenosine tetraphosphate (Ap4A). Half maximal activation of the enzyme was accomplished with 5 microM Ap4A. The Km (S 0.5) values of the nucleotidase for IMP, GMP, AMP, XMP and CMP decreased about 10 fold in the presence of 10 microM Ap4A. Maximum velocity of the enzyme was not affected by Ap4A. ATP had been previously described as an activator of the enzyme. However, comparatively with Ap4A, concentrations of ATP two orders of magnitude higher are needed to elicit similar effects on the enzyme. Preliminary results indicate that Ap4A is also an activator of the cytosol 5'-nucleotidase from rat liver.