Diadenosine Tetraphosphate Hydrolase from Mouse Liver

An enzyme hydrolyzing diadenosine 5’,5”’-P1,P4-tetraphosphate (Ap4A) to AMP and ATP has been purified to apparent homogeneity from mouse liver cell extracts. The isolation procedure comprised ammonium sulfate precipitation, chromatography on Sephadex G75, DEAEhellulose, blue Sepharose and AMP-Sepharose. The enzyme is a single polypeptide chain with a native M, = 64,000 with a K,,, of 1.66 PM and Vmax of 1.25 pmol/min. AMP, ADP, Ap, GTP, Gp4, ApzA, Ap6A, Gp,G, and Gp,G are noncompetitive inhibitors of the Ap4A hydrolase activity, whereas Gp4G inhibits Ap4A hydrolysis competitively with a Ki of 6 ELM. Theophylline, caffeine, and isobutylmethylxanthine do not or only slightly inhibit Ap4A hydrolysis. Mitogenic factors have no effect on the enzymatic activity of Ap4A hydrolase, excluding that a direct influence of internalized mitogens on Ap4A degradation could be responsible for mitogen-dependent fluctuation of intracellular Ap4A pool sizes.