Biochemical and immunochemical characterisation of human diadenosine triphosphatase provides evidence for its identification with the tumour suppressor Fhit protein

We describe here the purification and characterisation of the human enzyme diadenosine triphosphatase isolated from human platelets and leukocytes, offering biochemical and immunochemical evidence to identify this enzyme with the novel tumour suppressor Fhit protein, a homodimer composed of ≈17 kDa monomers. It catalyses the Mg2+-dependent hydrolysis of diadenosine triphosphate, Ap3A, to AMP + ADP. The fluorogenic substrate di-ethenoadenosine triphosphate, ε-(Ap3A), and Fhit antibodies were used for enzymatic and immunochemical characterisations, respectively. Human Ap3Aase presents a native molecular mass of ≈32 kDa and no significant differences were found in Km values (2 μM), activating effects by Mg2+, Ca2+, and Mn2+, optimum pH (7.0-7.2) or inhibition by Zn2+ and diethyl pyrocarbonate between the human enzyme and the recombinant Fhit protein. Suramin is a very potent competitive inhibitor of both human Ap3Aase and Fhit protein with Ki values in the range 20-30 nM. Both human and rat Ap3Aase activity co-purifies with Fhit immunoreactivity under gel filtration, ion-exchange and affinity chromatography. Homogeneous human Ap3Aase preparations analysed by SDS-PAGE and Western blot analysis with Fhit antibodies elicit immunochemical responses corresponding to a ≈17 kDa polypeptide, indicating a dimeric structure for the enzyme Ap3Aase. The strong inhibition of Fhit enzyme by the drug suramin, supports the need to investigate the therapeutic potential of Fhit-Ap3Aase mediated by its interaction with suramin or related drugs.