Selective Degradation of 2′-Adenylated Diadenosine Tri- and Tetraphosphates, Ap3A and Ap4A, by Two Specific Human Dinucleoside Polyphosphate Hydrolases

It is known that the interferon-inducible 2′,5′-oligoadenylate synthetase can catalyze the 2′-adenylation of various diadenosine polyphosphates. However, catabolism of those 2′-adenylated compounds has not been investigated so far. This study shows that the mono- and bis-adenylated (or mono- and bis-deoxyadenylated) diadenosine triphosphates are not substrates of the human Fhit (fragile histidine triad) protein, which acts as a typical dinucleoside triphosphate hydrolase (EC 3.6.1.29). In contrast, the diadenosine tetraphosphate counterparts are substrates for the human (asymmetrical) Ap4A hydrolase (EC 3.6.1.17). The relative rates of the hydrolysis of 0.15 mM AppppA, (2′-pdA)AppppA, and (2′-pdA)AppppA(2‴-pdA) catalyzed by the latter enzyme were determined as 100:232:38, respectively. The asymmetrical substrate was hydrolyzed to ATP + (2′-pdA)AMP (80%) and to (2′-pdA)ATP + AMP (20%). The human Fhit protein, for which Ap4A is a poor substrate, did not degrade the 2′-adenylated diadenosine tetraphosphates either. The preference of the interferon-inducible 2′-5′ oligoadenylate synthetase to use Ap3A over Ap4A as a primer for 2′-adenylation and the difference in the recognition of the 2′-adenylated diadenosine triphosphates versus the 2′-adenylated diadenosine tetraphosphates by the dinucleoside polyphosphate hydrolases described here provide a mechanism by which the ratio of the 2′-adenylated forms of the signalling molecules, Ap3A and Ap4A, could be regulated in vivo.