Diadenosine polyphosphates inhibit adenosine kinase activity but decrease levels of endogenous adenosine in rat brain

Findings in peripheral tissues that diadenosine polyphosphates (Ap(n)As) activate 5'-nucleotidase activity and inhibit adenosine kinase activity in vitro led us to test the hypothesis that Ap(n)As and analogues thereof, through such actions on purine enzymes, increase brain levels of endogenous adenosine in vivo. Accordingly, we tested Ap(n)As for their effects on the in vitro activities of adenosine kinase, adenosine deaminase, AMP deaminase and 5'-nucleotidase and, following unilateral microinjections in rat striatum, on in vivo levels of endogenous adenosine. Adenosine kinase activity was not affected significantly by 5',5'''-P¹,P²-diadenosine pyrophosphate (Ap₂A) or by 5',5'''-P¹,P³-diadenosine triphosphate (Ap₃A), but was inhibited by 5',5'''-P¹,P⁴-diadenosine tetraphosphate (Ap₄A), 5',5'''-P¹,P⁵-diadenosine pentaphosphate (Ap₅A) and 5',5'''-P¹,P⁶-diadenosine hexaphosphate (Ap₆A); apparent IC₅₀ values were 5.0, 3.3 and 500 μM, respectively. Inhibition of adenosine kinase activity by Ap₄A and the four metabolically stable analogues of Ap₄A tested was uncompetitive. Following unilateral intrastriatal injections, adenosine levels, relative to uninjected contralateral striatum, were decreased significantly (P < 0.05) by 48% with Ap₄A and by 37% with AppCH₂ppA, a metabolically stable analogue of Ap₄A. Striatal levels of adenosine were not affected significantly by Ap₅A or Ap₆A. Cytosolic, but not particulate 5'-nucleotidase activity was inhibited and AMP deaminase activity was increased by some Ap(n)As. Although adenosine kinase inhibitors increase levels of endogenous adenosine and we showed here that Ap(n)As were potent inhibitors of this enzyme, these particular actions of Ap(n)As were not consistent with their effects on levels of endogenous adenosine.