ENZYMATIC PHOSPHORYLATION AND PYROPHOSPHORYLATION OF 2',3'-DIDEOXYADENOSINE-5'-MONOPHOSPHATE, A KEY METABOLITE IN THE PATHWAY FOR ACTIVATION OF THE ANTI-HIV (HUMAN-IMMUNODEFICIENCY-VIRUS) AGENT 2',3'-DIDEOXYINOSINE

2',3'-Dideoxyadenosine-5'-monophosphate (ddAMP) is a key intermediate in the metabolic pathway involved in the activation of the anti-retrov iral agent 2'3'-dideoxyinosine (ddI) to 2',3'-dideoxyadenosine-5'-trip hosphate (ddATP). The potential phosphorylation of ddAMP by adenylate kinase (myokinase) and pyrophosphorylation by the reverse reaction of 5-phosphoribosyl-1-pyrophosphate (PRPP) synthetase were investigated. Using ATP as phosphate donor, ddAMP was phosphorylated by adenylate ki nase with an efficiency of 8.8% of that for AMP, as estimated from the V-max/K-m ratios. In the presence of PRPP, Escherichia coli and rat P RPP synthetases catalysed the pyrophosphorylation of ddAMP with effici encies of 52 and 35% of that determined for AMP, respectively. Two car bocyclic phosphonate analogues of ddAMP were not substrates of adenyla te kinase. Yet, they were pyrophosphorylated by both PRPP synthetases, albeit less efficiently than ddAMP. In vivo, the usual function of PR PP synthetase is to synthesize PRPP from ribose-5-phosphate and ATP. I n the forward reaction ddATP proved to be a substrate as efficient as ATP for rat PRPP synthetase. ddATP was also studied as a potential pho sphate donor in the reaction catalysed by adenylate kinase with AMP as phosphate acceptor and found to be as efficient as ATP. The relevance of these in vitro results to the in vivo situation is discussed.