American Society for Pharmacology and Experimental Therapeutics
Molecular Pharmacology vol:84 pages:865-75
A mycoplasma-encoded purine nucleoside phosphorylase (designated PNPHyor) has been cloned and characterized for the first time. Efficient phosphorolysis of natural 6-oxopurine and 6-aminopurine nucleosides was observed with adenosine being the preferred natural substrate (Km = 61 μM). Several cytostatic purine nucleoside analogues proved to be susceptible to PNPHyor-mediated phosphorolysis and a markedly decreased or increased cytostatic activity was observed in Mycoplasma hyorhinis-infected human breast carcinoma MCF-7 cell cultures (MCF-7.Hyor), depending on the properties of the released purine base. We demonstrated a ~10 fold loss of cytostatic activity of cladribine in MCF-7.Hyor cells and observed a rapid and complete phosphorolysis of this drug when exposed to the supernatant of mycoplasma-infected cells. This conversion (inactivation) could be prevented by a specific PNP inhibitor. These findings correlated well with the high efficiency of PNPHyor-catalyzed phosphorolysis of cladribine to its less-toxic base 2-chloroadenine (Km = 80 μM). In contrast, the cytostatic activity of nucleoside analogues carrying a highly toxic purine base and being a substrate for PNPHyor, but not human PNP, was substantially increased in MCF-7.Hyor cells (~130-fold for fludarabine and ~45-fold for 6-methylpurine-2'-deoxyriboside). Elimination of the mycoplasmas from the tumor cell cultures or selective inhibition of PNPHyor by a PNP inhibitor restored the cytostatic activity of the purine-based nucleoside drugs. Since different prokaryotes, including mycoplasmas, preferentially colonize or are associated with tumor tissue in cancer patients, the data presented here may be of relevance for the optimization of purine nucleoside-based anti-cancer drug treatment.