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 the preferred natural substrate (Km = 61 µM). Several
cytostatic purine nucleoside analogs 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 an ∼10-fold loss of
cytostatic activity of
cladribine in MCF-7.Hyor cells and observed a rapid and complete phosphorolysis of this
drug when it was 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 analogs 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 mycoplasma 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 several studies suggest a high and preferential colonization or association of
tumor tissue in
cancer patients with different prokaryotes (including mycoplasmas), the data presented here may be of relevance for the optimization of
purine nucleoside-based anticancer
drug treatment.