An improved method for the enzymatic synthesis of purine nucleosides is described. Pyrimidine nucleosides were used as pentosyl donors and two phosphorylases were used as catalysts. One of the enzymes, either uridine phosphorylase (Urd Pase) or thymidine phosphorylase (dThd Pase), catalyzed the phosphorolysis of the pentosyl donor. The other enzyme, purine nucleoside phosphorylase (PN Pase), catalyzed the synthesis of the product nucleoside by utilizing the pentose 1-phosphate ester generated from the phosphorolysis of the pyrimidine nucleoside. Urd Pase, dThd Pase, and PN Pase were separated from each other in extracts of Escherichia coli by titration with calcium phosphate gel. Each enzyme was further purified by ion-exchange chromatography. Factors that affect the stability of these catalysts were studied. The pH optima for the stability of Urd Pase, dThd Pase, and PN Pase were 7.6, 6.5, and 7.4, respectively. The order of relative heat stability was Urd Pase greater than PN Pase greater than dThd Pase. The stability of each enzyme increased with increasing enzyme concentration. This dependence was strongest with dThd Pase and weakest with Urd Pase. Of the substrates tested, the most potent stabilizers of Urd Pase, dThd Pase, and PN Pase were uridine, 2'-deoxyribose 1-phosphate, and ribose 1-phosphate, respectively. Some general guidelines for optimization of yields are given. In a model reaction, optimal product formation was obtained at low phosphate concentrations. As examples of the efficiency of the method, the 2'-deoxyribonucleoside of 6-(dimethylamino)purine and the ribonucleoside of 2-amino-6-chloropurine were prepared in yields of 81 and 76%, respectively.
Twenty purine 2'-deoxy-2'-fluororibosides were synthesized by enzymic pentosyl transfer from 2'-deoxy-2'-fluorouridine. Each nucleoside analogue was assayed for cytotoxicity in uninfected Madin-Darby canine kidney cells and for their ability to suppress influenza A virus infections in these cells. The most potent antivirial activity was observed with analogues having an amino group in the 2-position of the purine moiety. All 2-unsubstituted analogues were less potent than their 2-amino counterparts. Furthermore, 2-methyl,2-methoxy, or 2-fluoro substitution obliterated antivirial activity. The most cytotoxic member of the series was the 2-fluoro-6-amino analogue (IC50 = 120 microM). 2'-Deoxy-2'-fluoroguanosine and those congeners readily converted to it by adenosine deaminase showed the most potent antivirial activity (IC50 = 15-23 microM). Little cytotoxicity was observed with this subgroup of analogues which renders them worthy of further investigation as potential antiinfluenza agents.
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