biosynthesis of nucleosides and nucleotides 315 



3. Purine Deoxyriboside Nucleosidases 



In view of the difficulty in the past of securing deoxyribonucleosides, it 

 is not surprising that nucleosidase studies on them are of more recent date 

 than those on ribosides. After the discovery of phosphorolysis of purine 

 ribosides, an analogous nucleosidase reaction with purine deoxyribosides 

 seemed very likely. Proof of this was furnished by Friedkin and Kalckar,^^ 

 who used procedures very similar to those employed in the study of purine 

 ribosides. Purified calf and rat liver^* and calf thymus-* nucleosidases were 

 used. These preparations seem to be identical with those acting in the 

 phosphorolysis of purine ribosides; their range of action extends to hypo- 

 xanthine and guanine deoxyriboside. In addition to the bases, deoxyribose- 

 1-phosphate was isolated as crystalHne cyclohexylamine salt.-* Experimen- 

 tation with this ester was complicated by its extreme lability, but Friedkin 

 was able to synthesize hypoxanthine deoxyriboside from it by combination 

 with hypoxanthine in presence of the enzyme. Synthesis is favored if the 

 phosphate concentration is kept low. The identity of the product obtained 

 could be corroborated by microbiological assay .^^ The enzyme from liver 

 shows activity also toward xanthine deoxyriboside, but the rate of reaction 

 is only about J^st'h of that with deoxyguanosine." 



The examination of the nucleosidases from microorganisms has been com- 

 plicated in some instances by peculiar effects of divalent ions such as phos- 

 phate, arsenate, sulfate, or succinate on these enzymes. ^^ A stabilizing effect 

 is exerted by these ions on pyrimidine nucleosidase, while the reverse holds 

 for purine nucleosidase from L. pentosus, which is rapidly inactivated in 

 their presence. Purine nucleoside phosphorylase, active on deoxyinosine 

 and on deoxyguanosine, has been obtained from Escherichia coliP It may 

 also act on deoxyadenosine.^^ 



4. Pyrimidine Riboside Phosphorylase 



Members of the Thannhauser school did the initial work on pyrimidine 

 nucleosidases. Bone marrow" and kidney'- were used as a source and 

 splitting was accomplished at a slow rate. Various pyrimidine riboside phos- 

 phorylases seem to exist, and the work of Klein'^ indicated phosphorolysis 

 rather than hydrolysis. 



" M. Friedkin and H. M. Kalckar, J. Biol. Chem. 181, 437 (1950). 



2' J. Wajzer, Arch. sci. physiol. 1, 485 (1947). 



" L. A. Manson and J. O. Lampen, J. Biol. Chem. 191, 95 (1951). 



" M. Friedkin, J. Biol. Chem. 148, 449 (1950). 



28 E. Hoff-J0rgensen, M. Friedkin, and H. M. Kalckar, /. Biol. Chem. 184, 461 (1950). 



"M. Friedkin, J. Am. Chem. Soc. 74, 112 (1952). 



2* J. O. Lampen, in "Phosphorus Metabolism" (McElroy and Glass, eds.), Vol. 2, 



p. 366, Johns Hopkins Press, Baltimore, 1952. 

 " L. A. Manson and J. O. Lampen, /. Biol. Chem. 193, 539 (1951). 



