VOL. 12 (1953) BIOSYNTHESIS OF NUCLEDOSIDES AND NUCLEOTIDES 25I 



We have for example : 



ribosyl-hypoxanthine -f phosphate ^ ribosyl-phosphate + hypoxanthine 

 or 



deoxyribos\d-hvpoxanthine + phosphate ^ deoxyribosyl-phosphate + hypoxanthine 



Hypoxanthine can be continuously removed by xanthine oxidase which catalyzes 

 an oxidation to uric acid. By addition of two enzymes, nucleoside phosphorylase and 

 xanthine oxidase, it is possible to follow the phosphorolysis of hypoxanthine-pentosyl 

 compounds spectrophotometrically since the uric acid formed manifests itself by a 

 conspicuously high absorbtion maximum at 2900 A ; at the same time the phosphorolysis 

 is brought to an end and the phosphopentosyl compounds which accumulate can be iso- 

 lated. If the phosphopentosyl compounds are incubated with hypoxanthine or guanine 

 and purine nucleoside phosphorylase, the purine ribosides are resynthesizedand inorganic 

 phosphate is liberated^-^. In the corresponding enzymic resynthesis of hypoxanthine 

 deoxyriboside it was shown that the product had high growth factor activity towards 

 Thermohacterium acidophilus R26 which is a deoxyribosyl requiring lactic acid bacte- 

 rium^. The enzymic equilibrium is towards purine incorporation both when studied 

 with ribosyl-phosphate and with deoxyribosyl-phosphate (see Fig.). 



This type of reaction clearty revealed that the reactive component was a type of 

 reactive ribosyl (or deoxyribosyl) as follows 



. ; 0— 



RO: C— 



H 



which can attach to a N-base or a phosphate anion. This means that the fission of ribosyl 

 phosphate takes place between the carbon number i of the ribose and the oxygen of the 

 phosphate. Later studies using ^^O have shown that fission occurs in an analogous way 

 between the carbon and oxygen in the enzymic splitting of glucose-i-phosphate in 

 polysaccharide synthesis'*. It has been demonstrated that arsenate exerts a hydrolytic 

 effect if added to glycosyl phosphorylases^-^. We are here once more reminded of the 

 corresponding observations made by Warburg and his group, i.e. the hydrolytic forma- 

 tion of 3-phosphoglycerate in the presence of arsenate. Apparently glycosyl arsenates 

 as well as acyl arsenates undergo spontaneous fission in water. Manson and Lampen 

 have also found that addition of arsenate to nucleoside phosphorylase in the presence 

 of deoxy ribosides yields free deoxy ribose'. 



Properties of pentosyl phosphates. The two pentose-i-phosphoric esters are highly 

 acid labile, especially deoxyribose-i-phosphate, which is 50% dephosphorylated at 20° 

 within 15 minutes at pH 4^ and thus is an example of one of the most acid labile com- 

 pounds of this type hitherto described. Ribo-furanose-i-phosphate is fairly stable at 

 pH 4 but highly acid labile in strong mineral acid (in 0.5 N sulfuric acid at 20° it is 

 hydrolyzed to the extent of 50% within 2.5 minutes^). Ribose-i-phosphate can therefore 

 be studied by the Lowry-Lopez phosphate determination at pH 4^. The properties of 

 the pentose-i-phosphates are summarized in Table V together with other phospho- 

 glycosyl compounds. Although the pentose-i-phosphatesare formed from ^-nucleosides^", 

 it is nevertheless difficult to make any suggestions with regard to the configuration of 

 the phosphopentosyl compounds. Enzymic phosphorolytic fission of the 1-4-a-glycosidic 

 linkages of glycogen and starch yields a-glucose-i-phosphate, as does the corresponding 

 fission of sucrose*^' ^^. In contrast, the a-i-4-glycosidic linkage of maltose yields ^- 



References p. 26JJ264. 

 16 



