VOL. 12 (1953) BIOSYNTHESIS OF NUCLEOSIDES AND NUCLEOTIDES 253 



AND Szilard23), as these compounds are able to reverse the mutagenic action of dimethyl- 

 xanthine, azoguanine, and other purine derivatives^*. If ribosides which are potential 

 ribosyl phosphate producers can incorporate some of the mutagenic purine derivatives 

 the latter will not appear in the deoxyribonucleic acids, which may again imply that 

 they are barred from producing mutations. In this case the ribosyl compounds may 

 function as a trap for mutagenic purines. The fact that the nucleosides also suppress the 

 normal or "spontaneous" mutation rate adds to the interest of the whole phenomenon. 

 Phosphorolysis of pyridinium nucleosides. Rowen and Kornberg^^ have found 

 that nicotine-amide riboside (the pyridinium (N+) riboside) undergoes a phosphorolysis 

 catalyzed by a liver enzyme which seems to be identical with the purine nucleoside 

 phosphorylase. The reaction is reversible. This observation is particularly surprising 

 because the pyridinium (N+) riboside linkage does not have the typical properties of a 

 substituted glycosidic linkage. It is, for instance, more labile towards alkali than towards 

 acid^^. The reduced nicotine-amide riboside (monohydropyridine riboside) is much more 

 labile towards acids than towards alkah^^., i.e. Hke a typical glycoside; yet the reduced 

 form is inert towards the enzyme. Since a pyridinium compound is converted by enzymic 

 phosphorolysis into a pyridine compound with a liberation of one hydrogen ion, it is 

 clear that the equilibrium : 



"pyridinium (N+) riboside" + phosphate ^ "pyridine" + ribose-i -phosphate + H+ 

 depends on pH. In a buffer at neutral pH the reaction will go predominantly from left to 

 right. At pH 7.4, expressing concentrations as mmols per ml K is ": 



N+-riboside X P 



K = = 10-3 



nicotinamide X rib-i-P 



The AF of hydrolysis of the N+ ribosyl bond at pH 7 is close to 8000 cals (c/.^^) of which 

 almost half is furnished by the dissociation of hydrogen ions. The AF hydrolysis of a 

 phosphoglycosyl compound like glucose-i-phosphate or ribose-i-phosphate amounts 

 presumably to 4500-5000 cals. 



Transglycosidascs. Phosphopentosyl compounds are not obligatory intermediaries 

 in nucleoside metabolism. MacNutt^^ has discovered the existence of a trans-N- 

 glycosidase in Lactobacillus helveticus and other lactic acid bacteria, requiring deoxy- 

 ribosides or Bi2- This enzyme catalyzes the following type of reaction: purine deoxy- 

 riboside + pyrimidine ^ pyrimidine deoxyriboside -\- purine. All the purines and 

 pyrimidines occurring in nucleic acids, as well as the corresponding deaminated deriva- 

 tives, can be participants. A corresponding trans-N-glycosidase for ribosides may also be 

 present but a study of such an enzyme is not possible in crude extracts of L. helveticus 

 due to the presence of a powerful hydrolytic enzyme, ribosidase^^. Ribosidases have also 

 been found in other micro-organisms^''. 



It has been shown that sucrose phosphorylase can also act as a trans-0-glycosidase 

 between ketoses [e.g. direct enzymic conversion of sucrose into a glucosido-sorboside^) . 

 We have been comparing a trans-N-glycosidase from L. delbruckii with a nucleoside 

 phosphorylase from L. casei. It can be seen from Table I that the trans-N-glycosidase 

 does not show phosphorylase activity nor does the phosphorylase show trans-N-glycosi- 

 dase activity^^. The interesting spleen enzyme trans-nicotinamide-glycosidase attacking 

 DPN^'^ has apparently no phosphorylase activity. 



The mechanism by which glycosyl phosphorylases or transferases operate will be 

 discussed elsewhere. Recent studies by Friedkin^" have shown that deoxyribosyl 

 References p. 263J264. 



