BIOSYNTHESIS OF NUCLEOSIDES AND NUCLEOTIDES 319 



riboside; future research will have to deal also with orotic acid deoxyriboside 

 and 5-methylorotic acid deoxyriboside, if such nucleosides should be dis- 

 covered. It is of interest to note that 5-methylorotic acid (thymine-4-car- 

 boxylic acid) was not found superior to thymine in tissue nucleic acid in- 

 corporation.'^ • '^* 



7. NUCLEOSIDE-A^'-TRANSGLYCOSIDASES 



The discovery of nucleoside-A^-transglycosidase stems from bacterial 

 growth experiments. It was found that several of the deoxyribose nucleo- 

 sides can substitute for the vitamin B12 requirement of certain bacterial 

 species.'^ '^^ Since it is apparently immaterial which of the deoxyribose nu- 

 cleosides is provided in the growth medium, one must assume that a mech- 

 anism for rapid interconversion exists to meet the needs of the cells for 

 deoxyribonucleic acid synthesis. This question was studied by McNutt,^** 

 who found nucleoside-A^'-transglycosidase action to be the underlying prin- 

 ciple. From earlier observations it seemed likely that the interconversion of 

 one deoxyriboside into another involves two steps: 



Ni-deoxyriboside -|- phosphate ;=^ Ni -(- deoxyribose-1 -phosphate (10) 



N2 + deoxyribose-1 -phosphate ;^ N2-deoxyriboside -f phosphate (11) 



If this mechanism were correct, deoxyribose- 1-phosphate together with 

 the requisite bases should have substituted for the deoxyribonucleosides in 

 promoting bacterial growth ; this, however, was not the case nor was phos- 

 phate needed. Likewise, it was possible to exclude a hydrolytic mechanism: 



Ni-deoxyriboside ^ Ni -f- deoxyribose (12) 



N2 -f deoxyribose ;=i Na-deoxyriboside (13) 



Purine and pyrimidine bases with added deoxyribose failed to substitute for 

 deoxyribose nucleosides in growth tests. It became clear, therefore, that the 

 organisms examined (L. helveticus, L. delbruckii, and Thermobacterium acido- 

 philum) contain one or several enzymes to bring about the interconversions. 

 The name nucleoside- N-transglycosidase has been proposed.'*" 



The analytical procedures in the study of this new enzyme included paper 

 chromatography, ultraviolet spectrophotometry, and a differentiation be- 

 tween purine and pyrimidine deoxyribosides based on the relative resistance 

 of the latter toward acid. Thus, starting with a purine deoxyriboside and a 



" G. B. Brown, P. M. Roll, and H. Weinfeld, in "Phosphorus Metabolism" (McElroy 



and Glass, eds.). Vol. 2, p. 385. Johns Hopkins Press, Baltimore, 1952. 

 "^ W. L. Holmes and W. H. Prusoff, J. Biol. Chem. 206, 817 (1951). 

 '8 V. Kocher and D. Schindler, Intern. Z. Vitarninforsch. 20, 441 (1949). 

 '9 E. Kitay, W. S. McNutt, and E. E. Snell, J. Biol. Chem. 177, 993 (1949). 

 " W. S. McNutt, Biochem. J. 50, 384 (1953). 



