334 F. SCHLENK 



The importance of the reactions outhned above for purine synthesis via 

 inosinic acid is discussed in Chapter 23, and the role of the citrovorum factor 

 is described on pages 335 and 336. Many details of inosinic acid synthesis 

 will have to be elaborated."^ It remains to be seen whether this is the main 

 path of nucleotide synthesis or merely an alternative. As yet, there is little 

 information about other nucleotides"^"^ and there are no data on a com- 

 parable system synthesizing deoxyribose nucleotides. The only indication 

 pointing in this direction is the tentative identification of carboxamide de- 

 oxyriboside in cultures of E. coli}^^ 



V. Role of B -Vitamins in Nucleoside and Nucleotide Sjrnthesis 



A function of several B-vitamins in nucleic acid metabolism has been 

 observed. Some of them act as coenzymes in the synthesis of the pentoses 

 and in the production of amino acid and other purine and pyrimidine pre- 

 cursors. This discussion will be restricted to vitamin cofactors participating 

 in the metabolism of the nucleosides, nucleotides, and closely related com- 

 pounds. From the preceding part of this chapter it is apparent that our 

 knowledge concerning the reaction pattern of these compounds is still very 

 limited; it is not surprising, therefore, that little can be reported about the 

 cofactors of these enzyme reactions. 



I. Vitamin B12 



The relationship of vitamin B12 to nucleoside and nucleic acid synthesis 

 was recognized in 1948.^^' "^'^o jt was found that deoxyribose nucleosides, 

 particularly thymidine, can replace vitamin B12 in promoting growth of 

 some Lactobacilli}'^^ •'^"-^^ Another effect was observed with L. leichmannii; 

 the phosphate incorporation into the deoxyribonucleic acid of this organism 

 was greatly increased, as measured by the use of radioactive phosphate. ^^^ 

 Thymine cannot usually replace thymidine as a substitute for vitaminBi2 , 

 which points to a role in deoxyribose formation or in the attachment of the 

 carbohydrate to the base.'^^ Contrary to expectations, however, the vitamin 

 has not been found in nucleosidase nor in the enzymes synthesizing deoxy- 

 ribose.'-^ The site of action may be in the nucleoside transglycosidase. 



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118 R. Ben-Ishai, E. D. Bergmann, and B. E. Volcani, Nature 168, 1124 (1951). 



119 W. Shive, J. M. Ravel, and R. E. Eakin, J. Am. Chem. Soc. 70, 2614 (1948). 



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