BIOSYNTHESIS OF NUCLEOSIDES AND NUCLEOTIDES 323 



furnished by Kalckar and Rittenberg.*^ After administration of ammonium 

 citrate, labeled with N^^, they found a high concentration of the isotope in 

 the 6-amino nitrogen of the adenylic acid of skeletal muscle. This indicated 

 a rapid reversible deamination. Under the same conditions the amino nitro- 

 gen of the glutamic acid of the muscle proteins showed an isotope concentra- 

 tion which was only one-fifth of that of the 6-amino nitrogen of the 5- 

 adenylic acid, while the amide nitrogen had a higher isotope value. The 

 acid amide group of glutamine (and perhaps asparagine) may be the im- 

 mediate precursor of the amino group. Weil-Malherbe^® has studied this 

 system with washed or dialyzed brain homogenates and believes that the 

 amination occurs at the expense of high-energy phosphate bonds. Elliott 

 and Gale have studied this system with Staphylococcus aureus.^'' 



d. Guanylic Acid Deaminase 



Guanylic acid deaminase was discovered by Schmidt;** it seems to be 

 different from guanase, but further characterization is needed. Guanylic 

 acid formation may occur by amination of xanthylic acid, deamination of 

 2,6-diaminopurine nucleotide,*^ ■*'' or by incorporation of an N-C unit in- 

 stead of formate into 4-amino-5-imidazolecarboxamide. These alternatives 

 are hypothetical; on the other hand, the occurrence of guanase in many 

 cells and the reactivity of nucleoside phosphorylase with guanine in presence 

 of the pentose esters suggest that guanine may be the precursor of the 

 guanine nucleosides. 



Glutamine may be suspected as a donor of the amino group as in the case 

 of adenine nucleotides, but no experiments are recorded. Deamination of 

 guanosine always seems to be attended by splitting into base and carbo- 

 hydrate.*^ The biosynthesis of guanyhc acid has barely been touched by 

 investigators.^"^ 



e. Cijtidine Deamination 



Deamination of cytidine has been observed in liver,** '^"^ in blood, ^^ in E. 

 coli, and in yeast.^° According to Wang ei al.,^- the action of the enzyme from 

 E. coli and from yeast extends to cytosine riboside and deoxy riboside, while 



" H. M. Kalckar and D. Rittenberg, J. Biol. Chem. 170, 455 (1947). 



"H. Weil-Malherbe, Biochem. J. 54, vi (1953). 



" W. H. Elliott and E. F. Gale, Nature 161, 129 (1948). 



58 G. Schmidt, Z. physiol. Chem. 208, 185 (1932). 



" A. Romberg and W. E. Pricer, Jr., /. Biol. Chem. 193, 481 (1951). 



«» G. P. Wheeler and H. E. Skipper, Federation J" roc. 12, 289 (1953). 



«°» B. Magasanik and M. S. Brooke, J. Biol. Chem. 206, 83 (1954). 



^"^ L. Grossman and D. W. Visser, /. Biol. Chem. 209, 447 (1954). 



" E. J. Conway and R. Cooke, Biochem. J. 33, 457 (1939). 



« T. P. Wang, H. Z. Sable and J. O. Lampen, J. Biol. Chem. 184, 17 (1950). 



