39. ANTIMETABOLITES AND NUCLEIC ACID METABOLISM 481 



which have been postulated to be the active inhibitors of growth. 181, 184, 185 

 Unfortunately, however, there is little if any difference between the biologi- 

 cal activity of the ribonucleoside and that of mercaptopurine as inhibitors 

 of sensitive or resistant populations of cells 184 , a result which suggested 

 either rapid cleavage of the ribonucleoside by nucleosidases to the free 

 analog or its further metabolism to the ribonucleotide at a rate comparable 

 to the formation of this derivative from mercaptopurine. A number of 

 workers have studied formation of the unnatural nucleotides in cell-free 

 systems. Mercaptopurine ribonucleotide has been prepared 186 by the con- 

 densation of the analog with pyrophosphorylribose-5-phosphate catalyzed 

 by inosinic pyrophosphorylase from beef liver, an enzyme which also forms 

 the ribonucleotides of hypoxanthineand guanine. Recently a purified pyro- 

 phosphorylase has been prepared from E. coli and found to exhibit approxi- 

 mately equal affinities for mercaptopurine and hypoxanthine, but a very 

 much lower affinity for guanine. 187 Crude extracts of mouse leukemic cells 

 form the ribonucleotide of mercaptopurine 173 ; the ribonucleotide also is 

 formed by a pyrophosphorylase from either yeast or liver 188 which appears 

 to be different from the enzyme responsible for the formation of the ribo- 

 nucleotides of guanine or hypoxanthine. Further phosphorylation of the 

 ribonucleotide of mercaptopurine to the di- and triphosphates has been 

 accomplished with ATP and an enzyme preparation from hog kidney. 189 

 Mercaptopurine deoxyribonucleoside has been formed by reaction of mer- 

 captopurine with deoxyribose-1 -phosphate catalyzed by nucleoside phos- 

 phorylase obtained from liver. 190 



Although convincing evidence is at hand concerning the conversion of 

 mercaptopurine to ribonucleotides, the incorporation of this analog into 

 nucleic acids has not been completely documented. Experiments with 

 mercaptopurine-S 35 and -C 14 in mice indicate significant labeling of the 

 nucleic acid fraction of rapidly proliferating tissues; upon hydrolysis to the 

 free purines, several radioactive derivatives were found, one of which had 

 the properties of mercaptopurine. 174 The other radioactive derivatives may 

 be thioguanine or acid-degradation products of mercaptopurine nucleotides; 

 incorporation into RNA appeared to be greater than that into DNA. 



184 H. E. Skipper, J. R. Thomson, D. J. Hutchison, F. M. Schabel, Jr., and J. J. 

 Johnson, Jr., Proc. Soc. Exptl. Biol. Med. 95, 135 (1957). 



185 M. E. Balis, D. H. Levin, G. B. Brown, G. B. Elion, H. C. Nathan, and G. H. 

 Hitchings, Arch. Biochem. Biophys. 71, 358 (1957). 



186 L. N. Lukens and K. A. Herrington, Biochim. et Biophys. Acta 24, 432 (1957). 



187 C. E. Carter, Biochem. Pharmacol. 2, 105 (1959). 



!88 J. L. Way and R. E. Parks, Jr., J. Biol. Chem. 231, 1467 (1958). 



189 J. L. Way, J. L. Dahl, and R. E. Parks, Jr., Federation Proc. 17, 418 (1958); J. Biol. 

 Chem. 234, 1241 (1959). 



190 M. Friedkin, Biochim. et Biophys. Ada 18, 447 (1955). 



