132 PTEROYLGLUTAMIC ACID 



occur with the free imidazole derivative. The latter may represent a break- 

 down product of an intermediate which may be a ribotide. This ribotide is 

 converted into inosinic acid (hypoxanthine ribose-5-phosphate). Evidence 

 for this is the acceleration in rate of incorporation of radioactive formate 

 into inosinic acid by the addition of ribose phosphate. 



This hypothesis fits the observations of Gots-^ that in sulfonamide-in- 

 hibited E. coli the p-aminobenzoic acid added at intervals after inocula- 

 tion is able to prevent the accumulation of imidazole derivative and to 

 increase growth but is unable to decrease the amount already formed. This 

 shows that imidazole derivative lies outside the active metabolic pool and 

 that the enzyme system formed from p-aminobenzoic acid is unal)le to 

 reutilize the imidazole once it has been converted to the free state. Green- 

 berg has recently obtained evidence that the aminoimidazolecarboxamide 

 as formed by sulfonamide-inhibited E. coli is originally formed as the 

 desoxyriboside.^^ 



Although aminoimidazolecarboxamide is not an intermediate per se in 

 the synthesis of hypoxanthine, Schulman et alr^ found that there was ap- 

 preciable disappearance of C^''-labeled aminoimidazolecarboxamide when 

 incubated with pigeon liver homogenates and that radioactive hypoxanthine 

 was formed. 



Buchanan" also found that the conversion of the imidazole derivative to 

 hypoxanthine is catalyzed by soluble proteins. The presence of insoluble 

 proteins appears unnecessary for optimal activity. When non-isotopic 

 inosinic acid and hypoxanthine are incubated with either isotopic glycine 

 or the isotopic imidazole derivative in pigeon liver homogenate, the inosinic 

 acid contains a concentration of C^^ three to five times greater than that of 

 hypoxanthine. These results were interpreted to mean that the element of 

 ribose is added to the imidazole derivative prior to ring closure with formic 

 acid. 



2, Inosinic Transformylase 



Buchanan^* has observed that incubation of formate with inosinate in a 

 pigeon liver system results in rapid exchange of the carbon in position 2 of 

 the hypoxanthine with the carbon of formic acid. This enzyme system was 

 termed "inosinic transformylase" and was found to be catalyzed by the 

 citrovorum factor. This represents the first partially purified enzyme sys- 

 tem in which the function of a PGA derivative can be clearly recognized. 



2U. S. Gots, Federation Proc. 9, 178 (1950). 



"^^ G. R. Greenberg, Information presented at American Society of Biological Chem- 

 ists, New York, 1952. Federation Proc. (In press, 1952). 

 2« M. P. Schulman, J. M. Buchanan, and C. S. Miller, Federation Proc 9, 225 (1950). 

 " J. M. Buchanan, Abstr. 119th Meeting, Am. Chem. Soc. p. 130, 1951. 

 28 J. M. Buchanan, cited by A. D. Welch, Pharmacol. Revs. 3, 345 (1951). 



