Biosynthesis of the purines.

4-Amino-5-imidazolecarboxamide (l), an amine found to accumulate in bacterial systems poisoned either by sulfonamides (2, 3) or aminopterin (4), has been suggested as an intermediate of purine synthetic reactions (5, 6). Although the metabolic inertness of the carboxamide’ in certain bacterial systems (7, 8) led to the belief by some (8) that this compound was an aberrant product of metabolism of sulfonamide-poisoned cells, certain mutant strains of Escherichia coli2 and Ophiostoma multiannulatum (9) as well as the normal strain of Lactobacillus arabinosus (7) may utilize the carboxamide for growth in a manner similar to purines. However, they usually require the carboxamide in greater concentration than the purines for equivalent growth. Williams (10) in this laboratory has more recently shown that labeled carboxamide is incorporated into purine compounds by the intact yeast cell and by yeast extracts. The carboxamide may be utilized in viva at a significant rate in the formation of uric acid by the pigeon (11) and in the synthesis of tissue purines as well as allantoin by the rat (12). In a preliminary communication (11, 13), it has been shown that the carboxamide may be converted into hypoxanthine by pigeon liver homogenates. Since Greenberg (14, 15) has shown that inosinic acid may be an intermediate in the de novo synthesis of hypoxanthine from formate, we have further studied the conversion of the carboxamide to hypoxanthine in pigeon liver homogenates and extracts in order to determine whether ribotide formation is involved in carboxamide metabolism. Evidence is presented indicating that the units of ribose and phosphate are added to the carboxamide prior to ring closure with formic acid, implicating the involvement of 4-amino-5-imid-