Enzymatic synthesis of the pteridine moiety of dihydrofolate from guanine nucleotides.

Much of the knowledge of pteridine biosynthesis from purines has come from radioactive tracer studies with whole organisms. Reports on the incorporation of the radioactivity of labeled precursors of purines (1) and of uniformly labeled adenine-1% (2) into riboflavin indicated the utilization of purines for riboflavin synthesis. Furthermore, the results of studies (2, 3) with adenine-814C showed that carbon 8 of purines was not involved in riboflavin synthesis. The isolation and identification of a pteridine, 6,7-dimethyl-8ribityllumazine, suggested that this compound was a precursor of riboflavin (3). A study of the biosynthesis of pteroyltriglutamate also showed that purines with the exception of carbon 8 are precursors of the pteridine moiety (4). Weygand et al. (5) proposed a pathway for pteridine biosynthesis from guanosine or a guanine nucleotide. Later, Reynolds and Brown (6) extended this pathway toward dihydrofolate synthesis. The over-all pathway proposed that 4(5’-phosphoribosyl)amino-2,5-diamino-6-hydroxypyrimidine and a phosphorylated form of n-dihydroneopterin (2-amino-4-hydroxy-6-(nerythro-trihydroxypropyl)dihydropteridine) are intermediary compounds. A recent study indicated that 2,4,5-triamino-6hydroxypyrimidine was a pteridine precursor (7). However, it has since been shown that this triaminopyrimidine is an inhibitor of dihydrofolate synthesis (6). n-Neopterin was first isolated and characterized by Rembold and Buschmann (8), and the dihydro derivative of this compound was shown to be a precursor of dihydropteroate (9). Reynolds and Brown (6,lO) demonstrated for the first time the enzymatic synthesis of the pteridine moiety of dihydrofolic acid by a cell-free system. They showed that radioactive dihydrofolate was synthesized from uniformly labeled guanosine-14C and from guanine plus ribose-1-l4C, and that no radioactivity was found in the dihydrofolate synthesized from guanosine-8-14C or from guanine plus ribose 5-phosphate-5-14C. We wish to present additional results from studies concerned with the enzymatic synthesis of the pteridine moiety of dihydrofolic acid by cell-free extracts of Lactobacillus plantarum.