The Nonenzymatic Preparation in Solution of N- (S-Phosphoribosyl) anthranilic Acid, an Intermediate in Tryptophan Biosynthesis*

Yanofsky (1) originally proposed N-(5’-phosphoribosyl)anthranilic acid as the first intermediate after anthranilic acid in tryptophan biosynthesis in microorganisms. Evidence was subsequently found which suggested that this compound is formed enzymatically from anthranilic acid and ribosylpyrophosphate 5-phosphate (2, 3). Although the dephosphorylated form of PRA,’ N-ribosylanthranilic acid, has been synthesized and characterized (4), repeated attempts to synthesize PRA have been unsuccessful (2,5, 6). The elusiveness of this intermediate has been ascribed to its lability. For example, tryptophanrequiring mutants of microorganisms which would be expected to accumulate PRA or its dephosphorylated form are found to accumulate primarily anthranilic acid, which is presumably a breakdown product. Synthetic N-ribosylanthranilic acid has a half-lie of approximately 30 min at pH 7.0 and 37” (5). It is the purpose of this communication to describe an extremely simple method of preparing PRA in solution which makes it available as a substrate for enzymatic studies.