Nonenzymatic Hydroxylation of Phenylalanine by Ascorbic Acid and Cu⁺⁺

Abstract

When phenylalanine was treated with ascorbic acid and Cu⁺⁺ in acetate buffer (pH 6.0), the decomposition of phenylalanine was observed. Cu⁺⁺ accelerated remarkably the decomposition of phenylalanine as compared with other transition metal ions. In contrast, nitrogen gas, other reducing agents and radical scavengers prevented the decomposition. The results indicate that a transient free radical intermediate, formed during the autoxidation of ascorbic acid, is responsible for the decomposition. The ascorbic acid-Cu⁺⁺ system was found to decompose phenylalanine giving hydroxy-phenylalanines such as dihydroxyphenylalanine, m-tyrosine, p-tyrosine and o-tyrosine.