The Photochemical Decomposition and Hydroxylation of Phenylalanine in the Presence of Riboflavin

Abstract

When phenylalanine was illuminated in the presence of riboflavin in 0.1 M citrate buffer (pH 5.0), loss of phenylalanine was observed. The photochemical reaction in the riboflavin system was found to reduce phenylalanine, giving hydroxyphenylalanines (p-tyrosine, m-tyrosine and o-tyrosine). The hydroxylation of phenylalanine was inhibited by radical scavengers, e. g., 1, 2-dihydroxybenzene-3, 5-disulfonic acid, catalase, potassium iodide, potassium bromide, sodium thiocyanate and mannose. On the other hand, 1, 4-diazabicyclo [2. 2. 2] octane and guanosine, which are known to react with singlet oxygen, had no significant effect. The findings suggest that superoxide radical and hydrogen peroxide are essential intermediate species in the hydroxylation reaction and that the active species responsible for the photochemical hydroxylation of phenylalanine in the riboflavin system is the hydroxyl radical.