Studies on the Hydroxylation of Phenylalanine by 6, 7-Dimethyl-5, 6, 7, 8-tetrahydropteridine

抄録

The hydroxylation of phenylalanine by 6, 7-dimethyl-5, 6, 7, 8-tetrahydropteridine (DMPH₄) was investigated. When phenylalanine was treated with DMPH₄ in citrate buffer (pH 6.0), p-tyrosine, m-tyrosine and o-tyrosine were identified as hydroxylated products. The hydroxylation was pH-dependent, and the maximum rate was found at around pH 6. Replacement of air with nitrogen gas and the addition of hydroxyl radical scavengers or catalase prevented the hydroxylation. In contrast, ferrous ions significantly accelerated the hydroxylation as compared with other transition metal ions. In an aqueous solution of DMPH₄ under aerobic conditions, the electron spin resonance (ESR) spectra of the hydroxyl radical spin adducts with spin traps such as α-phenyl N-tert-butyl nitrone (PBN) and α-4-pyridyl 1-oxide N-tert-butyl nitrone (4-POBN) were observed. The results indicate that the hydroxylating effect of DMPH₄ is caused by hydroxyl radicals formed during the autooxidation of DMPH₄.