Hydroxylation of 4-methylphenylalanine by rat liver phenylalanine hydroxylase

• Published in: The Journal of biological chemistry 1991-02, Vol.266 (5), p.2903-2910
• Main Authors: SIEGMUND, H.-U, KAUFMAN, S
• Format: Article
• Language: English
• Subjects: Analytical, structural and metabolic biochemistry; Animals; Biological and medical sciences; Enzymes and enzyme inhibitors; Fundamental and applied biological sciences. Psychology; Hydroxylation; Liver - enzymology; Oxidoreductases; Phenylalanine - analogs & derivatives; Phenylalanine - chemistry; Phenylalanine Hydroxylase - metabolism; Rats; Substrate Specificity
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1016/S0021-9258(18)49933-4

Rat liver phenylalanine hydroxylase that has been activated with lysolecithin catalyzes the hydroxylation of 4-methylphenylalanine in the presence of a pterin cofactor. Two products, 4-hydroxymethylphenylalanine and 3-methyltyrosine, can be detected. The total amount of amino acids hydroxylated is equal to the amount of tetrahydropterin oxidized. Isotopic labeling studies with 18O2 and H2(18)O show that the hydroxyl groups of both products are derived from molecular oxygen and not from water. Results obtained with 2H-labeled substrates support the conclusion that these products are formed via different mechanistic pathways. Our previous investigations on substrate analogs, as well as the present results, indicate that a highly reactive oxygen-containing intermediate, such as an enzyme-bound iron-oxo compound, must be the hydroxylating species. Our present results could stimulate further discussion of the possibility that the reaction mechanism for the "NIH-shift" of the methyl group may not involve the spontaneous opening of an epoxide intermediate.