The non-enzymic hydroxylation of phenylalanine to tyrosine by 2-amino-4-hydroxy-6,7-dimethyl-5,6,7,8-tetrahydropteridine.
1. Phenylalanine is converted into tyrosine by incubation in air with 6,7-dimethyltetrahydropterin, which is a cofactor for the enzymic hydroxylation. This can cause serious inaccuracies in assays of phenylalanine hydroxylase. 2. The non-enzymic reaction is not specific for l-phenylalanine. 3. m-Tyrosine, o-tyrosine and dihydroxyphenylalanines are formed in addition to p-tyrosine; their chromatographic separation and assay are described. 4. l-[(14)C]Phenylalanine as purchased or soon after purification contains p- and m-tyrosine, both of which can cause errors in the assay of phenylalanine hydroxylase. 5. Catalase prevents the non-enzymic hydroxylation. Thiol compounds in low concentrations stimulate the reaction but in high concentrations are inhibitory. Fe(2+) and metal complexing agents have small stimulatory effects. 6. The mechanism of the non-enzymic reaction and its possible relation to the enzymic hydroxylation of phenylalanine are discussed; it is suggested that phenylalanine is attacked by a peroxide of the cofactor.
D000388:Air; D002247:Carbon Isotopes; D002374:Catalase; D055598:Chemical Phenomena; D002621:Chemistry; D002854:Chromatography, Paper; D003067:Coenzymes; D003486:Cyanides; D004295:Dihydroxyphenylalanine; D004492:Edetic Acid; D006861:Hydrogen Peroxide; D006900:Hydroxylation; D007501:Iron; D010545:Peroxides; D010649:Phenylalanine; D010651:Phenylalanine Hydroxylase; D011621:Pteridines; D013438:Sulfhydryl Compounds; D014443:Tyrosine
D002247:Carbon Isotopes; D003067:Coenzymes; D003486:Cyanides; D010545:Peroxides; D011621:Pteridines; D013438:Sulfhydryl Compounds; D014443:Tyrosine; D010649:Phenylalanine; D004295:Dihydroxyphenylalanine; D004492:Edetic Acid; D006861:Hydrogen Peroxide; D007501:Iron; D002374:Catalase; D010651:Phenylalanine Hydroxylase