Oxygenation of fluorinated tyrosines by mushroom tyrosinase releases fluoride ion

The reactions of 2-fluoro- and 3-fluoro-L-tyrosine with mushroom tyrosinase have been investigated. Both fluorinated tyrosines are good substrates for tyrosinase, with Vmax and Vmax/Km values similar to those for L-tyrosine. Oxygenation of 2-fluorotyrosine is regioselective, and only 6-fluorodopa was detected by HPLC in reaction mixtures. Oxygenation of both isomers of monofluorotyrosine results in fluoride ion production in the absence of ascorbic acid; however, 2-fluorotyrosine also produces fluoride in the presence of ascorbic acid. These results are consistent with previous studies demonstrating rapid intramolecular cyclization of nascent 6-fluorodopaquinone (M.E. Rice, B. Moghaddan, C.R. Creveling, and K.L. Kirk, 1987, Anal. Chem. 59, 1534-1538), which is competitive with reduction by ascorbate, resulting in elimination of the aromatic fluorine as fluoride ion.