Preparation of radioactively labelled isodityrosine

Oxidation of L‐[U‐ 14 C]tyrosine with 2.7 molar equivalents of alkaline hexacyanoferrate(III) yielded at least 12 chromatographically mobile oxidation products and a large amount of immobile material. Use of 0.23 molar equivalents of hexacyanoferrate(III) yielded [ 14 C]dityrosine (∼1.3% of added [ 14 C]tyrosine) and [ 14 C]isodityrosine (∼0.6%). A chromatographic method is described for the isolation of these two products from the mixture. A method is also described for the prepration of [ 3 H]isodityrosine from non‐radioactive isodityrosine by catalytic exchange with 3 H 2 . The [ 3 H]isodityrosine formed was essentially stable in 6M HCI at 110°C, indicating that the tritiation occurred at the benzylic groups.