Carbon isotope effects in the studies of the mechanism of action of tyrosine phenol-lyase

12C/14C kinetic isotope effects on the hydrolytic cleavage of tyrosine to phenol and ammonium pyruvate catalyzed by Citrobacter freundii tyrosine phenol-lyase have been determined in positions 2, 3 and ring-1' of L-tyrosine. The competitive method with dual-label approach was applied with 3',5'-ring 3H as remote label. The results revealed the change of the effect on carbon atom in position 2 during the reaction course from the high normal values to low inverse values. On the other hand, the effect values on 3 and ring-1' position remained constant during the reaction course. The discussion of these results regarding the reaction mechanism is presented. (author)