Kinetics and mechanism of the peroxidase-catalyzed iodination of tyrosine.

The kinetics of iodination of tyrosine by hydrogen peroxide and iodide, catalyzed by both horseradish peroxidase (HRP) and lactoperoxidase (LPO), were studied. The initial rates of formation of both molecular I2 and monoiodotyrosine (MIT) were measured with stopped flow techniques. The following reactions occur in both systems. Enzymatic: FeIII + H2O2-->Fev = O + H2O; Fev = O + I(-)-->FeIII-O-I-; FeIII-O-I- + H(+)-->FeIII + HOI; FeIII-O-I- + I- + H(+)-->FeIII + I2 + HO-. Iodine equilibria: I2 + I-<-->I3-; I2 + H2O<-->HOI + I- + H+. Nonenzymatic iodination, one or both of the following: Tyr + HOI-->MIT + H2O; Tyr + I2-->MIT + I- + H+, where FeIII is native peroxidase, Fev = O is compound I and Tyr is tyrosine. The big difference in the two systems is that the following reaction also occurs with LPO: FeIII-O-I- + Tyr-->MIT + FeIII + HO-, which is the dominant mechanism of iodination for the mammalian enzyme. The overall rate of formation of MIT is about 10 times faster for LPO compared to HRP under comparable conditions. A small decrease in rate occurs when D-tyrosine is substituted for L-tyrosine in the LPO reaction. Thus LPO has a tyrosine binding site near the heme. A kinetically controlled maximum is observed in I3- concentration. Once equilibrium is established, I2 is the dominant form of inorganic iodine in solution. However, hypoiodous acid may be the inorganic iodination reagent.