The mechanism of the nonenzymatic iodination of tyrosine by molecular iodine

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Abstract
Over the pH range 7-10, at very low buffer concentration, the nonenzymatic iodination of tyrosine obeys the rate law kapp=k''/[H+][I-](1+[I-]/K2) where kapp is the measured second order based upon the total initial concentrations of molecular iodine and tyrosine and K2 (units M) is the equilibrium constant for I3- .dblarw. I2 + KI-. The value of k'' is 3.5 .times. 10-8 M .cntdot. s-1. There are three plausible mechanisms that fit the experimental data. One, the simplest, is a concerted process in which hypoiodous acid attacks tyrosine with its phenolic group unionized. The other two involve the formation of an iodinated quinoid reactive intermediate species in a rapid pre-equilibrium between unionized tyrosine and either hypoiodous acid or molecular iodine. The pre-equilibrium, if it occurs, favors the initial reactants. It is followed by a slow step in which the quinoid is converted to mono-iodinated tyrosine. Positive deviations from the rate law for pH dependence indicate that some specific acid catalysis (H3O+) is occurring in the pH range 5-7. In the presence of sufficient buffer, general acid-base catalyst in the vicinity of pH 5 and carbonate acting as a general base at pH .apprx. 9.5. The nonenzymatic iodination of tyrosine occurs more rapidly as the pH is increased, in marked contrast to the peroxidase-catalyzed iodination, which has its optimum at low pH.

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