Mechanism of enzymatic and non-enzymatic tyrosine iodination Inhibition by excess hydrogen peroxide and/or iodide

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Abstract
Non-enzymatic (I2-mediated) and lactoperoxidase [EC 1.11.1.7]-catalyzed iodination of tyrosine are inhibited by excess I- and/or H2O2. This phenomenon is a consequence of the concentration-dependent dual role of I- and H2O2 in the iodinating system. I- and H2O2, in addition to their function as primary substrates of peroxidase, may act as alternative iodine acceptors and therefore compete with tyrosine for the active iodinating agent, irrespective of whether this compound is an enzyme-associated iodinium cation (E .cntdot. I.delta.+) or an equivalent oxidized iodine species (IOH, ICI, I2). The competitive reaction pathways resulting from excess I- and/or H2O2 in the iodination system are I2/I3- generation and/or pseudo-catalatic degradation of H2O2, respectively. I2 (and alternative medium-dependent oxidized iodine species such as IOH and ICI) generated in the iodination system may play an important role as iodinating agent(s). They serve as a substitute for the enzyme-bound iodinium species (E .cntdot. I.delta.+), if the prevailing I- concentration favors this pathway. The proposed mechanism of the various antagonistic and interactive reaction pathways is summarized in a scheme. [These results may be useful in studying thyroid hormone biosynthesis.].