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

Non-enzymatic (I2-mediated) and lactoperoxidase-catalyzed iodination of tyrosine are inhibited by excess iodide (I-) and/or hydrogen peroxide (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 X I delta +) or an equivalent oxidized iodine species (IOH, IC1, I2). The competitive reaction pathways resulting from excess I- and/or H2O2 in the iodination system are I2/I-3 generation and/or pseudo-catalatic degradation of H2O2, respectively. Our results also demonstrate that I2 (and alternative medium-dependent oxidized iodine species such as IOH and IC1) 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 X I delta +), if the prevailing I- concentration favours this pathway. The proposed mechanism of the various antagonistic and interactive reaction pathways is summarized in a scheme.