The control of peroxidase-catalysed iodination and de-iodination

It has been demonstrated that the H2O2/l ratio is a critical factor in the control of iodination and de-iodination of covalently bound tyrosyl residues in proteins and free iodotyrosines by peroxidase enzymes. This has been shown for myeloperoxidase (MPO) isolated from normal human polymorphonuclear lymphocytes in particular, and also for peroxidase of animal origin such as thyroid peroxidase (TPO) and lactoperoxidase (LPO). It has been shown that the H2O2/l ratio exerts a controlling influence on MPO-catalysed reactions of fully iodinated tyrosines, e.g. di-iodotyrosine, and of partially and completely iodinated thyronines such as thyroxine and tri-iodothyronine. Using an in vivo model system it has been shown that MPO catalyses the sequential events of iodination, iodine exchange and de-iodination of tyrosines and, furthermore, that all three reactions are influenced by the rate of H2O2 generation and the iodide concentration of the reaction medium. The action of MPO on iodothyronine substrates only affects de-iodination irrespective of whether the iodothyronine is partially iodinated, as in tri-iodothyronine, or completely iodinated, as in thyroxine. This MPO-catalysed de-iodination of thyroxine and tri-iodothyronine can also be regulated by the H2O2/l ratio. Moreover, the results show that MPO-catalysed iodine exchange can only occur in completely iodinated tyrosines such as di-iodotyrosine (DIT). Iodine exchange in partially iodinated tyrosines such as mono-iodotyrosine (MIT) or in iodothyronines (T3 and T4) cannot be catalysed by MPO irrespective of the H2O2/l ratio. These results introduce a new concept which may be important in understanding the control of thyroid activity in thyroid disease and the control of MPO activity in biological defence mechanisms in man.