Analyses of catalytic intermediates of hog thyroid peroxidase during its iodinating reaction.

Abstract

The formation of Compounds I, II, and III of hog thyroid peroxidase was demonstrated in its reaction with H2O2 by spectrophotometric and kinetic methods combined with a stopped flow technique. Hog thyroid peroxidase reacted with H2O2 to form Compound I with a rate constant of 5.2 X 10(6) M-1 s-1. Compound I was then spontaneously converted to Compound II with a rate constant of 8 s-1. In the presence of 200 microM H2O2, Compound II ws further converted to Compound III (or oxyform) with a half-life of 0.8 s. During the peroxidative oxidation of tyrosine, thyroid peroxidase was present as Compound I, and the rate constant for the reaction of Compound I with tyrosine was measured at 1.4 X 10(4) M-1 s-1. The addition of iodide to the above reaction system markedly decreased the steady state concentration of Compound I. From the kinetic data, it was concluded that the reaction between Compound I and iodide occurred by way of a 2-electron transfer and the rate constant was roughly estimated at 2.1 X 10(7) M-1 s-1. This value was much higher than that obtained from overall kinetic data. Similar reactions were carried out with bovine lactoperoxidase and the kinetic data for the two enzymes were compared. The primary oxidation product of iodide was suggested to be iodinium cation (I+) in either enzyme reaction.