The formation of sulfenyl iodides as intermediates during the in vitro iodination of tyrosine by calf thyroid homogenates

Abstract

Following dialysis against chlorinated water, the soluble protein fraction of calf thyroid homogenates readily iodinated tyrosine when supplemented with iodide at 37 °C. Addition of iodide to an active preparation also resulted in a rapid increase in optical density at 355 mμ, characteristic of sulfenyl iodide formation. The thyroidal sulfenyl iodide was stable for long periods at 5 °C, but at 20 °C or 37 °C it disappeared with the formation of monoiodotyrosine (MIT) and diiodotyrosine (DIT). The decomposition of the reactive thyroidal species in the presence of 2-mercaptoethanol was characteristic of the behaviour of sulfenyl iodides but not I3−. Excess iodide increased the stability of the sulfenyl iodide but inhibited the iodination of tyrosine. Thiourea inhibited both sulfenyl iodide formation and iodination.Evidence is presented that the chlorine activation of thyroid protein involved sulfenyl chloride formation. During dialysis of thyroid protein against 36Cl-chlorinated water a significant proportion of the 36Cl became bound to protein. Protein-bound and nonprotein-bound 36Cl were separated by filtration through Sephadex G-50. Incubation of the protein-bound fraction with 131I-iodide resulted in displacement of 36Cl by 131I and the formation of 131I-MIT and 131I-DIT. These studies support the hypothesis that thyroidal iodotyrosine formation may involve sulfenyl iodide intermediates.