STIMULATION OF THIOURACIL BINDING AND IODINATION SYSTEM IN BEEF THYROID MICROSOMES

1. 1. It has been shown previously by many investigators that beef thyroid microsomal fractions enzymically iodinate in vitro in the presence of glucose and glucose oxidase. [14C]Thiouracil has now been found to be maximally bound to microsomal fractions under these same conditions. Both iodination and thiouracil binding are inhbited in a very similar pattern by thiols (mercaptoethanol, glutathione) as well as thioureylenes (1-methyl-2-mercaptoimidazole, thiourea). 2. 2. Both iodination and thiouracil binding still occur if glucose oxidase is replaced by 0.1 mM H2O2. 3. 3. Thiouracil has been shown previously1 to react rapidly and at low concentrations with the sulfenyl iodide group in a model protein. This reaction of model sulfenyl iodide was inhibited by specific thiols and thioureylenes at concentrations comparable in all respect to that effective in the thyroidal system. 4. 4. After I3-pretreatment of the microsimal fraction the subsequent uptake of [14C]thiouracil was increased, consistent with previously reported data on model sulfenyl iodides. The relative inhibition by thiols and thioureylenes of this [14C]-thiouracil binding by iodine pretreated microsomal fractions was also characteristic of the sulfenyl iodide group. 5. 5. These data provide additional support for the hypothesis1-3 that a sulfenyl iodide group takes part in the iodinating mechanism of the thyroid gland. © 1968.