The reaction of thiouracil with beta-lactoglobulin sulfenyl iodide.

Abstract

1. 1. The binding of [14C]thiouracil to β-lactoglobulin is stimulated by prior treatment of the protein with I3− or H2O2plus I−. The highest value are found when all of the thiol groups are converted into sulfenyl iodide groups prior to [14C]thiouracil treatment and when excess of the oxidant is avoided. The amount of radioactivity bound to the protein corresponds with the stoichiometry of the formation of a mixed disulfide. 2. 2. The product of the reaction of β-lactoglobulin and thiouracil is decomposed by azide, cyanide, H2O2, I−, I3−, sulfite, thiols, thiosulfate and thiourea derivatives. 3. 3. About 60% of the original thiol groups in β-lactoglobulin were recovered after treatment of the mixed disulfide solution with cyanide or mercaptoethanol. 4. 4. H2O2 alone does not stimulate subsequent thiouracil binding to β-lactoglobulin. 5. 5. The stimulation of thiouracil binding by I3− or by H2O2plus I− occurs also with ovalbumin but is not observed in proteins which do not form stable sulfenyl iodide derivatives. 6. 6[14C]Thiouracil can be used as a reagent for the detection of small quantities of sulfenyl iodide groups which have been generated in pure proteins or in tissues.