Thyroxine degradation during lipoxidase-catalyzed peroxidation of linoleic acid.

The possibility has been examined that thyroxine may be degraded by an intermediate free radical in the oxidation of linoleate catalyzed by lipoxidase. Such a reaction has been observed and the lipid radical with which thyroxine reacts is suggested to be enzyme attached RO; proposed by Tappel (in W. 0. LUIVDBERG (Editor), A&oxidation and antioxidants, Vol. I, Interscience Publishers, New York, 1961, p. 325). A similar reaction has been inferred to occur during thyroxine degradation by peroxidizing microsomal lecithin. Although thyroxine serves as an antioxidant during autoxidation of lecithin, it was predicted that a similar reaction with enzyme-bound RO; might not influence oxidation rate or extent. The results bear out the prediction. The purpose of this and prior studies of thyroxine degradation in systems in vitro has been to investigate the mechanism of reaction of thyroxine with biological materials. In light of recent studies of the mechanism of deiodination of iodoaryl compounds, it is suggested that the lipid radical RO; might either initiate degradation by electron abstraction or react with a terminal product of thyroxine after deiodination. The enabling property of thyroxine making these reactions probable is its predicted capacity to take up, transiently hold, and yield electrons.