THYROXINE DEIODINATION BY A MICROSOMAL PREPARATION REQUIRING FE++, OXYGEN, AND CYSTEINE OR GLUTATHIONE1

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Abstract
A microsomal system of liver and kidney is described which deiodinates thyroxine, and which requires Fe++ and oxygen. The activity of the system is much enhanced by addition of cysteine, reduced glutathione, or ascorbate. Deiodination by whole homogenates or by microsomal suspensions is much increased by preheating the tissue to 100[degree]C for a few minutes. Prolonged heating destroys deiodinating activity. Optimal deiodination was observed at an incubation temperature of 37[degree]. The tissue factor has been obtained in an acetone powder and in soluble form. Dialysis of homogenates against phosphate buffer activates the deiodinating enzyme and makes it thermolabile, but addition of Fe++ restores the heat stability of the dialyzed homogenate. Deiodination is inhibited by cyanide, p-hydroxymercuribenzoate, and several substances which chelate iron. The deoidinating system may belong to a class of reactions which requires Fe++, oxygen, and a reducing agent, and which opens phenolic rings. The tissue factor is thought to be enzymatic in nature because its action is catalytic, it is destroyed by pretreatment with crude trypsin, and because it has tissue specificity.