Conversion of thyroxine to 3,3′,5′-triiodothyronine (reverse-T3) by a soluble enzyme system of rat liver

The reaction by which thyroxine (T 4 ) undergoes monodeiodination in the nonphenolic ring to form 3,3′,5′-triiodothyronine (reverse-T 3 ) has been studied in rat liver. In whole homogenate the conversion of T 4 to rT 3 is obscured by rapid degradation of the product, whereas in cytosol accumulation of rT 3 is linear for 60 min of incubation. In the cytosol system, the rate of rT 3 formation is maximal at pH 8.2, is thiol-dependent, is inactivated by heat, but is not inhibited by anaerobiosis or absence of light. Propylthiouracil is a potent inhibitor of the reaction. The higher pH-optimum of the rT 3 -forming system compared to that of T 4 -to-T 3 conversion indicates that the former reaction is mediated by an enzyme which is distinct from that controlling 3,5,3′-triiodothyronine (T 3 ) formation.