The reaction by which thyroxine (T4) undergoes monodeiodination in the nonphenolic ring to form 3,3′,5′-triiodothyronine (reverse-T3) has been studied in rat liver. In whole homogenate the conversion of T4 to rT3 is obscured by rapid degradation of the product, whereas in cytosol accumulation of rT3 is linear for 60 min of incubation. In the cytosol system, the rate of rT3 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 rT3-forming system compared to that of T4-to-T3 conversion indicates that the former reaction is mediated by an enzyme which is distinct from that controlling 3,5,3′-triiodothyronine (T3) formation. © 1977.
Conversion of thyroxine to 3,3',5'-triiodothyronine (reverse-T3) by a soluble enzyme system of rat liver.
BUI, FRANCO;
1977
Abstract
The reaction by which thyroxine (T4) undergoes monodeiodination in the nonphenolic ring to form 3,3′,5′-triiodothyronine (reverse-T3) has been studied in rat liver. In whole homogenate the conversion of T4 to rT3 is obscured by rapid degradation of the product, whereas in cytosol accumulation of rT3 is linear for 60 min of incubation. In the cytosol system, the rate of rT3 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 rT3-forming system compared to that of T4-to-T3 conversion indicates that the former reaction is mediated by an enzyme which is distinct from that controlling 3,5,3′-triiodothyronine (T3) formation. © 1977.
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