Thyroxine monodeiodination in normal human kidney tissue in vitro

Abstract

Abstract. The present study deals with thyroxine monodeiodination in normal human kidney. To allow for comparison with previous reports, the present methods are similar to those used by others in rat tissue studies. The microsomal cell fraction of normal human kidney tissue was obtained by differential ultracentrifugation. The microsomes were incubated under various conditions and the deiodination products assayed with radioimmunoassay. A type I 5'-monodeiodinase was demonstrated, pH optimum around 6.5. Competitive inhibition was observed of T3 generation from T4 by rT3 with a Km of 3.0 μm and a Ki of 4 μm. Vmax was 26.1 pmol/min/mg protein. Likewise rT3 was generated from added T4, but it was rapidly degraded, while T3 was relatively stable as is the case in rat tissue preparations. Propylthiouracil inhibited 5'-deiodination in a dose dependent fashion with complete abolishment of deiodination at propylthiouracil concentration of 10−4m. Ipodate inhibited the reaction with complete inhibition at 10−2 m. The data demonstrate that a human kidney particulate cell-fraction contained considerable amounts of T4 deiodinases, very similar to the type I deiodinase of various rat tissue, although the handling of rT3 and the inhibitory action of this iodothyronine on T4 to T3 conversion seem to be slightly different in the two species.