[Monodeiodination of thyroxine to 3,3', 5-triiodothyronine and 3,3', 5'-triiodothyronine in human kidney homogenate (author's transl)]

Abstract

The renal cortex, later proved by histology to be free of carcinoma, was obtained from the kidneys of 9 patients undergoing nephrectomy for hypernephroma or ureteral carcinoma. The cortex was homogenized in a cold 50mM Tris-HCl buffer, pH 7.5, and centrifuged at 800 X g. Supernatants (1.8 mg protein, referred to as "homogenate") were enriched with 2 micrograms of T4 and were incubated for varying periods at different temperatures. The T3 and rT3 formed were extracted into ethanol and measured by RIA. Reaction mixtures contained 5mM dithiothreitol, without which formation of T3 and rT3 from T4 was negligible. The production of T3 and rT3 from T4 was abolished by prior heating of the homogenate to 56 degrees C for 30 min. In fresh homogenates, the production of T3 and rT3 increased with an increased concentration of homogenate (0.2-1.8 mg protein), increased incubation temperature up to 37 degrees C, increased incubation time up to 60 min, and increased T4 concentration up to 8 micrograms/tub. T3 production from T4 was near maximal at pH 6.5 and rT3 production at pH 10. At the standard pH of 7.5, rates of net T3 and rT3 production were 58 and 45%, respectively, of those at the optimum pH. Degradation of rT3 was rapid, while degradation of T3 was negligible. Both T3 and rT3 production from T4 were inhibited in a dose dependent manner by ipodate, propylthiouracil and salicylate. The apparent Km values for monodeiodination of T4 to T3 was 10 microM. Among the usual subcellular fractions of the kidney homogenate, microsomes were most potent in deiodinating T4 to T3 and to rT3. These results indicate that the human renal cortex contains the enzymes generating T3 and rT3 from T4.