Observations on the Nature of the Heat-Resistant Thyroxine Deiodinating System of Rat Liver

Publisher Website
Google Scholar
Abstract
Experiments were performed to investigate the nature of the heat-resistant deiodinating mechanism which is sometimes observed in preparations of rat liver. As shown previously, deiodinating activity in homogenates of rat liver persisted after, or was enhanced by, boiling. In contrast, the activity in slices prepared by standard procedures (i.e., suspended in KRP (Krebs-Ringer Phosphate Buffer prior to their transfer to the incubation medium) was almost completely inhibited by heat. This loss of heat-resistant deiodinating activity in slices appeared to be due to the loss of diffusible components during preparation, since slices placed immediately after slicing into the KRPG (Krebs-Ringer phosphate glucose buffer) in which they were to be incubated with T4 demonstrated heat resistance. The compounds involved in the heat-stable deiodinating mechanism diffused rapidly from the tissue and were also dialyzable; heat-resistant activity was restored to leached or dialyzed slices when they were resuspended for incubation with T4 either in KRPG previously exposed to unleached sUces or in KRPG containing the dialyzable factors from unleached slices. In unboiled systems, deiodinating activity was reduced by leaching or dialysis. Deiodinating activity was also observed in the leaching medium, but not in the dialysate, incubated without slices. It was therefore concluded that rat liver contains at least 2 mechanisms for the deiodination of T4. One is heat resistant and is probably not physiological. The other is destroyed by heat and thus may represent enzymic deiodination. It is not clear to what extent the heat-resistant deiodinating system contributes to the total activity observed in the unboiled, unleached slice system and in homogenates. It is apparent, therefore, that the physiological significance of T4-deiodination studies performed in vitro must be considered with caution.