Evidence for the kidney as an important source of 5'-monodeiodination activity and stimulation by somatostatin in Oreochromis niloticus L.

Radioimmunoassay of 3,5,3'-triiodothyronine (T3) and thyroxine (T4) in the thyroidal region of mature male Oreochromis niloticus revealed stores of T4 but negligible levels of T3, yielding a very low T3/T4 ratio (0.3%). 5'-Deiodination (5-D) of T4 into T3 was examined in liver and kidney homogenates in vitro by radioimmunoassay of T3 with T4 as substrate. In both organs, the 5'-D activity was temperature dependent: at 4 degrees C, T3 production was below the level of detection and maximal in both tissues at 37 degrees C; and at 45 degrees C, the enzymatic activity was reduced. T3 production seemed to reach a plateau after 60 min of incubation. The reaction required exogenous thiol cofactor (dithiothreitol) and was inhibited partially or completely by propylthiouracil depending on the concentrations used. Hepatic and renal 5'-D activities were stimulated by somatostatin (SRIF) within 4 hr, but a subsequent increase in plasma T3 was observed only when SRIF was injected together with T4, while the magnitude of rT3 production decreased. It is concluded that almost all the circulating T3 is provided by peripheral T4 to T3 conversion since T3 RIA in thyroidal follicles demonstrated insignificant T3 production. The kidney may contain the large part of the functional deiodinase which converts T4 into T3. As in mammals and unlike in other fishes, there is not only 5'-D activity, but also 5-D activity, and both may be influenced by SRIF.