Zinc inhibits the in vitro conversion of thyroxine to triiodothyronine in brown adipose tissue.

The effect of in vitro addition of zinc sulphate on T-4 deiodination in bro wn adipose tissue (BAT) of rats exposed to 4 degrees C or 22 degrees C temp erature during 24 h, was studied. Animals were killed by cervical dislocati on and BAT was immediately removed and homogenized in sucrose buffer (320 m M) containing HEPES (10 mM) pH 7.4. The preparation was centrifuged at 4 de grees C during 10 min. Aliquots were separated adding 50, 100 mu M, 1 o 5 m M zinc sulphate plus 0, 5, 10 or 25 mM dithiothreitol plus 1 mu Ci I-125-T4 . The mixture was incubated at 37 degrees C during 60 min. Aliquots were ap plied to Whatman paper and chromatographed. BAT from control rats kept at 2 2 degrees C produced 79 +/- 30 pg T-3/g protein/h. This value was significa ntly reduced in homogenates containing 1 or 5 mM zinc. In rats exposed to 4 degrees C, T-3 production increased to 248 +/- 37 pg/mg protein/h. The add ition of 100 mu M, 1 or 5 mM zinc significantly decreased T-3 production. T he inhibitory action of zinc on BAT T4 deiodination may have a deleterious effect on BAT thermogenesis.