El cinc inhibe la conversión de tiroxina en triiodotironina en grasa parda in vitro. / [Zinc inhibits the in vitro conversion of thyroxine to triiodothyronine in brown adipose issue]

The effect of in vitro addition of zinc sulphate on T4 deiodination in brown adipose tissue (BAT) of rats exposed to 4 degrees C or 22 degrees C temperature during 24 h, was studied. Animals were killed by cervical dislocation and BAT was immediately removed and homogenized in sucrose buffer (320 mM) containing HEPES (10 mM) pH 7.4. The preparation was centrifuged at 4 degrees C during 10 min. Aliquots were separated adding 50, 100 microM, 1 o 5 mM zinc sulphate plus 0, 5, 10 or 25 mM dithiothreitol plus 1 microCi 125I-T4. The mixture was incubated at 37 degrees C during 60 min. Aliquots were applied to Whatman paper and chromatographed. BAT from control rats kept at 22 degrees C produced 79 +/- 30 pg T3/g protein/h. This value was significantly reduced in homogenates containing 1 or 5 mM zinc. In rats exposed to 4 degrees C, T3 production increased to 248 +/- 37 pg/mg protein/h. The addition of 100 microM, 1 or 5 mM zinc significantly decreased T3 production. The inhibitory action of zinc on BAT T4 deiodination may have a deleterious effect on BAT thermogenesis.

Changes induced by zinc on thyroxine deiodination by rat liver in vivo and in vitro

Se estudió el efecto de la administración in vivo o del agregado in vitro de zinc sobre la deiodinación 5de la tiroxina (T4) por el hígado de rata y sobre la concentración hepática de grupos sulfhidrilos libres (NPSH). Se usaron ratas Wistar macho de 200-240g de peso corporal. A un grupo de 12 ratas se les inyectó i.p. sulfato de zinc 2mg/Kg de peso, 24h antes de iniciar el estudio. Se sacrificaron los animales por dislocación cervical y el hígado fue inmediatamente homogeneizado. Se agregó a los homogenatos dithithreitol (DTT) (0,2.5,5 o 10mM concentración final) y 1ACi de 125I-T4. Para los estudios in vitro en animales sin tratar, se agregó al homogenato de hígado sulfato de zinc o cloruro de cadmio (2.5 o 5mM) más DTT y T4 marcada. Todos los homogenatos fueron incubados durante 90 min a 37ºC y luego cromatografiados en papel Whatman 1. Las ratas inyectadas con zinc tuvieron una disminución significativa (p<0.01) de la deiodinación de T4, de la producción de 125 iodo (P<0.02) y de triiodotironina (T3) (P<0.05). En los estudios in vitro, el agregado de zinc o cadmio disminuyó significativamente la degradación de T4 (P<0.02) y la producción de iodo (P<0.02 para el zinc y P<0.05 para el cadmio) y de T3 (P<0.05). La concentración hepática de NPSH en los animales inyectados con zinc fue normal. La concentración sérica de T4 y T3 en los animales inyectados con zinc fue normal pero en los inyectados con cadmio se redujo significativamente (P<0.01 para T4 y P<0.02 para T3). Los resultados indican que el zinc inhibe la actividad de la 5-deioidnasa hepática, por um mecanismo probablemente relacionado con la unión del metal a los grupos sulfhidrilos de la enzima (AU)

Changes induced by zinc on thyroxine deiodination by rat liver in vivo and in vitro.

The effects of in vivo administration or in vitro addition of zinc on 5-deiodination of thyroxine (T4) and the concentration of nonprotein sulfhydryl groups (NPSH) in rat liver were studied in 200-240 g body weight male Wistar rats. Twelve rats were injected i.p. with zinc sulphate 2 mg/kg body weight 24 h before the experiments were started. Animals were killed by cervical dislocation and the liver was immediately removed and homogenized. Dithiothreitol (DTT) (0, 2.5, 5 or 10 mM, final concentration) and 1 microCi 125I-T4 were added to homogenates. For the in vitro studies, animals were killed by cervical dislocation and the liver removed and added zinc or cadmium (2.5 or 5 mM) plus DTT and labelled T4. Homogenates were incubated for 90 min at 37 degrees C and thereafter chromatographed in Whatman 1 paper. Zinc-injected rats had a significant (P < 0.01) decrease in T4 deiodination and in the generation of iodine (P < 0.02) and T3 (P < 0.05). In the in vitro studies, both zinc and cadmium reduced (P < 0.02) the deiodination of T4, and the generation of iodine (P < 0.02) for zinc and P < 0.05 for cadmium) as well as the generation of T3 (p < 0.05). The NPSH in zinc-injected rats were within normal levels. Serum T4 and T3 in zinc-treated rats were normal, whereas in cadmium-treated rats were both significantly decreased (P < 0.01 for T4 and P < 0.02 for T3). The data indicate that zinc blocks the activity of liver 5-deiodinase through a mechanism probably related to its binding to the sulfhydryl groups of the enzyme.