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.
González Pondal
M
M
CONICET, Buenos Aires, Argentina.
Paier
B
B
Noli
M I
MI
Tudino
M B
MB
Stripeikis
J D
JD
Hagmüller
K
K
Zaninovich
A A
AA
D000704:Analysis of Variance; D000818:Animals; D002104:Cadmium / Q000494:pharmacology*; D008099:Liver / Q000378:metabolism*; D008297:Male; D051381:Rats; D017208:Rats, Wistar; D013431:Sulfates / Q000494:pharmacology*; D013974:Thyroxine / Q000378:metabolism*; D014284:Triiodothyronine / Q000097:blood; D017967:Zinc Compounds / Q000494:pharmacology*; D019287:Zinc Sulfate
D013431:Sulfates; D017967:Zinc Compounds; D002104:Cadmium; D014284:Triiodothyronine; D019287:Zinc Sulfate; D013974:Thyroxine