SUZUKI, M., YOSHIDA, K., SAKURADA, T., KITAOKA, H., KAISE, K., KAISE, N., FUKAZAWA, H., YAMAMOTO, M., SAITO, S. and YOSHINAGA, K.
Monodeiodination of Thyroxine to 3, 3', 5'-Triiodothyronine in the Human Placenta. Tohoku J. exp. Med., 1983,
140 (3), 311-318-We studied the characteristics of monodeiodination of thyroxine to T
3 and reverse T
3 in the human placenta which was obtained at normal delivery. The placentas were homogenized in cold sucrose Tris-HCl buffer, pH 7.5. The microsomal fraction was incubated at 37°C in the air for 1hr with 2μg of T
4 in the presence of 0.05M DTT. The T
3 and reverse T
3 generated in the reaction mixture were extracted into cold ethanol and measured by RIA. Among the usual subcellular fractions of the placental homogenate, microsomas were most potent in deiodinating T
4 to reverse T
3, 17.9ng/mg protein/μg T
4/60min. In microsome, production of reverse T
3 from T
4 was dependent upon protein concentration, incubation temperature, incubation time, pH and T
4 concentration, and unstable to prior heating of the microsomal fraction. The production of T
3 from T
4 was negligible in the present system. Degradation of T
3 in the human placenta was rapid. Although addition of anti-T
3 antibody to the reaction mixture suppressed the degradation of T
3, it had no effect on the net production of T
3, suggesting that the obtained net T
3 production rate had not been influenced by its degradation. Degradation of reverse T
3 was negligible. These results indicate that the human placenta actively deiodinates T
4 to reverse T
3 enzymatically. This enzyme system might have some influence on the transplacental passage of thyroid hormone from the mother to the fetus.
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