Monodeiodination of thyroxine in the maternal and fetal dog kidneys and placenta

We studied the monodeiodinating activities of the dog placenta and the pregnant dog kidney and compared them with those of the non-pregnant dog kidney and fetal kidney. Each tissue was homogenized in 50 mM Tris/HC1 buffer, pH 7.5. The homogenate (1 mg protein) was incubated with 1 microgram of T4 at 37 degrees C in the air for 15 min or 60 min in the presence of 5 mM DTT. The T3 and reverse T3 generated in the reaction mixture were extracted into cold ethanol and then measured by RIA. Sulfhydryl group content in each tissue was determined. The characteristics of monodeiodinating activity of the dog placenta were in good concordance with those of the rat and human placentas. Net reverse T3 production in the dog placenta was 1.4-13.2 (6.1 +/- 5.3, mean +/- S.D.) ng/mg protein/micrograms T4/60 min but net T3 production was negligible. Although it seemed that the T3 production/reverse T3 production ratio in the pregnant dog kidney was lower than that in the non-pregnant dog kidney, there was no statistical significance. It is conceivable that the characteristics of deiodination in the placenta are tissue specific, independent of the changes in internal conditions induced by pregnancy. In the fetal dog kidney homogenate, T3 and reverse T3 were metabolized only minimally and productions of T3 and reverse T3 were also very low. An increase in DTT concentration up to 100 mM had no effect on the production rates of these triiodothyronines . Total and non-protein SH group contents in the fetal dog kidney homogenate were about the half of those in the maternal tissue.(ABSTRACT TRUNCATED AT 250 WORDS)