Type II Iodothyronine 5'-Deiodination by Human and Rat Placenta in Vitro*

T4 and rT3 5'-deiodination were measured in human and rat placental homogenates. rT3 5'-deiodination rates were reduced 5-40% by 1 mM 6-n-propylthiouracil (PTU), but T4 5'-deiodination was not affected by 1 mM PTU. In human tissue, homogenates of the chorionic membrane decidua had greater PTU-insensitive activity than homogenates of trophoblastic tissue or amnion. In the rat placenta, basal zone homogenates had greater activity than homogenates of placental labyrinth or amnion. The distribution of 5'-deiodinating activity in placental structures differed from that of T3 tyrosyl ring deiodinating activity in both species. PTU-insensitive rT3 5'-deiodinating activity was enriched in particulate subcellular fractions, but not in cytosol. Kinetic studies of T4 5'-deiodination using particulate fractions from human chorionic membrane decidua and rat basal zone showed a sequential kinetic pattern when the dithiothreitol concentration was varied, with apparent Michaelis constants (Km) and 50% inhibitory concentrations for T4 of 2.1-3.2 nM, and for rT3 of 9-15 nM. Thus, PTU-insensitive iodothyronine 5'-deiodination in rat and human placenta is very similar to, or the same as, that termed type II activity in rat anterior pituitary, central nervous system, and brown adipose tissue. These results extend the range of tissues in which type II iodothyronine 5'-deiodination has been identified, and strengthen the applicability to man of studies of iodothyronine deiodination in the rat.