The kinetics of type III iodothyronine deiodinase (5-D) in rat placenta and brain and the role of phospholipids in enzyme activity were determined. Pregnant Sprague-Dawley rats were given either vehicle (control group) or T
4 (15μg/100g bw/day; hyperthyroid group) from the 14th to the 21st day of gestation. Mitochondrial-microsomal fractions of the placenta and brain were used as the source of T
4 5-D. Placental T
4 5-D activity in the hyperthyroid group was increased when determined at 13nM T
4, but it was not significantly different from the control value when assayed at 1.3μM T
4. In contrast, T
4 5-D in the brain was significantly increased in the hyperthyroid group regardless of the substrate concentration. Hyperthyroid rats showed decreased Km for placental 5-D and increased Vmax for brain 5-D. CM-Sephadex chromatography of solubilized placental microsomes was performed to determine whether phospholipids cause a reduction in the Km of placental 5-D in hyperthyroid rats. T
3 5-D activity was undetectable unless protein-containing fractions were combined with phospholipid-containing fractions in the reaction mixture. Kinetic studies revealed that phospholipids had no effects on either Km or Vmax of placental T
3 5-D. These data indicate that 5-D activity in the rat placenta is increased in hyperthyroidism with different kinetic changes from those in the brain, and that phospholipids have no effects on the kinetic parameters of placental 5-D whereas they are essential for the enzyme activity.
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