Ontogenic characterization of type I and II thyroxine 5'-monodeiodinases in brown adipose tissue from fetal and newborn rabbits.

The ontogenic changes in thyroxine 5'-monodeiodinase (5'-MD) activity were examined in brown adipose tissue (BAT) of fetal (26, 28 and 30 days of gestation) and newborn (1-14 days old) rabbits. A near-maximum stimulation of BAT 5'-MD activity was reached at 1 mM of dithiothreitol (DTT). The kinetics of 5'-MD inhibition by propylthiouracil (PTU) was uncompetitive in fetal and noncompetitive in neonatal tissue. In fetal tissue the Ki was 3.2 mM independently of DTT concentration, in newborns from 0.5 to 8.25 mM at 1-10 mM of DTT. An apparent Km constant for T4 to T3 conversion in BAT of fetal rabbit amounted to 0.2, 0.67, 1.9 and 2.0 microM in the presence of 1, 2, 5 and 10 mM DTT, respectively. In neonatal rabbits Km was constant (0.72 microM) in various DTT concentrations. The presence of 5'-MDI and 5'-MDII in BAT of fetal and neonatal rabbit was identified. Using iopanoic acid and amiodarone as inhibitors, a decrease in the 5'-MDII:5'-MDI ratio from 8.9 to 2.7 after birth was observed. Changes in 5'-MD activity with advancing age were not correlated with the SH group content in the BAT.