Biochemical and ontogenic characterization of thyroxine 5'-monodeiodinase in brown adipose tissue from fetal and newborn lambs.

We characterized T4 5'-monodeiodinase (5'-MDI) and its ontogenic changes in brown adipose tissue (BAT) of fetal lambs from a gestational age of 99 days to the newborn period. BAT 5'-MDI activity in fetal or newborn lamb was markedly enhanced by dithiothreitol (DTT); a near-maximum stimulation was reached at a DTT concentration of 1 mM. The enzyme is heat-labile (inactivated when heated to 56 C for 5 min) and has a pH optimum at 6.5. In the presence of 1 mM DTT, BAT 5'-MDI was extremely sensitive to propylthiouracil (PTU): 0.04 mM PTU resulted in 94% inhibition of the T4 to T3-converting activity. An increase of DTT concentration to 20 mM in the incubation media reduced the inhibition by 0.04 mM PTU to 59%. Kinetic analysis, in the presence of 4 mM DTT, revealed a Michaelis-Menten constant (Km) of 1.43 microM for T4 and an inhibitory constant (Ki) of 22 microM for PTU. A marked and progressive rise of BAT 5'-MDI was noted from the 99-day fetus to the newborn period [6.0 vs. 94 +/- (SE) 12 pmol T3/mg protein X h]. The total sulfhydryl content in BAT varied from 3.8 to 5.4 mmol/kg wet weight equivalent, whereas nonprotein sulfhydryl content varied from 0.5 to 0.9 mmol/kg in groups of lambs between 99 days and the newborn period, without a significant change with age. These results indicate that 5'-MDI in fetal lamb BAT increases progressively between 99 days gestation and the neonatal period. Ovine fetal BAT 5'-MDI is highly sensitive to the inhibitory effect of PTU at low DTT concentration (1 mM) and is relatively insensitive to PTU only at high DTT concentration (20 mM). Finally, 5'-MDI in BAT from the fetal lamb reaches maximal activity near term and seems similar in its biochemical characteristics to the 5'-MDI of liver, kidney, or thyroid.