Characterization of Thyroid Hormone 5′-Monodeiodinase Activity in the Turtle (Trachemys scripta)

Thyroid hormone metabolism by 5′-monodeiodinase enzymes (5′MD) was characterized in peripheral tissues of the turtle,Trachemys scripta,and compared with activity measured in the rat. Based on differences in pH dependence, sensitivity to inhibitors, substrate affinity, and cofactor requirements, at least two types of enzyme activities have been identified in the turtle. A 5′MD activity was measured in liver and kidney microsomal fractions that exhibits inhibition by 2n-propyl-6-thiouracil (PTU), a higher affinity for rT3(Km=2 μM) than for T4(Km=6.5 μM), a low cofactor dependence, and a high pH optimum for T4metabolism. The characteristics of this turtle low affinity T4activity correspond to the mammalian type I monodeiodinase. A second type of monodeiodinase (MD) activity that is less sensitive to PTU, has a higher affinity for T4(Km=1 nM), a higher cofactor requirement, and a lower pH optimum was colocalized with the first form. Both turtle MD activities remain active over a range of temperatures, allowing for activity at the preferred body temperature of this species (28 to 37°C compared to the 37°C optimum in the rat). Based on limited comparative data of MD systems from several fish and birds, the turtle most closely resembles avian species. Like birds, turtles possess a mammalian-like type I activity and have colocalized MD forms in the liver. However, the second turtle MD form (MDH) is not comparable to the mammalian or avian MDII-like activity. Analysis of the deiodinase products from both turtle MDs by high-performance liquid chromatography confirmed that the putative turtle MDI produces T3from T4as expected. The MDH produces rT3from T4as does the mammalian type III form, but MDH has a wider tissue distribution (kidney, liver, pancreas, heart, ovary, and brain) and distinct enzyme kinetics. Moreover, MDH activity in the turtle kidney is 100-fold higher than in the liver, indicating that the kidney may play a critical role in the metabolism of thyroid hormones in the turtle; this high renal activity distinguishes the turtle from all other vertebrates studied.