Characterization of thyroid hormone 5'-monodeiodinase activity in the turtle (Trachemys scripta)

Area of Science:

• Comparative endocrinology
• Biochemistry
• Molecular biology

Background:

• Thyroid hormones are crucial for vertebrate development and metabolism.
• 5'-monodeiodinase (5'MD) enzymes regulate active thyroid hormone (T3) production.
• Understanding 5'MD in reptiles like turtles provides insights into vertebrate evolution of hormone regulation.

Purpose of the Study:

• To characterize thyroid hormone 5'-monodeiodinase (5'MD) activities in the turtle, Trachemys scripta.
• To compare turtle 5'MD activities with those found in mammals (rat).
• To elucidate the specific roles and characteristics of different 5'MD enzyme types in turtles.

Main Methods:

• Enzyme activity assays using liver and kidney microsomal fractions.
• Analysis of pH dependence, inhibitor sensitivity, substrate affinity (Km), and cofactor requirements.
• High-performance liquid chromatography (HPLC) to identify deiodinase products.

Main Results:

• Two distinct 5'MD activities were identified in turtles, differing in PTU sensitivity, substrate affinity, and pH optimum.
• One activity resembles mammalian type I 5'MD, converting T4 to T3.
• A second activity (MDH) converts T4 to rT3, similar to mammalian type III, but with distinct kinetics and broader tissue distribution, notably high in the kidney.

Conclusions:

• Turtles possess both mammalian-like type I 5'MD and a unique MDH enzyme.
• The high activity and broad distribution of MDH, especially in the kidney, suggest a critical role in turtle thyroid hormone metabolism, distinguishing them from other vertebrates.
• Turtle 5'MD enzymes function across a physiological temperature range, supporting their ectothermic lifestyle.