Rat liver microsomal UDP-glucuronosyltransferase activity toward thyroxine: characterization, induction, and form...

Area of Science:

• Biochemistry
• Pharmacology
• Toxicology

Background:

• Thyroxine (T4) is a crucial thyroid hormone.
• UDP-glucuronosyltransferase (UDP-GT) enzymes are key in drug and xenobiotic metabolism.
• Glucuronidation is a major pathway for T4 deactivation in the liver.

Purpose of the Study:

• To characterize in vitro T4 UDP-GT activity in rat liver microsomes.
• To investigate if T4 glucuronidation is mediated by a specific UDP-GT form.
• To determine if microsomal enzyme inducers can enhance T4 glucuronidation.

Main Methods:

• Assessed T4 UDP-GT activity in hepatic microsomes from Wistar and Gunn rats.
• Administered four classes of UDP-GT inducers (phenobarbital, 3-methylcholanthrene, pregnenolone-16 alpha-carbonitrile, clofibrate) to Sprague-Dawley rats.
• Measured changes in T4 glucuronidation rates per milligram of microsomal protein and per kilogram of body weight.

Main Results:

• Gunn rats, deficient in certain UDP-GT forms, showed significantly lower T4 glucuronidation rates compared to Wistar rats.
• All four tested UDP-GT inducers (PB, 3MC, PCN, CLO) substantially increased T4 UDP-GT activity.
• Induction effects varied between per-milligram-protein and per-kilogram-body-weight measurements, but all showed significant increases.

Conclusions:

• T4 glucuronidation is mediated by multiple UDP-GT forms, not a single specific isoform.
• The deactivation of T4 can be significantly influenced by UDP-GT enzyme induction.
• These findings have implications for understanding T4 metabolism and drug interactions affecting thyroid hormone levels.