Characterization and postnatal development of 5'-deiodinase activity in goat perirenal fat.

The capacity to generate the active thyroid hormone 3,3',5-triiodothyronine (T3) from thyroxine (T4) has been assessed in the major adipose tissue depot (perirenal) of newborn goats through measurements of iodothyronine 5'-deiodinase activity. High activity of the enzyme was found in the perirenal adipose tissue. Developmental studies demonstrated that there was a rise, then a fall, in iodothyronine 5'-deiodinase activity (per milligram of protein) over the first week of postnatal life, with a further decline between the second and third weeks. At 3 wk of age, the activity per gram of tissue was only 2% of that of the newborn; however, because of age-related increases in the lipid content of the perirenal adipose tissue, the activity per cell (per microgram of DNA) at 3 wk of age was approximately 10% of the activity in the newborn. Kinetic studies and inhibition characteristics with propylthiouracil, gold thioglucose, and T4 indicated that the iodothyronine 5'-deiodinase in goat perirenal adipose tissue is of the type I form; there was no evidence for the type II enzyme. Affinity labeling of the iodothyronine 5'-deiodinase with bromoacetyl-rT3 or bromoacetyl-T4 was consistent with this view. In contrast to goats, only the type II enzyme is evident in rats and mice. It is concluded that the perirenal (brown) adipose tissue is likely to be a significant source of T3 for other tissues in newborn goats and that there are major species variations in the form of iodothyronine 5'-deiodinase present in brown fat, with the goat exclusively exhibiting type I.