CHARACTERIZATION AND POSTNATAL-DEVELOPMENT OF 5'-DEIODINASE ACTIVITY IN GOAT PERIRENAL FAT

The capacity to generate the active thyroid hormone 3,3',5-triiodothyr onine (T-3) from thyroxine (T-4) has been assessed in the major adipos e tissue depot (perirenal) of newborn goats through measurements of io dothyronine 5'-deiodinase activity. High activity of the enzyme was fo und in the perirenal adipose tissue. Developmental studies demonstrate d that there was a rise, then a fall, in iodothyronine 5'-deiodinase a ctivity (per milligram of protein) over the first week of postnatal li fe, 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 ne wborn; however, because of age-related increases in the lipid content of the perirenal adipose tissue, the activity per cell (per mu g of DN A) at 3 wk of age was similar to 10% of the activity in the newborn. K inetic studies and inhibition characteristics with propylthiouracil, g old thioglucose, and T-4 indicated that the iodothyronine 5'-deiodinas e in goat perirenal adipose tissue is of the type I form; there was no evidence for the type II enzyme. Affinity labeling of the iodothyroni ne 5'-deiodinase with bromoacetyl-rT(3) or bromoacetyl-T-4 was consist ent with this view. In contrast to goats, only the type II enzyme is e vident in rats and mice. It is concluded that the perirenal (brown) ad ipose tissue is likely to be a significant source of T-3 for other tis sues in newborn goats and that there are major species variations in t he form of iodothyronine 5'-deiodinase present in brown fat, with the goat exclusively exhibiting type I.