Types I and II iodothyronine 5'-deiodinase activities in perirenal brown adipose tissue of neonatal goats.

Abstract

The perirenal adipose tissue is the main depot in newborn goats, and it has recently been shown to represent functional fat [l]. In the rat, brown adipose tissue contains a high activity of the iodothyronine 5'deiodinase enzyme system which converts thyroxine to the active thyroid hormone, 3,3',5-triiodothyronine [see 21. Iodothyronine 5'deiodinase exists in two major forms, termed types I and 11. The type I contains a selenocysteine residue at the active site, and both enzyme activity and enzyme protein are highly sensitive to deficiency of the trace element selenium [3.4]. Although the type I1 enzyme does not contain selenocysteine, its activity is also decreased in selenium deficiency [4]. The iodothyronine 5'deiodinase in rat brown adipose tissue, which is rapidly induced on cold exposure of the animal, appears to be exclusively of the type I1 form [see 2.31. High iodothyronine 5'-deiodinase activity has been reported in brown of neonatal lambs, cattle, and rabbits, but the form of the enzyme varies in these species. Only the type I has been reported in newborn cattle [5], while both types I and I1 are present in lambs and rabbits with type I predominating in the former, and type I1 in the latter [6,7]. In the present study we have determined the iodothyronine Sdeiodinase activity of the perirenal adipose tissue of newborn goats, and examined the changes in activity during the transition of the tissue from brown to white fat. We were particularly concerned to establish the type of iodothyronine Sdeiodiase present, and whether this varies during early postnatal development. Such information is important for assessing the likely impact of selenium deficiency on brown thermogenesis and cold responsiveness in neonatal agricultural animals. Male goats, British Saanen breed, were obtained from the Rowett herd, within 12 h of birth, and at 2.5,5,7, 14 and 21 days of age (2-4 goats each). Adipose tissues were removed from five sites: perirenal, pericardial, omental, and subcutaneous (hind limb and neck regions) although only the perirenal tissue was used in the present study. The tissues were frozen in liquid nitrogen, and stored at -80°C. After thawing, the tissues were homogenized in a sucrose/HEPES/dithiothreitol buffer, pH 7.0, and centrifuged at 80 xg for 10 min at 4°C. as described previously [2]. The infranatant was collected, and stored at -70°C until required for analysis. The total iodothyronine 5'deiodinase activity was subsequently measured at 37°C. as previously [2]. High total iodothyronine 5'deiodmase activity was found in perirenal adipose tissue of newborn goats, indicating that the tissue has a substantial capacity for the conversion of thyroxine to 3.3'5-t~iiiodothyronine at birth (Table 1). The activity remained high over the first 5 days of life, but fell progressively thereafter. By 3 weeks of age the total activity (per g of tissue) was only 2% of that in the newborn (Table 1). Propylthiouracil inhibition studies showed that the major portion (-70 80%) of the total activity was due to the seleniumcontaining type I form, the remainder being type 11. Although iodothyronine 5'-deiodinase activity in the perirenal pad fell markedly over the first 3 weeks of postnatal life, the relative proportion of types I and I1 remained essentially constant. Table 1. Developmental changes in iodothyronine 5'deiodinase activity in goat penrenal adipose tissue