Adipose tissue, brown-type

The major types of adipose tissue are (1) white adipose tissue, which manufactures, stores, and releases lipid and (2) brown adipose tissue, which dissipates energy via uncoupled mitochondrial respiration. Obesity research includes evaluation of the activity of adrenergic receptors and their effect on adipose tissue with respect to energy storage and expenditure or thermogenesis. 

Thiolester hydrolases are present in most tissues and cell compartments. High concentrations are found in liver microsomes and in brown adipose tissue mitochondria and peroxisomes. Several acyl-CoA hydrolases have shown a close relationship to the nonspecific carboxylesterases EC 3.1.1.1. Thus, palmitoyl-CoA hydrolase purified from rat liver microsomes was found to be identical to esterase pI 6.2I6A (ES4 type). An acyl-CoA hydrolase was isolated that showed high similarity to esterase pI 6.1 [74a] [129] [130]. These few examples are further illustrations of the unsatisfying situation of the traditional classification of esterases. 

There is a remarkable and instructive exception to the general rule that respiration slows when the ATP supply is adequate. Most newborn mammals, including humans, have a type of adipose tissue called brown fat in which fuel oxidation serves not to produce ATP but to generate heat to keep the newborn warm. This specialized adipose tissue is brown because of the presence of large numbers of mitochondria and thus large amounts of... 

Type II deiodinase occurs in the brain and brown adipose tissue of rats (but not in muscle of rats), and in the brain, skeletal muscle, heart, and thyroid gland in humans (Fallud et al 1997). This enzyme catalyzes the conversion of T4 to T3. When the thyroid gland is stimulated, the type II deiodinase takes on an increased importance in the conversion of T4 to T3 (Salvatore et at., 1996). Type II deiodinase is unique among the deiodinases in that it appears to contain two selenium atoms, rather than just one. The physiological role of the enzyme is to utilize T4 acquired from the bloodstream and to convert it to T3 within the target tissue. 

Quite a few studies have been published examining potential relationships between indices of obesity or type n diabetes and the Arg64 /i3AR polymorphism (74). The basis for such studies is the expression of the /i3AR in brown adipose tissue, where receptor activation increases thermogenesis, and in white adipose tissue,... 

In extrathyroidal tissues, the thyroid hormones are catabolized in a series of deiodination, oxidative, or conjugation reactions. Deiodination of circulating T4 occurs mainly in the liver and kidney by type I deiodinases with either 5 -deiodination yielding T3 or 5-deiodination yielding inactive rTj. The production of reverse Tj is thought to be another extrathyroidal control mechanism regulating the delivery of free T3 to the tissues. Type II deiodinases carry out 5 -deiodination in the cerebral cortex and brown adipose tissues. 

Raasmaja, A., Viluksela, M., Rozman, K.K. (1996). Decreased liver type 1 3 -deiodinase and increased brown adipose tissue type II 3 -deiodinase activity in 2,3,7,8-tetrachlorobibenzo-p-dioxin (TCDD)-treated Long-Evans rats. Toxicology 114, 199-203. 

Thus the Type I 5 deiodinase activity in cerebral cortex, like that in the liver, requires an active sulfhydryl group, the carboxymethylation of which causes enzyme inactivation. In hypothyroid animals, most of the rT3 is deiodinated by the Type II pathway, since the Type I activity is reduced, and Type II activity increased several fold. Further studies with brain and brown adipose tissue microsomes have shown that the sensitivity of Type II activity to PTU is inversely related to the DTT concentration used during the assay, so that it is important to keep this factor in mind when assessing the sensitivity of a particular enzymatic activity to inhibition by this agent (15,16). Interested readers are referred to these references for a more thorough discussion of this complex area. While the two 5 deiodinase activities are quite distinct enzymatically, until such time as the protein sequences are determined, a definitive answer as to their structural similarities cannot be given. 

The other major role of selenium is in the production of the thyroid hormones (see p. 127), for which it is a component of the enzyme type I iodothyronine deiod-inase (IDi), which convertsT4 to the physiologically activeT3.AVhen there is a deficiency of selenium the ratio ofT4 T3 increases. The enzyme is found primarily in the liver and kidney and not in the thyroid of farm animals. Type II iodothyronine deio-dinase (ID2) does not contain selenium and also converts T4 toT3, but as it is imder feedback control from T4 an increase in the latter, when selenium is deficient, compounds the problem. The major enzyme in ruminants is ID and in non-ruminants ID2. A third selenium-containing enzyme, ID3, has been found in the placenta. ID is particularly important in the brown adipose tissue of newborn ruminants and releases T3 for use in other tissues. 

Two types of adipose tissue are now recognized, namely ordinary white adipose tissue and brown adipose tissue, which is found in hibernating animals and newborn infants but is very poorly defined in adult humans. Ordinary white adipose tissue acts as an energy reserve and a heat insulator, while brown adipose tissue is specialized for the production of heat instead of ATP. 

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