Effect of hyperthyroidism on lipogenesis in brown adipose tissue of young rats

Fatty acid synthetic capacity, investigated both in subcellular fractions and in vivo, is very active in brown adipose tissue of room temperature-acclimated rats. In hyperthyroid animals this tissue, analogously to the liver, exhibits an increased activity of acetyl-CoA carboxylase, fatty acid synthetase and microsomal fatty acid chain elongation, this last mechanism remaining unaffected in mitochondria. An enhancement of reducing capacities of a group of cytoplasmic NADP-dependent enzymes has also been observed in brown adipose tissue of hyperthyroid rats, probably due to a greater use of NADPH in lipogenesis under these conditions. An increase in palmitate oxidation and in polyenoic fatty acids was observed in mitochondria of brown adipose tissue from hyperthyroid animals. The latter increase is related to the importance of these compounds in the regulation of membrane fluidity and probably to an increased resistance to cold in the hyperthyroid state.