Brown adipose tissue (BAT) is a specialized form of adipose tissue involved in metabolism, and in thermogenesis (heat generation). Thus, it is extremely active in some species, for example, during arousal from hibernation, in animals exposed to cold (nonshivering thermogenesis), and in heat production in the newborn. Though not a prominent tissue in humans, it is present in normal individuals. BAT is characterized by a well-developed blood supply and a high content of mitochondria and cytochromes, but low activity of ATP synthase. Metabolic emphasis is placed on oxidation of both glucose and fatty acids. Norepinephrine liberated from sympathetic nerve endings is important in increasing lipolysis in the tissue and increasing synthesis of lipoprotein lipase to enhance utilization of triacylglycerol-rich lipoproteins from the circulation. Oxidation and phosphorylation are not coupled in mitochondria of this tissue because of the presence of a thermogenic uncoupling protein, thermogenin (also called uncoupling protein 1 [UCP1]) and the phosphorylation that does occur is at the substrate level, for example, at the succinate thiokinase step and in glycolysis. Thus, oxidation produces much heat, and little free energy is trapped in ATP. Thermogeninacts as a proton conductance pathway dissipating the electrochemical potential across the mitochondrial membrane (Figure1). Recent research has shown that BAT activity is inversely related to body fat content, and thus is a potential target for the treatment of obesity and related metabolic disorders.

Fig1. Thermogenesis in brown adipose tissue. Activity of the respiratory chain results in the translocation of protons from the mitochondrial matrix into the intermembrane space. The presence of thermogenin (UCP1) in brown adipose tissue enables the protons to flow back into the matrix without passing through the F1 ATP synthase and the energy generated is dissipated as heat instead of being captured as ATP. The passage of H+ via thermogenin is inhibited by purine nucleotides when brown adipose tissue is unstimulated. Under the influence of norepinephrine, the inhibition is removed by the production of free fatty acids (FFA) and acyl-CoA. Note the dual role of acyl-CoA in both facilitating the action of thermogenin and supplying reducing equivalents for the respiratory chain. ⊕ and ⊖ signify positive or negative regulatory effects.

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مواضيع ذات صلة

Hormones Regulate Fat Mobilization

Adipose Tissue is The Main Store of Triacylglycerol in The Body

Classification of Proteins

Precipitation Reactions of Proteins

Study of Protein Structure

Imbalance in the Rate of Triacylglycerol Formation & Export Causes Fatty Liver

The Liver Plays A Central Role in Lipid Transport & Metabolism

HDL Takes Part in Both Lipoprotein Triacylglycerol & Cholesterol Metabolism

Tertiary Structure of Proteins

Secondary Structure of Proteins

Primary Structure of Proteins

Chylomicrons & Very-low Density Lipoproteins are Rapidly Catabolized