The Release of Fatty Acids from Adipose Tissue Is Regulated

The Release of Fatty Acids from Adipose Tissue Is Regulated 

Comparison between synthesis and degradation of fatty acids in the liver. Both processes involve two carbons at a time and very similar intermediates, even though they go in opposite directions. CoA is also heavily involved in both processes. Here the similarities end. The enzymes used in the two processes are totally different, and the coenzymes are also different. In the degradative direction FAD and NAD+ are used, whereas in the synthetic direction the coenzyme NADPH is used. Degradation occurs in the mitochondrial matrix, and synthesis occurs in the cytosol. 

Scanning electron micrograph of white adipocytes from rat adipose tissue (600 X). (Courtesy of Dr. A. Angel, University of Manitoba, and Dr. M. J. Hollenberg of the University of Toronto.) 

When certain hormones (e.g., epinephrine) bind to their receptors in adipose tissue, adenylate cyclase is activated. The cAMP that is formed activates protein kinase A, which phosphorylates triacylglycerol lipase. The phosphorylated form of this enzyme is the active species, and triacylglycerols are degraded to fatty acids. 

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The release of fatty acids from adipose tissue is regulated by the rate of hydrolysis of triacylglycerol and the rate of esterification of acyl-CoA with glycerol 3-phosphate. The rate of hydrolysis is stimulated by hormones that bind to cell-surface receptors and stimulate adenylate cyclase (which catalyzes the production of cAMP from ATP). Hormone-sensitive lipase (Sec. 13.4) can exist in two forms, one of which exhibits very low activity and a second which is phosphorylated and has high activity. Before hormonal stimulation of adenylate cyclase, the low-activity lipase predominates in the fat cell. Stimulation of protein kinase by an increase in cAMP concentration leads to phosphorylation of the low-activity lipase. An increase in the rate of hydrolysis of triacylglycerol and the release of fatty acids from the fat cell follows. This leads to a greater utilization of fatty acids by tissues such as heart, skeletal muscle, and liver. 

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