Lipolysis // is the breakdown of lipids and involves hydrolysis of triglycerides into glycerol and free fatty acids. The following hormones induce lipolysis: epinephrine, norepinephrine, ghrelin, growth hormone, testosterone, and cortisol. These trigger 7TM receptors (G protein-coupled receptors), which activate adenylate cyclase. This results in increased production of cAMP, which activates protein kinase A, which subsequently activates lipases found in adipose tissue.
"Glucagon increases blood glucose and causes breakdown of fat and protein. It acts on specific G protein-coupled receptors to stimulate adenylyl cyclase, and its actions are somewhat similar to beta-adrenoceptor-mediated actions of adrenaline. Unlike adrenalin, however, its metabolic effects are more pronounced than its cardiovascular actions. Glucagon is proportionately more active on liver, while the metabolic actions of adrenalin are more pronounced on muscle and fat" (Rang and Dale's Pharmacology, 2015).
Triglycerides are transported through the blood to appropriate tissues (adipose, muscle, etc.) by lipoproteins such as VLDL (Very-Low-Density-Lipoproteins). Triglycerides present on the VLDL undergo lipolysis by the cellular lipases of target tissues, which yields glycerol and free fatty acids. Free fatty acids released into the blood are then available for cellular uptake. Free fatty acids not immediately taken up by cells may bind to albumin for transport to surrounding tissues that require energy. Serum albumin is the major carrier of free fatty acids in the blood. The glycerol also enters the bloodstream and is absorbed by the liver or kidney where it is converted to glycerol 3-phosphate by the enzyme glycerol kinase. Hepatic glycerol 3-phosphate is converted mostly into dihydroxyacetonephosphate (DHAP) and then glyceraldehyde 3-phosphate (GA3P) to rejoin the glycolysis and gluconeogenesis pathway.
While lipolysis is triglyceride hydrolysis (the process by which triglycerides are broken down), esterification is the process by which triglycerides are formed. Esterification and lipolysis are, in essence, reversals of one another.
- Michael W King, PhD. "Oxidation of Fatty Acids". themedicalbiochemistrypage.org, LLC. Retrieved 9 April 2012.
- Tom Brody, Nutritional Biochemistry, (Academic Press, 2nd edition 1999), 215-216. ISBN 0121348369
- Baldwin, Kenneth David Sutherland; Brooks, George H.; Fahey, Thomas D. (2005). Exercise physiology: human bioenergetics and its applications. New York: McGraw-Hill. ISBN 0-07-255642-0.