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Lipolysis is the hydrolysis of lipids. Metabolically it is the breakdown of triglycerides into free fatty acids within cells. When fats are broken down for energy the process is known as beta oxidation. Ketones are produced, and are found in large quantities in ketosis (a state in metabolism occurring when the liver converts fat into fatty acids and ketone bodies which can be used by the body for energy). Lipolysis testing strips such as Ketostix are used to recognize ketosis.

The following hormones induce lipolysis: epinephrine, norepinephrine, glucagon, growth hormone and cortisol (though cortisol's actions are still unclear[1]). 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 activate lipases found in adipose tissue.

Triglycerides are transported through the blood to appropriate tissues (adipose, muscle, etc) by lipoproteins such as chylomicrons. Triglycerides present on the chylomicrons 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.[1] 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.[2] 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 mostly converted 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 essentially reversals of one another.[3]

References

  1. ^ Journal of Endocrinology (2008) 197, 189-204
  2. ^ Tom Brody, Nutritional Biochemistry, (Academic Press, 2nd edition 1999), 215-216. ISBN 0121348369
  3. ^ 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.  

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