Lipolysis

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Example of a triglyceride in fat. The fatty acid residue marked blue is saturated, the green marked is single, the red marked triple unsaturated . In the center, the triple esterified glycerine can be seen in black .
A diglyceride, here with a saturated fatty acid residue ( marked in blue ) and an unsaturated fatty acid residue ( marked in green ).
A monoglyceride, here with a saturated fatty acid residue ( marked in blue ).

Lipolysis ( English lipolysis , from Greek lipos , fat and lysis , dissolution ) is the hydrolytic splitting of saponifiable lipids , i.e. H. of triglycerides ( fats ) and Cholesterinestern by enzymes from the group of lipases . In addition to mono- and diglycerides as intermediate products, the breakdown produces free fatty acids that are released into the blood , as well as one of the alcohols glycerol or cholesterol . In animal organisms, lipolysis occurs primarily during the mobilization of depot fat from the adipocytes of the adipose tissue , and to a lesser extent during fat digestion , the enzyme pancreatic lipase being used. The fatty acids formed are broken down further by means of the so-called β-oxidation , generating energy. The process of lipolysis is reversible; In lipogenesis, organisms can synthesize fatty acids from carbohydrates such as glucose, and then fats.

Localization

About 15% of the ester bonds absorbed by the triacylglycerides are broken down in the stomach, the majority in the intestine, but mainly monoacylglycerides and fatty acids are formed. The remaining Monoester overlap with longer chain fatty acids to micelles together, which passively through the cell membranes in the intestinal mucosa ( mucosa diffuse). Here the fatty acids, mono- and diacylglycerides are converted back into fats and stored together with cholesterol and cholesterol esters , phospholipids and lipoproteins to form the chylomicrons . These represent the actual form of transport for lipids; This transports the lipids taken in from food mainly into the fat cells, the so-called adipocytes , and partly also into the liver and muscle tissue. The actual lipolysis takes place mainly in the fat cells. Fats are the most important energy reserve in humans. Almost all animals use lipids as a long-term storage form and in almost all animals in the form of triglycerides, only some fish store energy for a longer period in wax , for example in the case of Escolar . Most animals have cells (adipocytes) specialized in storing fats, while some fish store their energy in muscle tissue or in the liver.

stomach

In the stomach, gastric lipases break down fats into mono- and diglycerides. Although these lipases have a pH optimum of 5-7, they also work very effectively in the strongly acidic range in the stomach; however, the proportion of the total ester cleavage is low at around 15%.

Small intestine

In the small intestine, pancreatic lipase breaks down the fats mainly into β-monoglycerides. In addition to fats, carboxylesterases and bile salt-activated lipases also completely split cholesterol esters and β-monoglycerides into fatty acids and the respective alcohol.

Adipocytes

The fat tissue serves as the body's own fat depot for energy delivery. If necessary, triglycerides are broken down into glycerine and fatty acids via a lipolytic cascade. The rate-determining enzyme is adipocyte triglyceride lipase ( ATGL ). This first splits off the first fatty acid from the triglyceride. In the second step, the diglyceride formed is cleaved by the hormone-sensitive lipase ( HSL ), which in turn releases a fatty acid, but this time with the formation of a monoglyceride. In the last step, the monoglyceride lipase ( MGL ) splits the monoglyceride and releases the last fatty acid and glycerine. The cholesterol and retinyl esters also present in adipose tissue are broken down by the HSL with the release of fatty acids.

Activation and inhibition

The control of lipolysis is closely related to the regulation of the blood sugar level , since on the one hand glucagon increases fat breakdown, but insulin inhibits it. The release of adrenaline , noradrenaline and other phenylalkylamines as well as cortisol activates lipolysis. Many drugs with so-called sympathomimetic effects, i.e. H. stimulating effect on the vegetative nervous system , act on fat breakdown, whereby the effect in the two different groups of sympathomimetics is opposite (see list). Prostaglandins , nicotinic acid and beta blockers inhibit lipolysis.

Activation of lipolysis occurs through

Inhibition of lipolysis takes place through

Lipolysis in food

Lipids as well as the enzymes that break them down are present in many foods such as milk and dairy products , oil seeds , grains , fruits and vegetables . This is why lipolysis takes place here too, which is undesirable in most foods. In cheese and raw sausage production, these processes are sometimes desirable. In addition to mono- and diesters of glycerine , short- and medium-chain fatty acids are also formed, which negatively affect the taste and smell of the food, such as the foul-smelling butyric acid . This breakdown is known as fat spoilage , whereby the fatty acids formed can also be oxidized to lipid peroxides when exposed to air . When food is heated, the lipases are structurally changed by denaturation and thus inactivated. This process is used to preserve milk, for example ( ultra-high temperature ).

literature

  • AK Lehninger: Biochemistry. 3. Edition. Springer, 2001, ISBN 3-540-41813-X .
  • G. Löffler, PE Petrides, PC Heinrich: Biochemistry & Pathobiochemistry. 8th edition. Springer, Heidelberg 2006, ISBN 3-540-32680-4 .

Individual evidence

  1. ^ A b Science Online Lexica: Entry on lipolysis in the lexicon of nutrition. Retrieved November 18, 2009.
  2. ^ Heinrich Kasper: Nutritional medicine and dietetics. "Elsevier, Urban & FischerVerlag", 2014, ISBN 978-3-437-16833-8 .
  3. ^ A b Michael I. Gurr: Lipid Biochemistry. John Wiley & Sons, 2008, ISBN 978-1-405-17270-7 , p. 93.
  4. ^ G. Löffler, PE Petrides, PC Heinrich: Biochemie & Pathobiochemie. 8th edition. Springer, Heidelberg 2006, ISBN 3-540-32680-4 , p. 1057.
  5. ^ G. Löffler, PE Petrides, PC Heinrich: Biochemie & Pathobiochemie. 8th edition. Springer, Heidelberg 2006, ISBN 3-540-32680-4 , pp. 393-395.
  6. ^ R. Zechner, PC Kienesberger, G. Haemmerle, R. Zimmermann, A. Lass: Adipose triglyceride lipase and the lipolytic catabolism of cellular fat stores. In: J Lipid Res. 50 (1), Jan 2009, pp. 3-21.
  7. ^ R. Zimmermann, JG Strauss, G. Haemmerle, G. Schoiswohl, R. Birner-Gruenberger, M. Riederer, A. Lass, G. Neuberger, F. Eisenhaber, A. Hermetter, R. Zechner: Fat mobilization in adipose tissue is promoted by adipose triglyceride lipase. In: Science . 306 (5700), Nov. 19, 2004, pp. 1383-1386.
  8. ↑ K. Ström, TE Gundersen, O. Hansson, S. Lucas, C. Fernandez, R. Blomhoff, C. Holm: Hormone-sensitive lipase (HSL) is also a retinyl ester hydrolase: evidence from mice lacking HSL. In: FASEB J . 23 (7), Jul 2009, pp. 2307-2316.

Web links

Wikibooks: Triacylglycerol Breakdown  - Learning and Teaching Materials