Also found in: Medical.
the aggregate processes of conversion and biosynthesis of neutral fats in the animal and human body.
Lipometabolism can be divided into the following stages: the decomposition of fats entering the body through food and absorption of these fats in the gastrointestinal tract; the transformation of the absorbed products of lipid catabolism in the tissues, leading to the synthesis of fats specific for the given organism; the processes of oxidation of fatty acids, accompanied by the liberation of biologically useful energy; and the excretion of the products of lipometabolism from the body.
Fats undergo no changes in the oral cavity, since saliva contains no lipolytic enzymes. Lipolysis begins in the stomach; however, it proceeds at a slow rate here, since the lipase of gastric juice can act only on preemulsified fats, and the conditions necessary for the formation of fat emulsion are absent in the stomach. Only in very young children receiving well emulsified fats in their food (milk) can lipolysis in the stomach reach even 5 percent. Most of the fats in food undergo lysis and absorption in the upper sections of the intestine. In the small intestine the fats are hydrolyzed by a lipase secreted by the pancreas and the intestinal glands to monoglycerides and, to a lesser extent, to glycerol and fatty acids. The extent of lipolysis in the intestine depends on the amount of bile admitted to the intestine and the bile-acid content of this bile. Bile acids activate intestinal lipase and emulsify the fats, making them more accessible to the action of the lipase. In addition, the bile acids promote the absorption of free fatty acids. The absorbed fatty acids in the intestinal mucosa are used partially for the resynthesis of fats and other lipids specific for the given tissue and partially in the form of free fatty acids, which are passed into the blood. The mechanism of the synthesis of triglycerides from fatty acids is associated with the activation of the latter by the formation of their compounds with coenzyme A (CoA). The newly synthesized triglycerides, as well as the triglycerides absorbed in unlysed form and the free fatty acids, can pass from the intestinal walls into both the lymphatic system and the portal vein system. Triglycerides entering the lymphatic system through the thoracic duct pass in small portions into the general circula-tory system and can be deposited in the body’s various fat depots (for example, subcutaneous adipose tissue, the omenturn, perirenal tissue). However, most of the triglycerides and fatty acids entering the portal vein system are retained in the liver, undergoing further transformations there. In the course of intermediary metabolism in the tissues the fats are lysed to glycerol and fatty acids under the effect of tissue lipases; the further oxidation of these compounds results in the release of large amounts of the energy stored in the form of adenosinetriphosphoric acid (ATP). The oxidation of glycerol is associated with the formation of acetic acid, which, as acetyl-CoA, is involved in the tricarboxylic acid cycle. At this stage, lipometabolism crosses pathways with the metabolism of proteins and carbohydrates.
The oxidation of the higher fatty acids in human and animal tissues follows a different route. Activated higher fatty acids in the form of compounds with Co A react with carnitine, forming derivatives of carnitine that are capable of permeating the membranes of mitochondria. Within the mitochondria the fatty acids are successively oxidized, with the release of active dicarbon constituents—acetyl-CoA, which is involved in the tricarboxylic acid cycle or used in other biosynthetic reactions.
Lipometabolism is regulated by the nervous system and the hormones of the pituitary, adrenals, and gonads. By damage inflicted, for example, to the hypothalamic region of the brain, an animal can be fattened.
In plants, fats are formed from carbohydrates. This process proceeds most intensely in maturing oil-bearing seeds and fruits. When seeds germinate, the reverse process occurs: fats are lysed (with the participation of lipases) to glycerol and fatty acids, and carbohydrates are formed from the products of decomposition. Therefore, as seeds germinate, their fat content decreases and their free fatty acid content increases. Glycerol is present in sprouts in negligible amounts, since it is easily and rapidly converted to carbohydrates. Fats are converted to carbohydrates in the germinating seeds of oil-bearing plants through the glyoxylate cycle.
REFERENCESFerdman, D. L. Biokhimiia, 3rd ed. Moscow, 1966. Pages 303–19, 460, 485.
Kretovich, V. L. Osnovi biokhimii rastenii, 5th ed. Moscow, 1971.
Mahler, H., and E. Cordes. Osnovy biologicheskoi khimii. Moscow, 1970. (Translated from English.)
G. A. SOLOV’EVA