Biochemistry

CHAPTER 10. LIPIDS METABOLISM 10.1 LIPOLYSIS IN HUMAN ALIMENTARY TRACT Lipolysis in the human alimentary tract has several stages. This process requires lipolytic enzymes (and the appropriate conditions for their activity) and emulsifiers (detergents). According to modem classification, lipolytic enzymes belong to the group of hydrolases that catalyze the breakdown of various lipids. In the human alimentary tract, the substrates of hydrolytic breakage are triacylglycerols, phospholipids, cholesterol esters. The optimal condition for the action of lipolytic enzymes is pH 7.8r8.2. Only emulsified fats undergo hydrolytic breakage in the stomach. Such lipids are found in milk and dairy products, egg yolk, mayonnaise. All other food fats need emulsifiers that reduce surface tension and prevent fat droplets from sticking together. Emulsifiers have hydrophilic and hydrophobic groups, they surround each fat droplet in such a way that hydrophilic groups are turned to water, and hydrophobic groups - to fat. The main emulsifiers of fats in the human alimentary tract are bile salts. In the oral cavity, lipid digestion does not occur due to the lack of lipases. In the stomach, there is a slight hydrolysis of emulsified fats under the action of low- active lipase of gastric juice. The main quantity of edible fats is hydrolyzed in the small intestine by the action of lipase, which is formed in the pancreas (it is possible only after emulsification of fats). In the cavity of the intestine under the influence of small bubbles of carbon dioxide, which are abundantly released by neutralizing the hydrochloric acid of food pulp with bicarbonate of pancreatic and intestinal juices, large fat droplets are crushed into small ones. In the process of gut motility, fats are crushed into very small droplets, which are emulsified with the participation of paired bile salts and monoacylglycerols. The main role is played by bile salts (soaps) secreted with bile into the intestinal lumen. They are adsorbed on the surface of the drops of fat, forming the thinnest film on them that prevents the droplets from merging into larger drops. However, bile saltsdramatically reduce the tension on the surface of two phases - water and fat, which contributes to crushing its droplets into smaller ones. This forms a thin emulsion (particle diameter does not exceed 0.5 microns), which facilitates the enzymatic hydrolysis of fat. At the same time, bile salts activate lipase and also contribute to the absorption of higher fatty acids, forming soluble complexes with them. 189