Biochemistry

3 Figure 10.2 - Lipoprotein Model 1 - cholesterol esters; 2 - triacylglycerols; 3 - phospholipids; 4 - cholesterol;5 - polar layer Cholesterol enters the human gastro-intestinal tract predominantly with egg yolk, meat, liver, and brains. With food, a person receives daily 0.1-0.3 g of cholesterol in free form or in the form of its esters. The last-mentioned, with the participation of the enzyme pancreatic juice, cholesterol esterase, are splitted into cholesterol, which can be absorbed as a complex with bile acids, and fatty acids. Phospholipids, in particular lecithins, under the influence of the corresponding hydrolases, are split into glycerol, higher fatty acids, choline and phosphoric acid. The components of phospholipids are absorbed by the intestinal wall and enter the blood (phosphoric acid mainly in the form of sodium and potassium salts). From the products of hydrolysis of food lipids in the cells of the intestinal epithelium, lipids specific for this species of animal are resynthed (Fig. 10.3). An important role in the metabolism of fat belongs to the liver. Its enzymatic systems catalyze the vast majority of lipid metabolism reactions. In the liver, triacylglycerols are synthesized, which either linger in it, or in the form of lipoproteins enter the blood. There are two types of lipoproteins formed in the liver: pre-0-lipoproteins (very low density lipoproteins - VLDL) and a-lipoproteins (high density lipoproteins - HDL). In human plasma, fl-lipoproteins are formed from pre-p- lipoproteins (low density lipoproteins - LDL). 193