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Биохимия Biochemistry (2 course)
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Лечебное дело GENERAL MEDICINE
2 course
Биохимия Biochemistry (2 course)
8-Digestion of lipids. Lipolysis, lipogenesis.
SYNOPSIS
SYNOPSIS
Fat digestion

Fat digestion (TG) occurs under the action of lipase enzymes (a class of hydrolases) that break down ester bonds.

The fat digestion does not occur in the oral cavity. It is established that the so-called lingual lipase is synthesized in the mucosa of the tongue's root, the activity of this lipase cannot manifest in the oral cavity, so it acts in the stomach. The optimum pH for this lipase is in the pH range of 4-4.5. It acts on TGs containing fatty acids (FA) with a short and medium chain length, which is typical for TG of milk. Thus, lingual lipase is important in infants. The activity of this enzyme in adults is extremely low.

The stomach synthesizes its own lipase, the value of which is insignificant in adults, since the content of this lipase in gastric juice is small. In addition, the pH of gastric juice is far from optimal (pH 5.5-7.5); there are no conditions for fat emulsification in the stomach, and lipase can only actively act on emulsified TG. Unlike adults, infants have moderate acidity in the stomach (pH 5.0), milk fat is emulsified, lipase is synthesized in sufficient quantities and is active. Thus, the fat digestion in the stomach mainly occurs in children.

The main place of TG digestion in adults is the small intestine.

Pancreatic juice lipase acts on TG. Pancreatic lipase is a glycoprotein with an optimum pH of 8-9, it enters the upper part of the small intestine in an inactive state in the form of prolipase. The transformation of prolipase into an active enzyme occurs with the participation of bile acids and colipase (a peptide with a molecular weight of 10 kDa). Colipase attaches to prolipase, which activates it and makes it resistant to the action of trypsin. Lipase breaks down emulsified TGs. Pancreatic lipase cleaves off fatty acids in the a-position. The products of fat digestion in the intestines are: glycerin, fatty acids, and monoglycerols.

The role of bile acids in the fat digestion:

Activate pancreatic lipase.
Emulsify fats.
Are necessary for the absorption of fat digestion products.
Absorption of fat digestion products:

Glycerol and short-chain fatty acids are highly soluble in water, so they are easily absorbed by the intestinal wall. For long-chain fatty acids (not soluble in water), it is necessary to create a soluble form at the beginning. To do this, fatty acids bind to bile acids (1:3), forming choleic acids, which are easily absorbed. Mono-, di- and triacylglycerols (the latter in small quantities) are absorbed in the form of cholein complexes. Inside the intestinal epithelial cells, fat resynthesis occurs mainly from monoacylglycerols and fatty acids.

Fat resynthesis is the synthesis of fat special for a given organism. Not all fatty acids can be esterified. In an unchanged form, they enter the bloodstream, where they bind to albumins and are carried by blood to organs and tissues in this form. Free fatty acids are called non-esterified or NEFA. TG formed in epithelial cells are covered with a shell of protein and phospholipid. As a result, chylomicrons (CM) are formed. CM are microscopic lipoprotein particles with a diameter of about 500 nm.

CMs consist of 1-2% protein and 98-99% lipids, of which 88% are TGs, 8% are PL (phospholipids) and 4% - cholesterol (CS). 90% of TG of food origin is transported in the form of CM, the remaining 10% is absorbed in the form of fatty acids. NEFA is transferred to the liver, where they are either oxidized or go to the synthesis of lipids. The liver is characterized by a limited ability to store TG and even a small excess of them is secreted in the form of VLDL (very low density lipoproteins). Unlike CM, VLDL contain 10% protein and 90% lipids.

Thus, CM and VLDL are transport forms. CM transport exogenous lipids, and VLDL - lipids synthesized in the liver. 70% of CM enter the lymph, and 30% - directly the blood capillaries. The "milky" blood plasma after taking greasy food is due to the presence of CM in it.

Chylomicrons' pathways:

CMs are broken down by lipoprotein lipase (LPL), which acts on the triglyceride part. This enzyme is located in the surface of endothelial cells of capillaries, especially adipose tissue, lactating mammary gland, skeletal and cardiac muscles. TGs of chylomicrons are broken down into glycerin and fatty acids. In CM, the amount of lipid lowers, the density of such a particle increases, it turns into a so-called residual CM. LPL is activated by heparin. The residual CM is transferred to the liver, where it is metabolized. The NEFA formed in this process in the heart and muscles are absorbed and oxidized.

The content of blood lipids may decrease due to their deposition in various tissues. The ability to store fat is characteristic of all tissues except the brain. The main role in lipid metabolism is played by adipose tissues and the liver. Approximately 65% of the adipose tissue weight is accounted for by TG. The amount of adipose tissue increases with age. There are white adipose tissue (subcutaneous adipose tissue and omentum) and brown adipose tissue (located in the interscapular region). White fat is a source of glycerin and fatty acids. Brown fat performs a thermogenic function, since fat burns in it with the formation of water, carbon dioxide and heat. There is a lot of brown fat in newborns and in winter-sleeping animals.

Lipolysis:

Lipolysis is the fat breakdown into glycerol and fatty acids. It is catalyzed by triglycerol lipase, diglycerol lipase and monoglycerol lipase, which act respectively on TG, DG and MG. Triglycerol lipase is a limiting enzyme of lipolysis. It has two forms: phosphorylated (active) and non-phosphorylated (inactive). The transformation of the inactive form into the active one occurs under the influence of protein kinase. Protein kinase, in turn, is activated as a result of the attachment of cAMP to its allosteric centers. The regulation of lipolysis occurs under the influence of hormones. Catecholamines, glucagon, STH, ACTH, thyroxine, pituitary lipotropin, cAMP activate lipolysis. Insulin inhibits lipolysis.

Lipogenesis:

Lipogenesis is the fat synthesis. Glycerin and fatty acids in active forms are necessary for the fat synthesis. The active form of glycerol is 3-phosphoglycerol (α-glycerophosphate). The active form of the fatty acid is acyl-CoA.

In adipose tissue, 3-phosphoglycerol is formed from DOAP by hydrogenation under the action of glycerophosphate dehydrogenase with the expenditure of NADH. Thus, the fat synthesis in adipose tissue occurs from carbohydrates. In the liver and kidneys, 3-phosphoglycerol is formed by phosphorylation of glycerol under the action of glycerol kinase with the ATP expenditure.

Acyl-CoA is formed as a result of interaction of a fatty acid with НSCоА with the ATP expenditure under the influence of acyl-CoA synthetase.

The assembly of triglyceride occurs from 3-phosphoglycerol and acyl-CoA under the action of acyltransferase, (see the course of reactions in a student's book or lecture).

CA, STH, IT, ACTH, ADP inhibit the lipogenesis. Insulin, estrogens and ADP activate.

Depending on the type of tissue where it enters, glycerin can:

1. 
   

   oxidize to CO2 and H2O to form 21 ATP (see the lecture);

   
   


2. 
   

   be used in the synthesis of lipids (TG and PL);

   
   


3. 
   

   be used for the synthesis of glucose in gluconeogenesis.

   
   

Student independent learning activities under the supervision of a lecturer

Laboratory work.

Determination of lipase activity in blood serum.

Equipment:

A tripod with test tubes.

2 ml and 0.1 ml pipettes.

Thermostat.

Reagents:

Distilled water.

Tris buffer pH 8.0.

Oil emulsion.

Phenolphthalein.

0,05 Н NaOH.

Ethanol.

Serum.

The principle of the method. An emulsion of vegetable oil is used as a substrate for lipase. The measure of the enzymatic activity is the amount of released fatty acids, which are titrated with alkali.

The work. Pour 0.25 ml of dist. water, 1.3 ml of oil emulsion and 0.1 ml of tris buffer pH 8.0 into 2 test tubes. 0.1 ml of serum is added to one test tube (experimental), the second is a control. The test tube is shaken and placed in a thermostat at 37°C for 15 minutes. During this time, 1 drop of phenolphthalein is added to the control tube and 0.05H NaOH is titrated to a weak pink staining.

The calculation of lipase activity is carried out according to the formula: (O-K)*10 = units of lipase activity in 1 ml of serum, where O is the amount of alkali that went to titrate the experimental sample, 10 is the conversion factor per 1 ml of serum.

The norm of lipase activity in serum is 0.5-1.5 units.

Diagnostic value of determining the activity of lipase:

The activity of lipase in serum can sharply increase in pancreatic diseases, especially in acute pancreatitis; chronic biliary tract disease; peptic ulcer perforating into the pancreas. The activity of lipase may decrease with acidic fibrosis of the pancreas.

Laboratory work study questions:

What is lipase? Name the localizations of lipase formation in the gastrointestinal tract.
The principle of the method for determining the activity of lipase in blood serum.
What disease is characterized by a sharp increase in lipase in the blood? Why?
When does the activity of lipase in the blood decrease?