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Fat and Cholesterol Metabolism

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Dietary Fat Lipogenesis is not very active in people on a Western diet Lipogenic enzyme expression is down-regulated by fat consumption Most of our fat comes from the diet ~100g/day Most fat in white adipose tissue will have come from dietary fat and not de novo lipogenesis Fat is hydrophobic Problems for digestion and transport Digestive enzymes need fat to be in an emulsion Fat needs to be carried around the bloodstream within lipoproteins

Formation of Emulsions Molecules have hydrophilic and hydrophobic characteristics amphiphilic amphipathic Phospholipids in cell membranes As is the phosphatidic acid and lysolecithin in salad dressing Amphiphilic molecules act as detergents emulsify fat into tiny ‘particles’ micelles

Bilayers and Micelles In both structures, polar heads are facing the aqueous environment while the hydrophobic tails are buried in the core Micelles can also be formed using bile salts

Fat Digestion Pancreatic Lipase Fat is contained in the core of micelles that formed with bile salts Churning of dietary fat with bile salts in the intestine Chyme Emulsion Easy for lipase to interact with Pancreatic Lipase Hydrolyses fat into FA and glycerol Plus mixture of mono- and di-acyl glycerols

FIGURE 17-1 Processing of dietary lipids in vertebrates FIGURE 17-1 Processing of dietary lipids in vertebrates. Digestion and absorption of dietary lipids occur in the small intestine, and the fatty acids released from triacylglycerols are packaged and delivered to muscle and adipose tissues. The eight steps are discussed in the text.

Bile Salts Produced in the liver Made from cholesterol Cholesterol itself it not amphiphilic enough to be a detergent - needs modification by addition of polar groups Stored in the gall bladder After digestion of fat Reabsorbed and taken back to the liver Via hepatic portal vein

Bile Salts Polar groups are added to cholesterol to make it more amphiphilic The only way to get rid of cholesterol is to make them into bile salts (chol cannot be oxidised)

Undigested Fat If gall bladder is blocked by gall stones no bile salts secreted  less fat digestion  lower calorie intake Inhibitors of fat digestion as weight-loss drugs Orlistat (brand name Xenical) Listed side effects include oily spotting; orange colored oil in your stool; gas with oily discharge; an urgent need to go to the bathroom; an inability to control bowel movements, an increased number of bowel movements.

Olestra Olestra (Olean) is a fat substitute FA attached to sucrose Not attacked by lipases Olestra will passes through gut undigested Carries with it fat soluble vitamins D, E, K need to be added as supplements See anti-Olestra sites eg. www.cspinet.org/olestra/

Lipoproteins Mixture of phospholipid and Apoproteins Apoproteins role - Enzymes, Structural, Docking Different types of lipoproteins characterised by size and by types of apoproteins First lipoproteins made by intestinal cells Chylomicrons Enter lymphatic system Contains fat and cholesterol esters

FIGURE 21-39 Lipoproteins. (a) Structure of a low-density lipoprotein (LDL). Apolipoprotein B-100 (apoB-100) is one of the largest single polypeptide chains known, with 4,636 amino acid residues (Mr 513,000). One particle of LDL contains a core with about 1,500 molecules of cholesteryl esters, surrounded by a shell composed of about 500 more molecules of cholesterol, 800 molecules of phospholipids, and one molecule of apoB-100.

FIGURE 21-38 Synthesis of cholesteryl esters FIGURE 21-38 Synthesis of cholesteryl esters. Esterification converts cholesterol to an even more hydrophobic form for storage and transport.

FIGURE 21-40 Lipoproteins and lipid transport FIGURE 21-40 Lipoproteins and lipid transport. (b) Blood plasma samples collected after a fast (left) and after a high-fat meal (right). Chylomicrons produced after a fatty meal give the plasma a milky appearance.

Delivery of Fat to Tissues Insulin stimulates LPL Also increases the supply of glycerol 3-phosphate for re-esterification Chylomicrons interact with tissues through lipoprotein lipase (LPL) LPL is on the surface of cells Fat in chylomicrons hydrolysed to fatty acids and glycerol

FIGURE 21-40a Lipoproteins and lipid transport FIGURE 21-40a Lipoproteins and lipid transport. (a) Lipids are transported in the bloodstream as lipoproteins, which exist as several variants that have different functions, different protein and lipid compositions (see Tables 21-1, 21-2), and thus different densities. Dietary lipids are packaged into chylomicrons; much of their triacylglycerol content is released by lipoprotein lipase to adipose and muscle tissues during transport through capillaries. Chylomicron remnants (containing largely protein and cholesterol) are taken up by the liver. Endogenous lipids and cholesterol from the liver are delivered to adipose and muscle tissue by VLDL. Extraction of lipid from VLDL (along with loss of some apolipoproteins) gradually converts some of it to LDL, which delivers cholesterol to extrahepatic tissues or returns to the liver. The liver takes up LDL, VLDL remnants (called intermediate density lipoprotein, or IDL), and chylomicron remnants by receptormediated endocytosis. Excess cholesterol in extrahepatic tissues is transported back to the liver as HDL. In the liver, some cholesterol is converted to bile salts.

Fate of FAs and Chylomicrons Fatty acids from chylomicron after lipolysis can be: Burnt in the heart and muscle Stored in WAT (hopefully not elsewhere) Build up of fat in the muscle associated with Type 2 diabetes FAs in WAT mainly re-esterified  FAT Re-esterification needs glycerol phosphate (Glyc3P) Glyc3P is made by glycolysis & glyceroneogenesis As fatty acids stripped out, chylomicron gets smaller And more cholesterol rich Form chylomicron remnants

Liver: Import/Export Chylomicron remnants taken up by liver Endocytotic process Internal digestion of remnants Release of cholesterol into liver Liver assembles VLDL from fat and cholesterol esters The fat could have been made by lipogenesis VLDL excreted into the blood stream

VLDL & LDL - Transport of Cholesterol LPL in peripheral tissues works on VLDL just as it did on chylomicrons  VLDL becomes fat depleted Remaining particle (LDL) relatively cholesterol rich Tissues take up LDL through LDL receptor Endocytotic process like chylomicron remnants. This is how cholesterol is delivered to the tissues

FIGURE 21-42 Uptake of cholesterol by receptor-mediated endocytosis.

FIGURE 21-40a Lipoproteins and lipid transport FIGURE 21-40a Lipoproteins and lipid transport. (a) Lipids are transported in the bloodstream as lipoproteins, which exist as several variants that have different functions, different protein and lipid compositions (see Tables 21-1, 21-2), and thus different densities. Dietary lipids are packaged into chylomicrons; much of their triacylglycerol content is released by lipoprotein lipase to adipose and muscle tissues during transport through capillaries. Chylomicron remnants (containing largely protein and cholesterol) are taken up by the liver. Endogenous lipids and cholesterol from the liver are delivered to adipose and muscle tissue by VLDL. Extraction of lipid from VLDL (along with loss of some apolipoproteins) gradually converts some of it to LDL, which delivers cholesterol to extrahepatic tissues or returns to the liver. The liver takes up LDL, VLDL remnants (called intermediate density lipoprotein, or IDL), and chylomicron remnants by receptormediated endocytosis. Excess cholesterol in extrahepatic tissues is transported back to the liver as HDL. In the liver, some cholesterol is converted to bile salts.

LDL Receptors Tissues express LDL receptors ONLY if want cholesterol Nearly all of our cells can make cholesterol themselves  when cells have enough cholesterol, they will stop making cholesterol & stop expressing LDL receptor Macrophages take up LDL without control  produce foam cells  form plaques HMG-CoA reductase is the rate limiting step in making cholesterol Can be inhibited by statins

Reverse Cholesterol Transport

FIGURE 21-40a Lipoproteins and lipid transport FIGURE 21-40a Lipoproteins and lipid transport. (a) Lipids are transported in the bloodstream as lipoproteins, which exist as several variants that have different functions, different protein and lipid compositions (see Tables 21-1, 21-2), and thus different densities. Dietary lipids are packaged into chylomicrons; much of their triacylglycerol content is released by lipoprotein lipase to adipose and muscle tissues during transport through capillaries. Chylomicron remnants (containing largely protein and cholesterol) are taken up by the liver. Endogenous lipids and cholesterol from the liver are delivered to adipose and muscle tissue by VLDL. Extraction of lipid from VLDL (along with loss of some apolipoproteins) gradually converts some of it to LDL, which delivers cholesterol to extrahepatic tissues or returns to the liver. The liver takes up LDL, VLDL remnants (called intermediate density lipoprotein, or IDL), and chylomicron remnants by receptormediated endocytosis. Excess cholesterol in extrahepatic tissues is transported back to the liver as HDL. In the liver, some cholesterol is converted to bile salts.

TABLE 21-1 Major Classes of Human Plasma Lipoproteins: Some Properties

TABLE 21-2 Apolipoproteins of the Human Plasma Lipoproteins

FIGURE 21-39b Lipoproteins FIGURE 21-39b Lipoproteins. (b) Four classes of lipoproteins, visualized in the electron microscope after negative staining. Clockwise from top left: chylomicrons, 50 to 200 nm in diameter; VLDL, 28 to 70 nm; HDL, 8 to 11 nm; and LDL, 20 to 25 nm. For properties of lipoproteins, see Table 21-1.

FIGURE 21-33 Summary of cholesterol biosynthesis FIGURE 21-33 Summary of cholesterol biosynthesis. The four stages are discussed in the text. Isoprene units in squalene are set off by red dashed lines.

FIGURE 21-34 Formation of mevalonate from acetyl-CoA FIGURE 21-34 Formation of mevalonate from acetyl-CoA. The origin of C-1 and C-2 of mevalonate from acetyl-CoA is shown in pink.

FIGURE 21-44 Regulation of cholesterol formation balances synthesis with dietary uptake. Glucagon promotes phosphorylation (inactivation) of HMG-CoA reductase; insulin promotes dephosphorylation (activation). X represents unidentified metabolites of cholesterol that stimulate proteolysis of HMG-CoA reductase.

Ways to Reduce Blood Cholesterol Reduce consumption of cholesterol Less meat, dairy products But intake of cholesterol is very small vs stores Inhibit absorption of cholesterol from gut Phytosterols as competitive inhibitors?  reabsorption of bile salts by using resins that bind to bile salts liver has to make more bile salts from cholesterol Inhibit cholesterol synthesis by using “statins” which inhibit HMG-CoA reductase Consume polyunsaturated fatty acids high saturated fat results in  HDL and LDL

Cholesterol Flux Total cholesterol in body ~140g ~1g of cholesterol enters the body each day from diet But only 0.5 g absorbed ~18g bile salts secreted into gut per day and 17.5g is reabsorbed per day the net loss of bile salt is very little (~0.5 g/day) So amount absorbed = amount lost as bile salts A reduction in intake will most likely be met by an increase in endogeous choleseterol synthesis But compare the store size to the intake 140 g to 0.5 g vs carbohydrate for which the store size and intake are similar magnitude vs fat – intake (100 g) < store (15,000 g)

Importance of Cholesterol Cholesterol is important for: Steroid hormone synthesis Regulating membrane fluidity Membrane fluidity is important for: Structural integrity Receptor/enzyme activity

Membrane with Saturated FA No double bond in FA Membrane crystalline

Membrane with Unsaturated FA Unsaturated FA  Kinks Membrane is less crystalline, more fluid and more permeable

Cholesterol & Membrane Fluidity Cholesterol “fine tunes” membrane fluidity