Lipid Digestion & Metabolism Part 1 COURSE: MEDICIMAL CHEMISTRY 1 COURSE CODE: 301

Lipid Digestion  Challenges  Lipids are not water soluble  Triglycerides too large to be absorbed  Digestive solution  Triglycerides mix with bile and pancreatic secretions  Emulsification and digestion

Bile  Produced in liver, stored in gallbladder  Alkaline solution composed of:  Bile salts  Cholesterol  Lecithin (phospholipids)  Bilirubin  Responsible for fat emulsification  Detergent action

Digestion of Lipid  Bile salts emulsify lipids  Pancreatic lipase acts on triglycerides  Triglycerides monoglyceride + 2 fatty acids  Pancreatic colipase  Activated by trypsin  Interacts with triglyceride and pancreatic lipase  Improves activity of pancreatic lipase

Pancreatic Colipase  Secreted from pancreas as procolipase  Activated (cleaved) by trypsin  Anchors lipase to the micelle  One colipase to one lipase (i.e., 1:1 ratio)

Emulsification  Produces small lipid spheres  Greater surface area  Lipases attack TG at 1 and 3 positions GlycerolGlycerol Fatty Acid 1 Fatty Acid 2 Fatty Acid 3 Lipase GlycerolGlycerol Fatty Acid 3 Fatty Acid 1 Fatty Acid 2 Triglyceride 2-Monoglyceride + 2 Free Fatty Acids 2 H 2 0

Digestion of Lipid  Phospholipase A 1 and A 2  Hydrolyzes fatty acids from phospholipids  Cholesterol esterase  Hydrolyzes fatty acids from cholesterol esters

Micelle Formation  Complex of lipid materials soluble in water  Contains bile salts, phospholipids & cholesterol, Bilirubin  Combines with 2-monoglycerides, free fatty acids and fat-soluble vitamins to form mixed micelles

Micelle Formation

Mixed micelle formed by bile salts, triacylglycerols and pancreatic lipase. Micelle Formation

Lipid Absorption  Mixed micelles move to intestinal mucosal cells (enterocytes) and release contents near cell  The bile salts are re-absorbed further down the gastrointestinal tract (in the ileum), transported to the liver, and finally recycled and secreted back into the digestive tract

Nutrient Absorption - Lipids  Fatty acids, 2-monoglycerides, cholesterol, and cholesterol esters move down by passive diffusion  Repackaged in intestinal cell  Some is reformed into triglycerides  Chylomicrons

In the Enterocyte...  Newly formed triglycerides accumulate as ‘lipid droplets’ at the endoplasmic reticulum  Coated with a protein layer  Stabilizes lipids for transport in lymph and blood (aqueous environment)  Glycerol and short chain fatty acids directly enter mesenteric blood These protein-coated lipid droplets are called Chylomicrons

Lipid Absorption Short and medium chain fatty acids simple diffusion exocytosis

Lipid Absorption (Chylomicrons)  Chylomicrons absorbed from enterocytes into lacteals (lymph vessels)  Ultimately enter blood via thoracic duct  Most long chain fatty acids absorbed into lymphatic system  Blood lipids transported as lipoproteins

Overview of Fatty Acid Uptake  Short- and medium-chain fatty acids  Enter portal blood directly from enterocytes  Bound to albumin in blood  Albumin–FFA complex  Oxidized in liver or elongated and used for triglyceride formation  Long-chain fatty acids  Form chylomicrons  Drain into the lymphatic system via the lacteal in mammals  Enter bloodstream at the thoracic duct

Overview of Lipid Digestion

1) Dietary fat (triglyceride) is hydrolyzed to free fatty acids and glycerol (actually monoacylglycerol) in the intestine by pancreatic lipase (PL). 2) Short chain fatty acids can enter the circulation directly, but most fatty acids are re-esterified with glycerol in the epithelial cells of the intestine. The resulting triglycerides enter the circulation as lipoprotein particles called chylomicrons through the lymphatic system. Lipid Metabolism at Fed state

3) The triglycerides in chylomicrons can be cleared by lipoprotein lipase (L) at the endothelial surface of capillaries. The resulting fatty acids can be: a)Adipose tissue: stored as fat in adipose tissue (Note: Triglycerides (fat) can also be made from excess glucose in the fed state); b)Muscle: used for energy in any tissue with mitochondria (Note: Most tissues will be relying on glucose as their primary energy source in the fed state. The exception would be exercising muscle) c) Liver: re-esterified to triglycerides in the liver and exported as lipoproteins called VLDL. (Note: Triglycerides and VLDL can also be synthesized from excess glucose and amino acids in the liver during the fed state). Lipid Metabolism at Fed state

4)VLDL has essentially the same fate as the chylomicrons. 5)Insulin simulates lipoprotein lipase. It also stimulates fatty acid and triglyceride synthesis in liver and adipose tissue and inhibits hormone- sensitive lipase in the adipose tissue. Lipid Metabolism at Fed state

1.Hormone-sensitive lipase is activated by glucagon (fasting) or epinephrine (exercise). Therefore, fat in adipose tissue is hydrolyzed to give glycerol and fatty acids during both fasting and exercise. 2.The fatty acids can be used directly as an energy source by most tissues with mitochondria. 3.Glycogen breakdown provides glucose and protein breakdown provides alanine, which is converted to glucose in the liver. The blood glucose is used by the brain and red blood cells. Exercising muscle will use both fatty acids and glucose for energy. The relative contribution these energy sources depends on the intensity of the exercise. Lipid Metabolism at Fasting & exercise state

4)The glycerol can be converted to glucose in the liver. This is a minor source of glucose. 5)Glucagon & epinephrine stimulate hormone-sensitive lipase and inhibit lipoprotein lipase and triglyceride synthesis 6) During prolonged starvation, the fatty acids can also be converted to ketone bodies in the liver. These ketone bodies can be used as an energy source by all tissues except those lacking mitochondria (eg. red blood cells). Brain adapts slowly to the use of ketone bodies during prolonged starvation. Lipid Metabolism at Fasting & exercise state

Oxidation of Fatty Acids is a Major Energy- Yielding Pathway in Many Organisms About one third of our energy needs comes from dietary triacylglycerols About 80% of energy needs of mammalian heart and liver are met by oxidation of fatty acids Efficient storage, better for slow, steady burn

Fatty Acids are converted into Fatty Acyl-CoA

Fatty Acid Transport into Mitochondria Fats are degraded into fatty acids and glycerol in the cytoplasm  -oxidation of fatty acids occurs in mitochondria Small (< 12 carbons) fatty acids diffuse freely across mitochondrial membranes Larger fatty acids are transported via acyl- carnitine / carnitine transporter

Acyl-carnitine / Carnitine Transport

Fatty Acid Transport into Mitochondria

Stages of Fatty Acid Oxidation  Stage 1 (  -Oxidation Pathway) consists of oxidative conversion of two-carbon units into acetyl-CoA with concomitant generation of NADH and FADH 2  Stage 2 (Oxidation of acetyl-CoA) involves oxidation of acetyl-CoA into CO 2 via citric acid cycle with concomitant generation NADH and FADH 2  Stage 3 (ATP generation) generates ATP from NADH and FADH 2 via the respiratory chain

Enoyl group  -Oxidation Pathway

 Repeating the four-step process six more times results in the complete oxidation of palmitic acid into eight molecules of acetyl-CoA  FADH 2 is formed in each cycle  NADH is formed in each cycle  Acetyl-CoA enters citric acid cycle and is further oxidizes into CO 2  This makes more GTP, NADH and FADH 2 Stages of Fatty Acid Oxidation

3 Stages of FA oxidation

Citric acid cycle

Beta Oxidation on 16C fatty Acid - Total Energy  7 rounds of beta oxidation 7 rounds X FADH2 X 1.5 ATP = 10.5 ATP 7 rounds X NADH X 2.5 ATP = 17.5 ATP  8 acetyl CoA / 8 Krebs Cycle 8 Krebs X 3 NADH X 2.5 ATP = 60 ATP 8 Krebs X FADH2 X 1.5 ATP = 12 ATP 8 Krebs X GTP X 1 ATP = 8 ATP Total = 108 ATP

Comparison of Fat and CHO  Fats provide about 9 kilocalories per gram and carbohydrates provide about 4 kilocalories per gram.  CHO provide energy more quickly  Fats are good fuel for endurance events, but not sprints

Glycerol from Fats Enters Glycolysis Glycerol kinase activates glycerol at the expense of ATP Subsequent reactions recover more than enough ATP to cover this cost Allows limited anaerobic catabolism of fats

Glycerol from Fats enters Glycolysis

Formation of Ketone Bodies  Entry of acetyl-CoA into citric acid cycle requires oxaloacetate  When oxaloacetate is depleted or consumed, acetyl-CoA is converted into ketone bodies  The first step is reverse of the last step in the  - oxidation: thiolase reaction joins two acetate units  Ketone Bodies: Acetoacetate, Acetone, β- Hydroxybutyrate

Ketone Bodies from 2 Acetyl-CoA

Β-Hydroxy butyrate as a fuel

Liver as the Source of Ketone Bodies  Production of ketone bodies increases during starvation  Ketone bodies are released by liver to bloodstream  Organs other than liver can use ketone bodies as fuels (absence of beta-ketoacyl Co-A transferase enzyme)  Too high levels of acetoacetate and  - hydroxybutyrate lower blood pH dangerously

 In healthy people acetone is formed in very small amounts from acetoacetate, which is easily decarboxylated either spontaneously, or by the action of acetoacetate decarboxylase.  Because individuals with untreated diabetes produce large quantities of acetoacetate, their blood contains significant amounts of acetone, which is toxic. Liver as the Source of Ketone Bodies

Ketone bodies are overproduced in diabetes & during starvation During starvation: Gluconeogenesis  depletes TCA cycle intermediates like oxaloacetate  diverts acetyl-CoA to ketone body formation Untreated Diabetes: Insufficient Insulin  extrahepatic tissue cannot take up glucose efficiently from blood, either for fuel or for conversion to fat.

Diabetes and Ketone Bodies  When there is not enough insulin in the blood then it must break down fat for its energy.  Ketones build up in the blood and then spill over into the urine so that the body can get rid of them. Acetone can be exhaled through the lungs. This gives the breath a fruity odor. Ketones that build up in the body for a long time lead to serious illness and coma. (Diabetic ketoacidosis)

Diabetic ketoacidosis (DKA)  Diabetic ketoacidosis ( DKA ) is a potentially life- threatening complication in patients with diabetes mellitus.  It happens predominantly in those with type 1 diabetes, but it can occur in those with type 2 diabetes under certain circumstances.  DKA results from a shortage of insulin; in response the body switches to burning fatty acids and producing acidic ketone bodies that cause most of the symptoms and complications.

Mechanism of DKA  Diabetic ketoacidosis arises because of a lack of insulin in the body.  The lack of insulin and corresponding elevation of glucagon leads to increased release of glucose by the liver (a process that is normally suppressed by insulin) from glycogen via glycogenolysis and also through gluconeogenesis.  High glucose levels spill over into the urine, taking water and solutes (such as sodium and potassium) along with it in a process known as osmotic diuresis.

Mechanism of DKA  This leads to polyuria, dehydration, and compensatory thirst and polydipsia.  The absence of insulin also leads to the release of free fatty acids from adipose tissue (lipolysis), which are converted, again in the liver, into ketone bodies (acetoacetate and β- hydroxybutyrate).  β-Hydroxybutyrate can serve as an energy source in the absence of insulin-mediated glucose delivery, and is a protective mechanism in case of starvation.  The ketone bodies, however, are acidic and therefore decrease the blood pH (metabolic acidosis).

Mechanism of DKA  The body initially buffers the change with the bicarbonate buffering system, but this system is quickly overwhelmed and other mechanisms must work to compensate for the acidosis.  One such mechanism is hyperventilation to lower the blood carbon dioxide levels (a form of compensatory respiratory alkalosis). This hyperventilation, in its extreme form, may be observed as Kussmaul respiration.

Mechanism of DKA  As a result of the above mechanisms, the average adult DKA patient has a total body water shortage of about 6 liters (or 100 mL/kg)  In addition, substantial shortages in sodium, potassium, chloride, phosphate, magnesium and calcium levels occur  Glucose levels usually exceed 13.8 mmol/L or 250 mg/dL.

Symptoms of DKA/HHS  Polyuria  Dehydration  Deep gasping breathing  Nausea/Vomiting  Abdominal Pain  Fatigue  Confusion  Coma

Diagnostic Criteria for DKA

Diabetic Coma  Diabetic coma is a reversible form of coma[1] found in people with diabetes mellitus. It is a medical emergency.  Diabetic ketoacidosis (DKA), if it progresses and worsens without treatment, can eventually cause unconsciousness, from a combination of a very high blood sugar level, dehydration and shock, and exhaustion.

Treatment of DKA/ Diabetic Coma  Hydration  Normal Saline – cc/hr for 4 hours, then 250 – 500 cc/hr for 4 hours, then cc/hr  Once glucose is < 200, should change fluids to D5 ½ NS (Dextrose 5% in 0.45% Normal Saline) until insulin drip is stopped  Insulin  Insulin drip: Bolus: 0.15 units/kg, then infuse at 0.1 mg/kg/hr  Ideally should decrease glucose mg/dL per hour  In DKA: Change to subcutaneous regimen  Need to give long-acting insulin dose several hours prior to stopping insulin drip.

Treatment of DKA/ Diabetic Coma  Serial Electrolytes  Potassium repletion  Should add potassium to IV fluids once potassium < 5  Sodium repletion