Lipid Digestion and Absorption PDF

LIPID DIGESTION AND ABSORPTION BY ASSISTANT PROFESSOR RASHA GHAZALA MEDICAL BIOCHEMISTRY  Students’ learning outcomes By the end of this lecture, the students should be able to: Illustrate lipid Digestion in mouth stomach and intestine Explain Mixed micelle forma...

LIPID DIGESTION AND ABSORPTION BY ASSISTANT PROFESSOR RASHA GHAZALA MEDICAL BIOCHEMISTRY  Students’ learning outcomes By the end of this lecture, the students should be able to: Illustrate lipid Digestion in mouth stomach and intestine Explain Mixed micelle formation and role of bile salts in lipid digestion Illustrate Lipid absorption and Chylomicron structure Define Abnormalities in lipid digestion CASE STUDY 61-year-old male patient with presented with a history of loose stools for 3 months and the stools were at times oily in nature without mucus or blood. His clinical examination was unremarkable. Seventy-two hour stool fat was increased , and computed tomography of the abdomen revealed a normal head and body and tail of pancreas associated with a nondilated biliary duct system. He was put on pancreatic enzyme supplements and he responded to the treatment, and on follow-up after 6 months, his stool frequency had declined Steatorrhea??? LIPID METABOLISM Dietary lipids are ingested in form of: *triglycerides (90%) *cholesterol *phospholipids *free fatty acids.  LIPID DIGESTION: Lipid digestion in the mouth The digestion by lipids begins in the mouth. Lingual lipase is Optimum pH of 2.5 – 5. The enzyme therefore continues to be active in the stomach. Acts on short chain triglycerides (SCT). Lipid digestion in the stomach The stomach produces the enzyme called gastric lipase. This enzyme helps in the digestion and disintegration of triglycerides. The lingual lipase from the mouth is still active in the stomach. However, these two lipases play only a minor role in digesting fats. Gastric lipase is acid stable with an optimum pH about 5.4 Up to 30% digestion of triglycerides occurs in stomach  Lipid digestion in the small intestine Solubilization (emulsification) of lipids in the aqueous intestinal environment needs: bile salts - mechanical agitation in the gut - colipase and - phospholipids. Emulsification of lipids Lipids Digestion - The emulsified fats are degraded by pancreatic lipases (lipase and phospholipase A2). pancreatic lipase - Triacylglycerols 2 fatty acids + mono-acylglycerol phospholipase A2 - Phospholipids fatty acid + lysophospholipid Cholesterol ester hydrolase (esterase) - Cholesterol ester FA Cholesterol  Isomerase shifts FA position 2 to 1. Than 1st position is then hydrolysed by lipase to form free glycerol and fatty acid Major end product: 2-MAG (78%) 1-MAG(6%) Glycerol and fatty acids (14%).. The particles which do not need to be enzymatically digested are- cholesterol and fat-soluble vitamins LIPID ABSORPTION The absorption of the digestive products of Lipids occurs by forming mixed micelles with bile salts  Micelles are molecules that help the fat products get close to the microvilli, which are situated in the small intestine. The microvilli in the small intestine aid in the absorption process. The fat digested product moves across the intestinal cell membranes by diffusion Inside the intestinal cell - Following absorption of the products of pancreatic lipases by the intestinal mucosal cells resynthesis of triacylglycerols. The triacylglycerols are then solubilized in lipoprotein complexes (chylomicron) A chylomicron contains lipid droplets surrounded by the more polar lipids and finally a layer of proteins  Chylomicron structure 48 48 48 Chylomicron structure Short chain fatty (SCF) and medium chain fatty (MCF) acid absorption SCF = Milk , Butter, Ghee MCF = Coconut oil and mother’s milk Do not need re-esterification. Directly enter into blood vessels, then to portal vein, finally to liver where they are immediately utilised for energy. Their absorption is rapid. They are better absorbed than long chain fatty acids. Chylomicrons in the circulation cause turbidity of serum that is cleared by the effect of lipoprotein lipase (LPL) = clearing factor. TGLPL 2 FFA+ MAG Glycerol +FFA  -glycerol passes mainly to the liver or other tissues having active glycerol kinase enzyme. -Free fatty acids *enter the adipose tissue stored *pass to other tissues to be oxidized and give energy  Absorption of lipids TG FA  -oxidation FA FA TG Abnormalities in Absorption of lipids 1. Defective digestion: Steatorrhea = daily excretion of fat in faeces is > 6 gm per day. In pancreatic diseases 2. Defective absorption: if the absorption is also defective steatorrhea Defective absorption may be due to diseases: Coeliac disease, Sprue, Crohn’s disease. Surgical removal of intestine. Obstruction of bile duct Since milk fat and coconut oil are made up of MCT, they are therapeutically useful in malabsorption syndrome Summary Lipids are important dietary constituents. They are ingested mainly in the form of triglycerides (90% of the dietary fat) The rest is made up of phospholipids, cholesterol, cholesterol esters and free fatty acids. Emulsification increases the surface area to perform enzyme functions.Lipolytic enzymes in intestine include Pancreatic lipase , Cholesterol esterase and Phospholipase A2 2-monoglycerides, long chain fatty acids, cholesterol, phospholipids and lysophospholipids are incorporated into molecular aggregates to form mixed micelle. The triacylglycerols are then solubilized in lipoprotein complexes (chylomicron) which help transport the triglycerides and the fat-soluble vitamins and cholesterol  Lipoprotein lipase (LPL) (clearing factor). This enzyme is located at the endothelial layer of capillaries of adipose tissue, muscles and heart Abnormalities in Absorption of lipids include defective digestion as pancreatic diseases or Defective absorption as malabsorption syndromes. Learning resources Harper illustrated biochemistry 31st edition Lippincott illustrated reviews biochemistry.