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Lipid Absorption

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BIOL 1840U
Peter Cheung

Lipid Absorption I. Introduction a. Fatty Acid Classification i. Long chain—12-18 C ii. Medium Chain—8-10 C iii. Short Chain—4-6 C b. Importance of Fats & Lipids i. Major source of energy (9.3 kcal/g) ii. Absorption of H2O insoluble vitamins iii. Signaling molecules iv. Substrates for synthesis (essential fatty acids—linoleic (omega-6) & linolenic (omega-3)) c. Definitions i. Fat—dietary term referring to lipidic material in diet ii. Lipid—chemical term that includes fat, cholesterol, TG, etc d. Hydrophobic Effect—in H2O, hydrophobic molecules arrange in such a fashion as to minimize their SA & contact w/ H2O (H2O molecules organize into clathrate around hydrophobic molecules => highly organized, energetically unfavorable structure) e. Amphiphile—hydrophobic and hydrophilic regions on same molecule i. In water seek to hide hydrophobic regions (away from H2O) ii. In apolar solvent hid polar groups iii. Can self-assemble into complex structures f. Critical Micellar Concentration (CMC) i. Lipids are not very soluble in H2O, but over a certain concentration (CMC) they organize into micelles ii. Micelle—extremely soluble! Aggregate of monomers in dynamic equilibrium; hides hydrophobic groups II. Lipid Absorption a. Challenge: absorbing and transporting hydrophobic/amphipathic molecules in a hydrophilic environment b. Luminal Phase i. EMULSIFICATION by gastric motility - Physical event to generate small fat droplets => ↑ SA ii. LIPOLYSIS by gastric lipase - Lingual lipase: cleaves short TGs, only important in infants - Gastric Lipase: optimum pH 3-6; frees 1FA from TG; secreted from chief cells iii. SOLUBILIZATION by bile salts - Bile salts form mixed micelles with FFA’s and 2-MG’s - Pancreatic lipase: active at fat/water interface only!; secreted in active form; cleaves 1 and 3 position of TG => 2FFA from TG * pancreatic lipase is inactivated by bile salts => overproduced & “helped by co-lipase 1 Pancreatic Co-lipase—facilitates pancreatic lipase binding to micelle protect lipase against bile acid inactivation - Phospholipase A2: cleaves 2 position of phospholipids => 1 FFA + monoglyceride - Non-speci
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