谷固醇血症：一种罕见的遗传性脂代谢紊乱ppt课件_精品.ppt

* How is cholesterol absorbed? To be absorbed, dietary cholesterol needs to be de-esterified by esterases, primarily secreted by the pancreatic exocrine glands. Free cholesterol is then incorporated into micelles, a physical process that is absolutely required for cholesterol absorption. Micelles are formed by the interaction of bile salts, phospholipids (some biliary cholesterol), and dietary cholesterol. In a dynamic process, the micelles interact with the enterocyte apical membrane, and its contents are transferred into the enterocyte, though the bile salts are not absorbed at this stage. The cholesterol is transferred to the enterocyte, esterified, and packaged into chylomicrons to be secreted into the lymphatic channels; chylomicrons are progressively digested and off-loaded in the periphery before the remnants are cleared by the liver. The liver uses some of the remnant cholesterol for bile acid synthesis, to be excreted, together with cholesterol, into bile and made available for the next round of dietary sterol absorption. Note that the bile salts are reabsorbed by the terminal ileum and, after circulation in the bloodstream, taken up by the liver and re-excreted into bile, completing the enterohepatic circulation. * Sterol absorption To determine how much cholesterol is absorbed under normal conditions, Ostlund and his colleagues measured rates of absorption in 100 healthy individuals, using the dual isotope tracer technique. The majority of individuals absorb about 55% of dietary sterols, with only a few individuals who hyperabsorb or hypoabsorb. Reference: Bosner MS, Lange LG, Stenson WF, Ostlund RE Jr. Percent cholesterol absorption in normal women and men quantified with dual stable isotopic tracers and negative ion mass spectrometry. J Lipid Res 1999;40:302-308. * Dietary sterols: normal physiology On a daily basis, we eat about 500 mg cholesterol and an almost equal amount of plant sterols. However, while we may absorb about 55% of dietary choleste