Lipoprotein/Cholesterol Metabolism

SREBP cleavage activating protein (SCAP) A cholesterol-sensing protein that, in the absence of cholesterol, escorts SREBP from the endoplasmic reticulum to the Golgi, where it is proteolytically activated. Statins Cholesterol-lowering drugs that act by inhibiting cholesterol synthesis and upregulating the LDL receptor. Sterol responsive element binding protein (SREBP) A transcription factor that postively regulates sterol-responsive […]

Important Terms (Lipoprotein/Cholesterol Metabolism)

ABCA1 A membrane lipid transport protein involved in cholesterol and/or phospholipid efflux from cells. Its action is necessary for the extracellular assembly of lipoprotein particles. Apo-A1 An apolipoprotein principally associated with HDL, an activator of lecithin choles-terol:acyltransferase. It interacts with cells to mediate delivery of cholesterol ester from HDL particles. Apo-B100 An apolipoprotein associated with […]

ANIMALS TRANSPORT LIPIDS (Lipoprotein/Cholesterol Metabolism)

ANIMALS TRANSPORT LIPIDS in an aqueous environment at concentrations up to one million times their solubility in water. They accomplish this task by surrounding water-insoluble lipids with amphipathic lipids and proteins to form plasma lipoproteins. A major part of lipid transport is to supply energy for muscle contraction and to deliver lipids to adipose tissue […]

LIPID ABSORPTION (Lipoprotein/Cholesterol Metabolism)

Animals absorb and transport large quantities of lipids. A major challenge is posed by the solubility characteristics of lipids. Because they are essentially insoluble in water, they must be packaged in order to move through an aqueous environment. Since many lipids are amphi-pathic (have water-soluble and water insoluble moieties), they have detergent-like properties that can […]


Plasma lipoproteins are uniquely endowed with the ability to transport large quantities of water-insoluble lipids through an aqueous environment. This because the non-polar lipids (triglyceride and cholesterol ester; Fig. 2) are "buried" in the core of the lipoprotein, surrounded by a monolayer of amphipathic lipids, phospholipid, and unes-terified cholesterol (Fig. 3). In addition to core […]


Since the intestine is primarily an absorptive organ, it must have the means of exporting newly absorbed lipids. The enterocyte re-esterifies fatty acids and monoglycerides to form triglycerides and phospholipids. Absorbed cholesterol is esterified to form cholesterol esters. Under ordinary circumstances, the vast majority of the core lipids in the chylomicron are triglycerides; however, after […]


VLDL assembly and secretion is similar to the corresponding pathway for chylomicrons. Triglycerides and cholesterol esters are packaged into the core of the lipopro-tein particle. However, in contrast to intestinal chylomi-cron secretion, hepatocytes secrete VLDL directly into the bloodstream. In the bloodstream, VLDL is acted upon by lipoprotein lipase, delivering its triglyceride cargo to muscle […]


Unlike chylomicron remnants, IDL has two competing metabolic fates: (1) uptake by the liver and (2) further processing to become LDL (Fig. 6). Since IDL can be cleared by the liver or can be processed to become LDL, this branch point represents an important stage where LDL concentrations can be regulated. Inefficient clearance of IDL […]


Chylomicrons and VLDL carry many apolipoproteins, among them apo-B and apo-E. Apo-B is a large (MW = 516 kDa) protein stably associated with the lipoprotein particle. The intestine produces a truncated form of the apoB, termed apo-B48, which is missing the carboxy-terminal half of apoB. The mechanism for the truncation involves a unique RNA editing […]

THE LDL RECEPTOR (Lipoprotein/Cholesterol Metabolism)

The discovery of the LDL receptor pathway by Michael S. Brown and Joseph L. Goldstein represents the most significant triumph in the field of atherosclerosis research. In an extraordinary collaboration begun in 1972, they discovered that cells possess a high-affinity receptor that binds to the apo-B100 moiety of LDL. (They were awarded the Nobel Prize […]