Page 77 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

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Encyclopedia of Physical Science and Technology EN008C-380 June 29, 2001 16:42

644 Lipoprotein/Cholesterol Metabolism

Cholesterol ester transfer protein (CETP) A blood- Reverse cholesterol transport A hypothetical pathway
borne protein that catalyzes the exchange of triglyc- by which cholesterol is transported from extrahepatic-
erides in VLDL for cholesterol esters in HDL. tissues to the liver via HDL.
Chylomicron remnants Chylomicron particles that have Scavenger receptors Receptors that bind to a wide range
been depleted of triglyceride after the lipoprotein of molecules, including modfied forms of LDL. They
lipase-mediated hydrolysis of their triglycerides. are involved in the accumulation of cholesterol in
Chylomicrons Lipoprotein particles produced in the in- macrophages and smooth muscle cells in the arterial
testine to package and secrete dietary lipids. Chylomi- wall.
crons are secreted into the mesenteric lymph. SREBP cleavage activating protein (SCAP) A chole-
Familial hypercholesterolemia An elevation in LDL sterol-sensing protein that, in the absence of choles-
cholesterol due to mutations at the LDL receptor terol, escorts SREBP from the endoplasmic reticulum
locus. to the Golgi, where it is proteolytically activated.
Familial hypoalphalipoproteinemia Adeficiency in Statins Cholesterol-lowering drugs that act by inhibit-
HDL due to mutations at the ABCA1 locus. ing cholesterol synthesis and upregulating the LDL
Fibric acid A class of drugs used to decrease the rate of receptor.
VLDL triglyceride secretion. Sterol responsive element binding protein (SREBP)
Gallstones Large crystals, usually made of cholesterol, A transcription factor that postively regulates sterol-
that form in the biliary tract and/or gall bladder when responsive genes, is attached to the endoplasmic retic-
cholesterol levels in bile are too high relative to phos- ulum membrane, and is released through a cholesterol-
pholipid and bile acids. sensitive proteolysis step.
HDL High density lipoprotein; carries about 20% of Tangier disease A severe HDL deficiency syndrome
plasma cholesterol. Its levels negatively correlate with caused by homozygous mutations at the ABCA1 locus.
risk of coronary heart disease. Is thought to mediate VLDL Very low density lipoprotein; a triglyceride-rich
“reverse cholesterol transport.” lipoprotein assembled in the endoplasmic reticulum
HMG-CoA reductase A major rate-limiting step in the and Golgi of hepatocytes and then secreted into the
cholesterol biosynthetic pathway; catalyzes the conver- bloodstream. While in the circulation, it gives rise to
sion of HMG-CoA to mevalonate. IDL and then LDL after it loses triglyceride.
IDL Intermediate density lipoprotein, also called “VLDL
remnant”; the VLDL particle that has been depleted of
triglyceride through the action of lipoprotein lipase. ANIMALS TRANSPORT LIPIDS in an aqueous envi-
LDL Low density lipoprotein; carries approximately ronmentatconcentrationsuptoonemilliontimestheirsol-
two-thirds of plasma cholesterol. Its levels are posi- ubility in water. They accomplish this task by surrounding
tively correlated with risk of coronary heart disease. water-insoluble lipids with amphipathic lipids and pro-
It is formed in the circulation from the catabolism of teins to form plasma lipoproteins. A major part of lipid
VLDL. transport is to supply energy for muscle contraction and
LDL receptor Receptor expressed in most tissues and to deliver lipids to adipose tissue for storage. Disorders in
mainly necessary for normal clearance of LDL from lipoprotein metabolism are a major risk factor for prema-
the bloodstream. ture coronary heart disease throughout the world. These
Lipoprotein lipase Enzyme that catalyzes the hydrolysis disorders arise from dysfunction in apolipoproteins, par-
of VLDL and LDL triglycerides to free fatty acids and ticular enzymes, and lipid transfer proteins, and also sec-
glycerol; present on the luminal surface of the capillary ondary to disorders in carbohydrate metabolism.
endothelium.
Lipoproteins Particles that transport lipids in the blood-
stream. I. LIPID ABSORPTION
Lp(a) An LDL-like lipoprotein particle in which apo-
B100isconnectedthroughadisulfidelinkagetoapo(a), Animals absorb and transport large quantities of lipids.
a plasminogen-related protein. A major challenge is posed by the solubility character-
Microsomal triglyceride transfer protein (MTP) A istics of lipids. Because they are essentially insoluble in
protein in the endoplasmic reticulum necessary for se- water, they must be packaged in order to move through
cretion of VLDL and chylomicrons. an aqueous environment. Since many lipids are amphi-
Nicotinic acid A coenzyme precursor that at pharmaco- pathic (have water-soluble and water insoluble moieties),
logical doses helps to lower triglyceride levels and raise they have detergent-like properties that can be harmful to
HDL levels. membranes.