2. lipoproteins
• The plasma lipoproteins are spherical macromolecular complexes of lipids and
specific proteins (apolipoproteins or apoproteins). include
• chylomicrons (CM),
• very-low-density lipo -proteins (VLDL),
• low-density lipoproteins (LDL), and
• high-density lipoproteins (HDL).
• They differ in lipid and protein composition, size, density and site of origin.
• Lipoproteins function both to keep their component lipids soluble as they
transport them in the plasma and to provide an efficient mechanism for
transporting their lipid contents to (and from) the tissues.
• In humans, the transport system is less perfect than in other animals and, as a
result, humans experience a gradual deposition of lipid—especially cholesterol—
in tissues. This is a potentially life-threatening occurrence when the lipid
deposition contributes to plaque formation, causing the narrowing of blood
vessels (atherosclerosis).
3.
4. Size and density of lipoprotein particles
Chylomicrons are the lipoprotein particles lowest in density and
largest in size, and contain the highest percentage of lipid and the
lowest percentage of protein.
VLDLs and LDLs are successively denser, having higher ratios of protein
to lipid.
• HDL particles are the densest.
• Plasma lipoproteins can be separated on the basis of their
electrophoretic mobility, or on the basis of their density by
ultracentrifugation.
5. Apolipoproteins
• The apolipoproteins associated with lipoprotein particles have a
number of diverse functions, such as
• providing recognition sites for cell-surface receptors, and
• serving as activators
• coenzymes for enzymes involved in lipoprotein metabolism.
Apolipoproteins are divided by structure and function into five major
classes, A through E, with most classes having subclasses, for example,
• apolipoprotein (or apo) A-I and apo C-II.
6. Metabolism of chylomicrons
• Chylomicrons are assembled in intestinal mucosal cells and carry dietary
triacylglycerol, cholesterol, fat-soluble vitamins, and choles - teryl esters
(plus additional lipids made in these cells) to the peripheral tissues. [Note:
TAGs account for close to 90% of the lipids in a chylomicron.]
• Modification of nascent chylomicron particles:
• The particle released by the intestinal mucosal cell is called a “nascent”
chylomicron because it is functionally incomplete. When it reaches the
plasma, the particle is rapidly modified, receiving apolipo - protein E (which
is recognized by hepatic receptors).
• The latter includes apo C-II, which is necessary for the activation of
lipoprotein lipase, the enzyme that degrades the triacylglycerol contained
in the chylomicron). The source of these apolipoproteins is circulating
HDL).
7.
8. Metabolism of VLDL
• VLDLs are produced in the liver. They are
Composed predominantly of endogenous triacylglycerol
(approximately 60%), and their function is to
carry this lipid from the liver (site of synthesis) to the
peripheral tissues. There, the triacylglycerol is degraded by
lipo protein lipase. [Note: “Fatty liver” (hepatic steatosis) occurs in
conditions in which there is an imbalance between hepatic
triacylglycerol synthesis and the secretion of VLDL. Such conditions
include obesity, uncontrolled diabetes mellitus, and chronic ethanol
ingestion.]
