936
UNIT 4
Maintenance of the Body
24
Cholesterol Transport
Because triglycerides and cholesterol are insoluble in water, they
do not circulate free in the blood. Instead, they are transported
to and from tissue cells bound to small lipid-protein complexes
called
lipoproteins
. Tese complexes solubilize the hydropho-
bic lipids, and contain signals that regulate lipid entry and exit
at specific target cells.
Lipoproteins vary in their relative fat-protein composition, but
they all contain triglycerides, phospholipids, and cholesterol in
addition to protein
(Figure 24.22)
. In general, the higher the
percentage of lipid in the lipoprotein, the lower its density; and
the greater the proportion of protein, the higher its density. On
this basis, there are
very low density lipoproteins (VLDLs), low-
density lipoproteins (LDLs),
and
high-density lipoproteins
(HDLs)
.
Chylomicrons
, which transport absorbed lipids from the
GI tract, are a separate class and have the lowest density of all.
Te liver is the primary source of VLDLs, which transport
triglycerides from the liver to peripheral (nonliver) tissues,
mostly to
adipose tissues
. Once the triglycerides are unloaded,
the residues are converted to LDLs, which are cholesterol-rich.
Te job of the LDLs is to transport cholesterol to
peripheral
tis-
sues
, making it available to tissue cells to synthesize membranes
or hormones, and to store it for later use. Docking of LDL to
the LDL receptor triggers receptor-mediated endocytosis of the
entire particle.
Te major function of HDLs, which are particularly rich
in phospholipids and proteins, is to scoop up and transport ex-
cess cholesterol
from peripheral tissues to the liver
, where it is
broken down and becomes part of bile. Te liver makes the pro-
tein envelopes of the HDL particles and then ejects them into
the bloodstream in collapsed form, rather like deflated beach
balls. Once in the blood, these still-incomplete HDL particles
About 15% of blood cholesterol comes from the diet. Te
liver—and to a lesser extent other body cells, particularly intes-
tinal cells—make the other 85% from acetyl CoA. Cholesterol
is lost from the body when it is catabolized and secreted in bile
salts, which are eventually excreted in feces.
Table 24.5
Summary of Normal Hormonal Influences on Metabolism
HORMONE’S EFFECTS
INSULIN
GLUCAGON
EPINEPHRINE
GROWTH
HORMONE
THYROXINE
CORTISOL
TESTOSTERONE
Stimulates glucose uptake
by cells
 
 
 
 
 
Stimulates amino acid
uptake by cells
 
 
 
 
 
Stimulates glucose
catabolism for energy
 
 
 
 
 
Stimulates glycogenesis
 
 
 
 
 
 
Stimulates lipogenesis and
fat storage
 
 
 
 
 
 
Inhibits gluconeogenesis
 
 
 
 
 
 
Stimulates protein synthesis
(anabolic)
 
 
 
Stimulates glycogenolysis
 
 
 
 
 
Stimulates lipolysis and fat
mobilization
 
 
Stimulates gluconeogenesis
 
 
 
Stimulates protein
breakdown (catabolic)
 
 
 
 
 
 
Triglyceride
Phospholipid
Cholesterol
Protein
2–7%
1–2%
Chylomicron
VLDL
LDL
HDL
3–6%
80–95%
55–65%
10%
5%
20%
30%
20%
45–50%
45%
25%
10–15%
5–10%
15–20%
From intestine
Made by liver
Returned to
liver
Figure 24.22
Approximate composition of lipoproteins that
transport lipids in body fluids.
VLDL
5
very low density lipoprotein,
LDL
5
low-density lipoprotein, HDL
5
high-density lipoprotein.
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