Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 4 Lipids and Ketones
The remaining enzymes of cholesterol synthesis are
located in the endoplasmic reticulum, perhaps because of
decreasing solubility of succeeding products formed in the
pathway. Next, HMG-CoA is reduced to mevalonate under
the catalysis of HMG-CoA reductase:
HMG-CoA 2 NADPH 2 H
⎯⎯⎯→mevalonate
2 NADP
CoA
HMG-CoA reductase is the primary control point for
cholesterol synthesis, and its control mechanisms will be
discussed later. Next, via three steps, isopentenylpyrophosphate
is formed. Six of these molecules, often called the
active isoprenoid units, are linked to form cholesterol in a
long and complex pathway, which is only partially understood
( Faust et al ., 1988 ; Liscum, 2002 ).
The control of HMG-CoA reductase is complex and not
completely understood. Artificially increasing plasma cholesterol
levels in vivo decreases the activity of the enzyme
in liver. However, cholesterol does not inhibit the enzyme
directly, but represses synthesis of the enzyme mRNA
( Liscum, 2002 ). Thus, if the amount of cholesterol consumed
in the diet increases, the amount synthesized by the
liver will decrease. This reciprocal relationship between
cholesterol consumed and hepatic synthesis limits the
extent to which plasma cholesterol levels can be decreased
by restricting the amount of cholesterol in the diet.
Hepatic HMG-CoA reductase is inhibited by phosphorylation
of the enzyme and reactivated by dephosphorylation.
The protein kinase system responsible for
the phosphorylation of HMG-CoA reductase is stimulated
by intracellular cAMP ( Liscum, 2002 ). Hepatic intracellular
cAMP levels are controlled in part by plasma glucagon,
which increases it, and by insulin, which decreases
it. Thus, conditions that increase insulin (e.g., eating) will
increase cholesterol synthesis, and conditions that decrease
insulin (e.g., diabetes) or increase glucagon (e.g., fasting)
will decrease cholesterol synthesis. Other hormones that
affect hepatic HMG-CoA reductase activity, but probably
not by altering intracellular cAMP levels, are thyroid hormones
(increase HMG-CoA reductase activity) and glucocorticoids
(decrease HMG-CoA reductase activity). Some
drugs, such as lovastatin and mevastatin, used in humans
to decrease plasma cholesterol levels, operate by inhibiting
HMG-CoA reductase ( Brown and Goldstein, 1990 ) .
Once cholesterol has been synthesized in the hepatocyte,
it can be secreted into the plasma as part of lipoproteins
(mostly in VLDL), it can be secreted into the
canaliculi and become part of bile micelles, it can be
degraded to bile acids, or it can be esterified to an LCFA
by acyl-CoA:cholesterol acyltransferase (ACAT), which is
located in the smooth endoplasmic reticulum. Cholesterol
esters are even less soluble than cholesterol and are found
in membranes and micelles wherever cholesterol itself is
found. Cholesterol ester can be exported as part of lipoproteins,
or it can be converted back to cholesterol plus
LCFA by cholesterol ester hydrolases, which are found
in the cytosol, endoplasmic reticulum, and lysosomes.
De-esterification is mandatory before cholesterol can be
catabolized to bile acids. Because enzymes for the final
steps of cholesterol synthesis and the first steps of its degradation
are colocated in the endoplasmic reticulum, it
might seem that most newly synthesized cholesterol would
be immediately degraded. However, the negative feedback
of bile acids on cholesterol degradation keeps this process
in check.
HDL contains lecithin:cholesterol acyltransferase
(LCAT), which esterifies cholesterol by transferring an
LCFA moiety from lecithin (phosphatidylcholine). The
cholesterol to be esterified by LCAT can be then secreted
with HDL at the time of its synthesis, or it can be cholesterol
from other lipoproteins or cell membranes that come
in contact with HDL at a later time.
VI. LIPOPROTEINS
A. Structure, Properties, and Assay of
Lipoproteins
Lipoproteins are very large noncellular conglomerations
(micelles) of lipids and proteins, which are suspended in
plasma or lymph. Their main function is to transport most
lipids (steroid hormones and LCFA being notable exceptions)
among tissues. Another function of lipoproteins is the
esterification of cholesterol. Lipoproteins have a micellar
structure in which the least polar molecules (triacylglycerol
and cholesterol) occupy the center and more polar molecules
(proteins and phospholipids) coat the exterior. Lipoproteins
are synthesized almost exclusively by liver and the small
intestine.
The main classes of lipoprotein are defined by their density
as determined by ultracentrifugation and are chylomicrons
(d 0.94 g/ml), very low density lipoproteins (VLDL,
d 0.94 to 1.006 g/ml), low-density lipoproteins (LDL,
d 1.006 to 1.063 g/ml), and high-density lipoproteins
(HDL, d 1.063 to 1.21 g/ml). Less commonly considered are
very high density lipoproteins (VHDL, d 1.21g/ml), which
are usually very low in concentration in plasma. It is common
to designate the lighter LDL (d 1.006 to 1.019g/ml)
as intermediate density lipoproteins (IDL), and some
schemes subdivide the HDL into HDL 1 , HDL 2 , and HDL 3 .
(Chapman, 1986 ; Gotto et al ., 1986 ).
The lipid component of lipoproteins is less dense than
the protein component, but the lipids have similar densities,
and the proteins have similar densities. Therefore, the
density of a lipoprotein is almost entirely dependent on its
ratio of lipid to protein, with the chylomicrons having the