194
5 Plasma Proteins, Yolk Proteins and Metal-Binding Proteins
as well as rats [252]. A smaller apoB form is missing in the chicken [250]. The human apolipoproteins A-I, A-II, A-IV, C-I, C-II, C-III, D and E
have lengths between 57 (apoC-I) and 377 amino
acids (apoA-I and apoA-IV). Just recently, the
rather curious apolipoprotein J was discovered in
a particular subclass of human HDL. This is synthesized as a 427-amino-acid polypeptide which is
cleaved post-translationally at 205-Arg/206-Ser;
the cleavage fragments apoJ a (205 amino acids)
and apoJ~ (222 amino acids) are held together by
disulphide bridges. In contrast to all other apolipoproteins, apoJ is synthesized in almost all cell
types, the highest concentrations being found in
brain, ovary, testis and liver [236]. Several isoforms are known for many human apolipoproteins, e.g. allelic sequence variants (apoE) or the
products of post-translational glycosylation
(apoA-II, apoB, apoC-III, apoE) , deamidation
(apoA-I), acylation or phosphorylation [68]. As
lipoprotein particles may contain hexoses, hexosamines and sialic acids up to a total concentration
of 9 %, they may be considered to be glycolipoproteins [39].
The different types of lipoprotein particles
have different transport functions. In most vertebrates, fats reabsorbed in the small intestine are
transported away by chylomicrons, which are
formed in the cells of the intestine and are introduced into the blood via the lymph. In birds, however, the reabsorbed lipids are included into
VLDL and directly enter the blood in the portal
vein. The chylomicrons are rapidly degraded in
the blood. Lipoprotein Iipases, located in particular on the surface of capillary endothelial cells,
release fatty acids and monoacylglycerols, which
are transported to the neighbouring cells with the
help of serum albumin. The remains of the chylomicrons, which retain only the proteins apoB and
apoE and are rich in cholesterol esters, are taken
up into liver cells. Fats synthesized in the liver are
secreted into the blood as VLDL; the HDL is also
formed in the liver. In contrast, the fatty acids
released from fat tissues are bound directly to
serum albumin. The VLDL particles formed in
the liver serve primarily to transport triacylglycerols or fatty acids to the peripheral tissues. Due to
the action of lipoprotein lipases, the VLDL particles lose triacylglycerols and are converted to
particles of intermediate density (IDL). Nonesterified cholesterol of the VLDL is transferred
to HDL, esterified by lecithin: cholesterol acyltransferase (LCAT) and returned to the IDL.
With the removal of further triacylglycerols and
all apoproteins except apoB, the IDL is converted to LDL, which transports cholesterol to
the peripheral tissues for utilization and to the
liver for degradation. ApoB and apoE are mainly
responsible for the binding to LDL receptors
[147]. At least two other proteins are encoded by
the same multi-gene family as the LDL receptor:
the LDL receptor-related protein (LRP) and glycoprotein 330. LRP appears to be the receptor for
the remnants produced by chylomicron lipolysis.
In the chicken, there are receptors on the oocyte
membrane for the yolk protein precursor VLDL
and vitellogenin, and a receptor on the body cells
which is mainly involved in cholesterol metabolism. Interestingly, only the oocyte receptor is
similar to the human LDL receptor [240]. The
liver cell receptors of the carp Cyprinus carpio
bind human and carp LDL and, conversely, carp
LDL also binds to human LDL receptors [71].
Comparative investigations of all classes of vertebrates have revealed large variation in LCAT
activity, e.g. compared with the value for man
there are lower values in the rat, sheep, cow and
dog, and higher values in the rabbit and rainbow
trout Salmo gairdneri; the activities are correlated to the cholesterol content of VLDL [91].
The activity of lipoprotein lipase also varies by up
to two orders of magnitude in the fat tissue, lung
and muscle of various mammals; there is no
apparent correlation with the nature of the lipid
metabolism in each species [53]. Apolipoproteins
are also exchanged between lipoprotein particles.
This is of particular importance as the apolipoproteins have regulatory effects on enzyme activities: apoC-II stimulates lipoprotein lipase; this
effect is inhibited by apoC-III; apoA-I, and also
apparently apoC-I, stimulate LCAT [151]. All in
all, the lipoproteins circulating in the blood
plasma may be considered as highly dynamic
structures. The few comparative studies clearly
show that the mode of action of the lipoproteins is
very similar amongst all vertebrates [213, 231].
Although no significant sequence similarity is
found between the different apolipoprotein
types, with the exception of apoD, they nevertheless appear to belong to the same protein superfamily. If, in assessing similarity, one takes into
account similar physicochemical properties of the
amino acids, periodic structures of 22 amino acids
can be detected in all members of the family.
These apparently form a-helices which on one
side carry polar side-chains and on the other side
non-polar side-chains (amphiphilic a-helices);
they can therefore bind both the polar groups of
phospholipids and the hydrocarbon chains of
the fatty acids. ApoB-100 also contains numerous
5 Plasma Proteins, Yolk Proteins and Metal-Binding Proteins
as well as rats [252]. A smaller apoB form is missing in the chicken [250]. The human apolipoproteins A-I, A-II, A-IV, C-I, C-II, C-III, D and E
have lengths between 57 (apoC-I) and 377 amino
acids (apoA-I and apoA-IV). Just recently, the
rather curious apolipoprotein J was discovered in
a particular subclass of human HDL. This is synthesized as a 427-amino-acid polypeptide which is
cleaved post-translationally at 205-Arg/206-Ser;
the cleavage fragments apoJ a (205 amino acids)
and apoJ~ (222 amino acids) are held together by
disulphide bridges. In contrast to all other apolipoproteins, apoJ is synthesized in almost all cell
types, the highest concentrations being found in
brain, ovary, testis and liver [236]. Several isoforms are known for many human apolipoproteins, e.g. allelic sequence variants (apoE) or the
products of post-translational glycosylation
(apoA-II, apoB, apoC-III, apoE) , deamidation
(apoA-I), acylation or phosphorylation [68]. As
lipoprotein particles may contain hexoses, hexosamines and sialic acids up to a total concentration
of 9 %, they may be considered to be glycolipoproteins [39].
The different types of lipoprotein particles
have different transport functions. In most vertebrates, fats reabsorbed in the small intestine are
transported away by chylomicrons, which are
formed in the cells of the intestine and are introduced into the blood via the lymph. In birds, however, the reabsorbed lipids are included into
VLDL and directly enter the blood in the portal
vein. The chylomicrons are rapidly degraded in
the blood. Lipoprotein Iipases, located in particular on the surface of capillary endothelial cells,
release fatty acids and monoacylglycerols, which
are transported to the neighbouring cells with the
help of serum albumin. The remains of the chylomicrons, which retain only the proteins apoB and
apoE and are rich in cholesterol esters, are taken
up into liver cells. Fats synthesized in the liver are
secreted into the blood as VLDL; the HDL is also
formed in the liver. In contrast, the fatty acids
released from fat tissues are bound directly to
serum albumin. The VLDL particles formed in
the liver serve primarily to transport triacylglycerols or fatty acids to the peripheral tissues. Due to
the action of lipoprotein lipases, the VLDL particles lose triacylglycerols and are converted to
particles of intermediate density (IDL). Nonesterified cholesterol of the VLDL is transferred
to HDL, esterified by lecithin: cholesterol acyltransferase (LCAT) and returned to the IDL.
With the removal of further triacylglycerols and
all apoproteins except apoB, the IDL is converted to LDL, which transports cholesterol to
the peripheral tissues for utilization and to the
liver for degradation. ApoB and apoE are mainly
responsible for the binding to LDL receptors
[147]. At least two other proteins are encoded by
the same multi-gene family as the LDL receptor:
the LDL receptor-related protein (LRP) and glycoprotein 330. LRP appears to be the receptor for
the remnants produced by chylomicron lipolysis.
In the chicken, there are receptors on the oocyte
membrane for the yolk protein precursor VLDL
and vitellogenin, and a receptor on the body cells
which is mainly involved in cholesterol metabolism. Interestingly, only the oocyte receptor is
similar to the human LDL receptor [240]. The
liver cell receptors of the carp Cyprinus carpio
bind human and carp LDL and, conversely, carp
LDL also binds to human LDL receptors [71].
Comparative investigations of all classes of vertebrates have revealed large variation in LCAT
activity, e.g. compared with the value for man
there are lower values in the rat, sheep, cow and
dog, and higher values in the rabbit and rainbow
trout Salmo gairdneri; the activities are correlated to the cholesterol content of VLDL [91].
The activity of lipoprotein lipase also varies by up
to two orders of magnitude in the fat tissue, lung
and muscle of various mammals; there is no
apparent correlation with the nature of the lipid
metabolism in each species [53]. Apolipoproteins
are also exchanged between lipoprotein particles.
This is of particular importance as the apolipoproteins have regulatory effects on enzyme activities: apoC-II stimulates lipoprotein lipase; this
effect is inhibited by apoC-III; apoA-I, and also
apparently apoC-I, stimulate LCAT [151]. All in
all, the lipoproteins circulating in the blood
plasma may be considered as highly dynamic
structures. The few comparative studies clearly
show that the mode of action of the lipoproteins is
very similar amongst all vertebrates [213, 231].
Although no significant sequence similarity is
found between the different apolipoprotein
types, with the exception of apoD, they nevertheless appear to belong to the same protein superfamily. If, in assessing similarity, one takes into
account similar physicochemical properties of the
amino acids, periodic structures of 22 amino acids
can be detected in all members of the family.
These apparently form a-helices which on one
side carry polar side-chains and on the other side
non-polar side-chains (amphiphilic a-helices);
they can therefore bind both the polar groups of
phospholipids and the hydrocarbon chains of
the fatty acids. ApoB-100 also contains numerous