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

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Chapter | 4 Lipids and Ketones
α -Oxidation, in which carbons are removed one at a time
from the carboxyl end of the LCFA, is used by brain tissue
to produce LCFA of varying lengths for synthesis of complex
lipids. ω -Oxidation, which is oxidation that occurs at
the methyl, rather than at the carboxyl, end is conducted
to a limited extent by the cytochrome P-450 system in the
endoplasmic reticulum of liver. The resulting dicarboxylic
acid can then undergo β-oxidation to a chain length of six
carbons (adipate), most of which will be excreted in the
urine ( Gurr et al ., 2002 ; Mortensen, 1990 ) .
III. TRIACYLGLYCEROL
A. Structure, Properties, and Assay of
Triacylglycerol
The main storage forms of LCFA are the triacylglycerols
(also called triglycerides), in which three LCFA are esterified
to glycerol. Triacylglycerols are even less soluble than
LCFA and also must be bound to proteins in complexes
called lipoproteins for transport through plasma.
Assay of triacylglycerol in plasma or serum is best
accomplished by enzymatic hydrolysis using lipase followed
by enzymatic determination of the released glycerol
( Klotzsch and McNamara, 1990 ; McGowan et al ., 1983 ).
If high plasma glycerol levels are likely, as it is in animals
that have not eaten lately, a plasma blank must be run. Older
methods that use alkaline hydrolysis require caustic reagents,
consume more time, and may assay phospholipids plus
triacylglycerol. Contamination of samples with glycerol,
which is sometimes used to lubricate stoppers of blood collecting
tubes, or with soap, which may contain glycerol or
fatty acids, will lead to falsely elevated values. If the sample
contains lipase, which is not uncommon, triacylglycerol
levels will decrease if it is allowed to stand. Prompt centrifugation
of blood samples followed by rapid analysis or freezing
of the plasma will prevent falsely low triacylglycerol levels.
B. Synthesis of Triacylglycerol
Although most cells can synthesize triacylglycerols, liver,
adipose, mammary gland, and small intestine are particularly
adept at it. LCFA-CoA are the building blocks for triacylglycerol
synthesis, and it should be realized that there
are two sources of LCFA-CoA for triacylglycerol synthesis:
LCFA in the plasma and LCFA synthesized locally.
Generally, physiological or pathological circumstances,
such as starvation or diabetes, which promote high plasma
levels of LCFA, suppress LCFA synthesis. Physiological
circumstances that promote LCFA synthesis, such as eating
a carbohydrate meal, also inhibit lipolysis in adipose,
so plasma LCFA levels are not elevated.
To form triacylglycerols, LCFA-CoA are esterified to
glycerol-3-P. Glycerol-3-P can be produced in the liver
from glycerol, which is absorbed from the plasma, and
ATP in a reaction catalyzed by glycerol kinase:
glycerol ATP ⎯⎯⎯→glycerol-3-P ADP
Glycerol is normally plentiful in plasma only when
there is active lipolysis occurring in adipose tissue.
When glucose is plentiful in the plasma and LCFA are
being synthesized from glucose via acetyl-CoA, glycerol-
3-P is also synthesized from glucose in liver, mammary
gland, and adipose. This process occurs via glycolysis to
dihydroxyacetone-P followed by a reduction catalyzed by
glycerol-3-P dehydrogenase:
dihydroxyacetone-P NADH H
glycerol-3-P NAD
←⎯⎯→
LCFA-CoA is esterified to glycerol-3-P by glycerol-P
acyltransferase:
glycerol-3-P LCFA-CoA ⎯⎯⎯→
1-acyl-glycerol-3-P CoA
This reaction occurs in both mitochondria and smooth
endoplasmic reticulum, but the smooth endoplasmic
reticulum enzyme is more plentiful and most important in
triacylglycerol synthesis. Next, another LCFA-CoA is
esterified by the enzyme, acylglycerol-P acyltransferase,
which is located in the smooth endoplasmic reticulum:
1-acyl-glycerol-3-P LCFA-CoA ⎯⎯⎯→
phosphatidate CoA
Phosphatidate (the ionized form of phosphatidic acid) is
1,2-diacyl-glycerol-3-P. Next, the phosphate is hydrolyzed
from phosphatidate by phosphatidate phosphohydrolase to
produce a diacylglycerol:
phosphatidate ⎯⎯⎯→diacylglycerol P
This reaction occurs in the smooth endoplasmic reticulum
and cytosol. Finally, a last LCFA-CoA is esterified by the
enzyme diacylglycerol acyltransferase, an enzyme located in
the smooth endoplasmic reticulum ( Bernlohr et al ., 2002 ):
diacylglycerol LCFA-CoA ⎯⎯⎯→
triacylglycerol CoA
If the triacylglycerol has been synthesized in adipose, it
will migrate into the large storage vesicle that each adipocyte
possesses. Most of the triacylglycerol synthesized in
liver normally will be incorporated into and exported from
the liver as part of very low density lipoproteins (VLDL).
However, if triacylglycerol synthesis exceeds hepatic export
capacity, triacylglycerol will accumulate in vesicles in hepatocytes,
leading to fatty liver. If the triacylglycerol has been
synthesized in mammary gland, the resulting triacylglycerols
will accumulate in vesicles of secretory cells, and the
vesicles will be extruded into the lumina of the gland acini.