in the liver also having the lowest rates of
triacylglycerol export (Pullen et al., 1990).
On the other hand, poultry and fish
actively synthesize fatty acids in the liver
and secrete VLDLs at very high rates. Rates
of VLDL export are intermediate for species
that have lipogenesis in both liver and
adipose tissue, such as rats and rabbits. In
rats, the origin of the fatty acids incor-
porated into triacylglycerol can affect the
rate of VLDL export. Dietary conditions
that promote lipogenesis in liver also
stimulate VLDL output. In contrast, high
fat diets or conditions that promote
mobilization of fatty acids from adipose
tissue decrease the rate of VLDL synthesis
but promote formation of a separate pool of
storage triacylglycerol (Wiggins and
Gibbons, 1996). Because the latter condi-
tion (uptake by the liver of fatty acids
mobilized from adipose tissue) is similar to
that usually encountered in ruminants,
similar factors may govern the rate of VLDL
synthesis in ruminants (Bauchart, 1993).
Consequently, conditions in ruminants
that promote extensive body fat mobiliza-
tion usually result in accumulation of
triacylglycerol within the liver, potentially
resulting in fatty liver. Problems with fatty
liver in dairy cows are more likely in over-
fattened cows, possibly as a result of high
insulin and its effects on fatty acid
esterification in the liver, and increased
insulin resistance in peripheral tissues
such as adipose tissue. The mechanism of
clearance of accumulated triacylglycerol
has not been determined definitively. No
hormone-sensitive lipase is present in the
liver of farm animals. In rats, the stored
lipid droplets do not contribute appreciably
to synthesis of VLDLs (Wiggins and
Gibbons, 1996). Rather, it appears that the
lipid droplet must be degraded by lysosomal
acid lipases to free fatty acids, which then
can be metabolized by the liver (Cadórniga-
Valiño et al., 1997).
Metabolism of Essential Fatty Acids
Animals can synthesize fatty acids with
double bonds no closer than nine carbons
from the methyl end of the fatty acyl chain.
For example, stearic acid (18:0) can be
desaturated to oleic acid (18:1) by
desaturase enzymes in liver, adipose tissue,
intestinal mucosa and mammary gland.
The convention for nomenclature of the
position of double bonds within the fatty
acyl chain refers to the carbon number
starting from the methyl carbon end of the
fatty acid, with the methyl carbon referred
to as the ‘-carbon’. Thus, oleic acid is
referred to in shorthand notation as 18:1
-9, because the double bond occurs at the
ninth carbon from the methyl end.
Alternate nomenclature refers to this as the
‘n-9’ or ‘9’ position. Likewise, the enzyme
activity responsible for conversion of
stearic to oleic acid is usually referred to as
9-desaturase.
Polyunsaturated fatty acids with
double bonds nearer to the end of the chain
are required for normal formation of cell
membranes and synthesis of other key
regulatory molecules such as prosta-
glandins (Sardesai, 1992). These fatty acids
fall into two groups, the -6 series and -3
series. Because animal tissues are unable to
synthesize fatty acids with double bonds in
the -6 or -3 positions, such fatty acids
must be supplied in the diet. In most
species, the parent compounds of these
families, linoleic acid (18:2 -6) and
linolenic acid (18:3 -3), respectively, are
the only fatty acids that are required from
dietary sources. Consequently, these are
referred to as dietary essential fatty acids.
These fatty acids can be elongated and
desaturated to produce longer chain fatty
acids that are more highly unsaturated. For
example, linoleic acid can be converted to
arachidonic acid (20:4 -6) beginning with
6-desaturation, followed by elongation
and 5-desaturation (Fig. 5.4). However,
because the position of the final double
bond in the chain is always fixed from the
methyl end, linoleic acid cannot be con-
verted to eicosapentaenoic acid (20:5 -3)
or docosahexaenoic acid (22:6 -3). Cats
and some other carnivores have very limited
activities of the 6-desaturase enzyme, and
thus require dietary arachidonic acid as
well as linoleic and linolenic acid.Lipid Metabolism 113