ally identical, and that little cross-reactivity exists
between FASs from birds and mammals [195].
FAS is localized in the cytosol; the necessary
acetyl-CoA, however, arises predominantly in the
mitochondria, the inner membranes of which are
impermeable to acetyl-CoA. Transport of acetyl
residues out of the mitochondria into the cytosol
in the rat is about 80 % in the form of citrate,
which is produced in the mitochondria by citrate
synthase and cleaved in the cytoplasm by citrate
lyase. This mechanism has also been demonstrated in the fat bodies of the locust Locusta migratoria. Acetyl carnitine as a transport form is of
relatively minor importance [63]. There are characteristic species-specific differences in the origin
of the acetyl-CoA and NADPH required for fatty
acid synthesis in the milk glands of various mammals. In contrast to other species, ruminants do
not incorporate 14C-glucose into fatty acids; free
acetate that can be turned over by acetyl-CoA
synthetase is apparently more important in this
case. Furthermore, the NADPH-dependent isocitrate dehydrogenase here is cytosolic and not
mitochondrial as in other mammals. In termites,
acetate delivered by symbionts can be converted
to acetyl-CoA, but pyruvate dehydrogenase, by
means of which carbohydrates can be introduced
into fatty acid synthesis, is missing [196].
The elongation substrate malonyl-CoA is provided by acetyl-CoA carboxylase, which has been
isolated from fatty acid-synthesizing tissues of the
mammals and birds as well as from higher plants,
yeast and bacteria. The enzyme is localized in the
cytoplasm and contains one biotin per mO'lf)r
The monomeric form is iJ1~~'
form is filamentous wit'
enzyme from chi ,-)
acids, corres'
nificiP-t'
J,.
15.1.2 Biosynthesis of Fatty Acids
571
the pigeon and goose, to 60:40 in the chicken.
Insect FAS also produces shorter fatty acids, e.g.
in the Mediterranean fruit fly Ceratitis capitata
there are fatty acids of 10-18 C atoms, and in
Drosophila melanogaster 14:0 occurs as well as
16:0 and 18:0 [219]. Pulse-labelling experiments
in Leishmania donovani have shown that the primary products are 12:0, 14:0 and 16:0 [112]. In
the species-specific very variable fatty acid spectrum of milk (Table 15.4), the rat, rabbit and goat
have considerable quantities of intermediatelength fatty acids (8: 0-12: 0). However, isolated
milk-gland FASs of the rat and rabbit only produce short- and long-chain fatty acids; those of
intermediate length arise by premature release
from the FAS complex through the action of a
soluble, 29-kDa thioesterase II. This soluble
enzyme is absent from the milk glands of the goat
and other ruminants, and in this case the formation of intermediate-length fatty acids involves
deviations in the specificity of the FAS thioesterase domain itself [195].
Soluble thioesterases that react only with acylCoA are widely found in animals and plants. In
contrast, a second type of enzyme that can also
cleave FAS-bound fatty acids is found in the milk
glands of some mammals and, in particular, in the
uropygial glands of many avian species. Here,
they are also responsible for the release of shorter
fatty acids from the FAS complex [40, 195]. Thus,
the uropygial gland of the pigeon contains no
fewer than four TE activities: the relevant FAS
domain, an acyl-FAS specific enzyme and two
~f)1""'1e acyl-CoA hydro lases [40]. The sequence
lJle :>~yl-FAS TE from the mallardAnas
at least in the region followabout 60 % with the TE
l-ole TE has also been
thosiphon pisum,
" mation of myr~es up 70% of
~lycerol-bound
the primer or
S accepts ace:quaUy as priCoA are also
y of the TE
ICt of even15:0 and 17:0
yryl-CoA is
,S of chicken
tty acids that
~, isoleucine
anched fatty
between FASs from birds and mammals [195].
FAS is localized in the cytosol; the necessary
acetyl-CoA, however, arises predominantly in the
mitochondria, the inner membranes of which are
impermeable to acetyl-CoA. Transport of acetyl
residues out of the mitochondria into the cytosol
in the rat is about 80 % in the form of citrate,
which is produced in the mitochondria by citrate
synthase and cleaved in the cytoplasm by citrate
lyase. This mechanism has also been demonstrated in the fat bodies of the locust Locusta migratoria. Acetyl carnitine as a transport form is of
relatively minor importance [63]. There are characteristic species-specific differences in the origin
of the acetyl-CoA and NADPH required for fatty
acid synthesis in the milk glands of various mammals. In contrast to other species, ruminants do
not incorporate 14C-glucose into fatty acids; free
acetate that can be turned over by acetyl-CoA
synthetase is apparently more important in this
case. Furthermore, the NADPH-dependent isocitrate dehydrogenase here is cytosolic and not
mitochondrial as in other mammals. In termites,
acetate delivered by symbionts can be converted
to acetyl-CoA, but pyruvate dehydrogenase, by
means of which carbohydrates can be introduced
into fatty acid synthesis, is missing [196].
The elongation substrate malonyl-CoA is provided by acetyl-CoA carboxylase, which has been
isolated from fatty acid-synthesizing tissues of the
mammals and birds as well as from higher plants,
yeast and bacteria. The enzyme is localized in the
cytoplasm and contains one biotin per mO'lf)r
The monomeric form is iJ1~~'
form is filamentous wit'
enzyme from chi ,-)
acids, corres'
nificiP-t'
J,.
15.1.2 Biosynthesis of Fatty Acids
571
the pigeon and goose, to 60:40 in the chicken.
Insect FAS also produces shorter fatty acids, e.g.
in the Mediterranean fruit fly Ceratitis capitata
there are fatty acids of 10-18 C atoms, and in
Drosophila melanogaster 14:0 occurs as well as
16:0 and 18:0 [219]. Pulse-labelling experiments
in Leishmania donovani have shown that the primary products are 12:0, 14:0 and 16:0 [112]. In
the species-specific very variable fatty acid spectrum of milk (Table 15.4), the rat, rabbit and goat
have considerable quantities of intermediatelength fatty acids (8: 0-12: 0). However, isolated
milk-gland FASs of the rat and rabbit only produce short- and long-chain fatty acids; those of
intermediate length arise by premature release
from the FAS complex through the action of a
soluble, 29-kDa thioesterase II. This soluble
enzyme is absent from the milk glands of the goat
and other ruminants, and in this case the formation of intermediate-length fatty acids involves
deviations in the specificity of the FAS thioesterase domain itself [195].
Soluble thioesterases that react only with acylCoA are widely found in animals and plants. In
contrast, a second type of enzyme that can also
cleave FAS-bound fatty acids is found in the milk
glands of some mammals and, in particular, in the
uropygial glands of many avian species. Here,
they are also responsible for the release of shorter
fatty acids from the FAS complex [40, 195]. Thus,
the uropygial gland of the pigeon contains no
fewer than four TE activities: the relevant FAS
domain, an acyl-FAS specific enzyme and two
~f)1""'1e acyl-CoA hydro lases [40]. The sequence
lJle :>~yl-FAS TE from the mallardAnas
at least in the region followabout 60 % with the TE
l-ole TE has also been
thosiphon pisum,
" mation of myr~es up 70% of
~lycerol-bound
the primer or
S accepts ace:quaUy as priCoA are also
y of the TE
ICt of even15:0 and 17:0
yryl-CoA is
,S of chicken
tty acids that
~, isoleucine
anched fatty