Lipiä metabolism
73
be
quantitatively reduced to ethanol in order to regenerate the NAD+
required in glycolysis and for the
of
lactate
(Shoubridge and
Hochachka,
1981).
A
mechanimn
by which substrate
could
be
diverted
to
lipogensis
remains
nuclear°
Furthermore, ethanol, and hence acetyl CoA,
production
lactate
is
limited
to
skeletal muscle
(Shoubridge
and
Hochachka, 1981, 1983) whereas the major deposition of labile fat appears
to
be in the visceral mesenteries
(Delahunty and de Vlaming, 1980).
Nagai and Ikeda (1972, 1973) demonstrated that radioactivity from [U —
14€] glutamate and [U — 14C] alanine could be incorporated into carp liver
neutral
lipids
and
that
this
process
was
depressed.
in.
fish
fed.
a
high
carbohydrate
diet.
They
suggested
that
amino
acids
rather
than.
stored
carbohydrates were the preferred source of substrate for lipogenesis.
The
quantitative
importance
of
lipogenesis
from.
substrate
derived
from
the
mobilization
of
(muscle)
protein
in
the
overall
accumulation
of
body or
gonad fat is unknown.
Production of
fatty acids from amino acids requires
removal
of
the
amino
group
from
the
amino
acid
and ultimate conversion of
the deaminated
product to acetyl CoA, frequently by the way of Krebs cycle
reactions°
The
acetyl
CoA
for
lipogenesis
is
normally
furnished
by
cleavage of citrate which has been transferred from the mitochondria to the
cytoplasmn
The other cleavage product, oxaloacetate, can then be converted
to
malate,
or
subsequent1y,
to
pyruvate,
both
of
which
can
re—enter
the
mitochondria
for
production
of
more
citrate
(Vance,
1983).
The
mitochondrial production of citrate is a portion of the Krebs cycle.
Goldfish under anoxic conditions do not display a complete functioning
Krebs
cycle (Shoubridge and Hochachka, 1981).
It
would be of
interest
to
determine
if
the
components of the cycle required for the above scheme do
function
under
anoxia.
Further
questions
to
be
addressed
include
which
amino
acids
can
serve
as
lipid precursors under anoxic conditions and how
cytoplasmic
and
mitochondrial
redox
balance
would
,be
maintained
during
anoxic lipogenesis.
Clearly, more work is required to fully understand the
mechanisms
behind
Blazka‘s
(1958)
original
observation.
Under
aerobic
conditions,
lipogenesis_ from
amino
acids
may
be
of
some
significance
depending on the availability and composition of food.
As
well
as
influencing the amount of fat deposited by cyprinids, cold
acclimation can profoundly influence the fatty acid composition of cyprinid
tissues.
ln
general, cyprinids show the typical poikilotherm response to
cold
acclimation
of
increasing
the
unsaturation
of
tissue
phospholipid
fatty acids (eg° Knipprath and Mead, 1968; Miller.et al., 1976; Farkas and
Csengeri, 1976; Farkas et al, 1980; Farkas, 1984).
Increased unsaturation
of membrane phospholipids fatty acids maintains membrane fluidity and hence
function
at
low
temperature.
The
polyunsaturated
fatty
acid
4,7,10,13,16,l9—docosahexaeneic acid.(22:6 (n—3)) is frequently the most
abundant polyunsaturate in cyprinid phospholipids.
It
has
also been found
to
be
the
polyunsaturated. fatty acid that is
the
most
during
acclimation
to
cold
in
phospholipids
from
goldfish
intestinal‘
microsomes (Miller et al. 1976) and red muscle mitochondria (Wodtke, 1981)
and
in
total
liver
phospholipids from several carps (Farkas et
al.
1980).'
An
exception to this rule is gold fish liver mitochondria (Wodtke, 1978).
This fatty acid is also deposited in developing eggs and the proportion of
22:6
(n-3)
in
eggs
has
been
linked
to
hatching
success
(Shimma et
al.,
1977).
To produce 22:6 (n+3), carp, like other fish, require a dietary source
of
(n—3) fatty acid,
such
as
linolenic acid, 18:3
(n—3)
(Farkas et al.,
1980), Which has
essential
fatty
acid
characteristics
(for
rev1ew,
see
Watanabe, 1982).
Farkas
et
al.
(1977, 1980) found that
carp fed abundant
quantities
of
linolenic
acid
do.
not
store
significant
amounts
of
73
be
quantitatively reduced to ethanol in order to regenerate the NAD+
required in glycolysis and for the
of
lactate
(Shoubridge and
Hochachka,
1981).
A
mechanimn
by which substrate
could
be
diverted
to
lipogensis
remains
nuclear°
Furthermore, ethanol, and hence acetyl CoA,
production
lactate
is
limited
to
skeletal muscle
(Shoubridge
and
Hochachka, 1981, 1983) whereas the major deposition of labile fat appears
to
be in the visceral mesenteries
(Delahunty and de Vlaming, 1980).
Nagai and Ikeda (1972, 1973) demonstrated that radioactivity from [U —
14€] glutamate and [U — 14C] alanine could be incorporated into carp liver
neutral
lipids
and
that
this
process
was
depressed.
in.
fish
fed.
a
high
carbohydrate
diet.
They
suggested
that
amino
acids
rather
than.
stored
carbohydrates were the preferred source of substrate for lipogenesis.
The
quantitative
importance
of
lipogenesis
from.
substrate
derived
from
the
mobilization
of
(muscle)
protein
in
the
overall
accumulation
of
body or
gonad fat is unknown.
Production of
fatty acids from amino acids requires
removal
of
the
amino
group
from
the
amino
acid
and ultimate conversion of
the deaminated
product to acetyl CoA, frequently by the way of Krebs cycle
reactions°
The
acetyl
CoA
for
lipogenesis
is
normally
furnished
by
cleavage of citrate which has been transferred from the mitochondria to the
cytoplasmn
The other cleavage product, oxaloacetate, can then be converted
to
malate,
or
subsequent1y,
to
pyruvate,
both
of
which
can
re—enter
the
mitochondria
for
production
of
more
citrate
(Vance,
1983).
The
mitochondrial production of citrate is a portion of the Krebs cycle.
Goldfish under anoxic conditions do not display a complete functioning
Krebs
cycle (Shoubridge and Hochachka, 1981).
It
would be of
interest
to
determine
if
the
components of the cycle required for the above scheme do
function
under
anoxia.
Further
questions
to
be
addressed
include
which
amino
acids
can
serve
as
lipid precursors under anoxic conditions and how
cytoplasmic
and
mitochondrial
redox
balance
would
,be
maintained
during
anoxic lipogenesis.
Clearly, more work is required to fully understand the
mechanisms
behind
Blazka‘s
(1958)
original
observation.
Under
aerobic
conditions,
lipogenesis_ from
amino
acids
may
be
of
some
significance
depending on the availability and composition of food.
As
well
as
influencing the amount of fat deposited by cyprinids, cold
acclimation can profoundly influence the fatty acid composition of cyprinid
tissues.
ln
general, cyprinids show the typical poikilotherm response to
cold
acclimation
of
increasing
the
unsaturation
of
tissue
phospholipid
fatty acids (eg° Knipprath and Mead, 1968; Miller.et al., 1976; Farkas and
Csengeri, 1976; Farkas et al, 1980; Farkas, 1984).
Increased unsaturation
of membrane phospholipids fatty acids maintains membrane fluidity and hence
function
at
low
temperature.
The
polyunsaturated
fatty
acid
4,7,10,13,16,l9—docosahexaeneic acid.(22:6 (n—3)) is frequently the most
abundant polyunsaturate in cyprinid phospholipids.
It
has
also been found
to
be
the
polyunsaturated. fatty acid that is
the
most
during
acclimation
to
cold
in
phospholipids
from
goldfish
intestinal‘
microsomes (Miller et al. 1976) and red muscle mitochondria (Wodtke, 1981)
and
in
total
liver
phospholipids from several carps (Farkas et
al.
1980).'
An
exception to this rule is gold fish liver mitochondria (Wodtke, 1978).
This fatty acid is also deposited in developing eggs and the proportion of
22:6
(n-3)
in
eggs
has
been
linked
to
hatching
success
(Shimma et
al.,
1977).
To produce 22:6 (n+3), carp, like other fish, require a dietary source
of
(n—3) fatty acid,
such
as
linolenic acid, 18:3
(n—3)
(Farkas et al.,
1980), Which has
essential
fatty
acid
characteristics
(for
rev1ew,
see
Watanabe, 1982).
Farkas
et
al.
(1977, 1980) found that
carp fed abundant
quantities
of
linolenic
acid
do.
not
store
significant
amounts
of