15.2.2 Energy Substrates in Muscles and Other Tissues
581
catabolized in the presence of sufficient oxygen.
As well as their major importance as energy reserves, fat stores can have other biological functions,
e.g. heat insulation in seals and whales, or the
improvement of buoyancy in the elasmobranchs,
which have no swim bladders [270], and also in
some teleosts, whales and aquatic invertebrates.
The lipid-rich forehead projection (melon) of dolphins and toothed whales presents a very unusual
function for lipids; it forms an ultrasound lens
with which to focus the sounds emitted in echo
navigation (p. 567; Table 15.5). Fats also serve as
energy supplies in the secretions intended for the
nutrition of young animals. The fat content of
mammalian milk is particularly high in species in
which the young have especially high energy
requirements (see Table 13.2, p. 467); this is the
case, for example, for young mice because of
their small body size, and for young whales
because of their unusual lifestyle. The nutritional
secretions discussed in the following sections
include the crop milk of pigeons, the stomach oils
of petrels and the secretion from the Dufour
glands of several hymenopterans.
15.2.1 Triacylglycerols (Triglycerides)
The most common reserve fats, the triacylglycerols (TAGs), have fatty acid compositions which
vary widely with the species and diet. In mammals, the main site of TAG production and storage is the fat-tissue cells (adipocytes), but the gut
mucosa, liver and milk glands are also capable of
intensive TAG synthesis. In the insects, the gut
wall and the fat-body cells are the main sites of
TAG synthesis and storage. The biosynthesis of
TAG can occur via one of three pathways. The
glycerol-3-phosphate (GP) pathway and the
dihydroxyacetone phosphate (DHAP) pathway
proceed via phosphatidic acids, which are also
the starting point for phospholipid synthesis
(Fig. 15.22, p.603). The phosphatidic acids are
dephosphorylated by an acidic phosphatase to
1,2-diacylglycerol (DAG), which can then be converted to TAG by acyl transfer. The third possibility, the monoacylglycerol (MAG) pathway,
leads from the product of lipase activity 2monoacylglycerol via DAG to TAG. The DHAP
pathway is of great importance in just a few mammalian organs, e.g. the brain. The MAG pathway
has been mainly investigated in the cells of the
small intestine, in which the MAG and fatty acids
extracted from the gut lumen are resynthesized to
give TAG and are given up into the lymph as chylomicrons after combination with phospholipids,
cholesterol and proteins (p. 194).
The fat tissues of the rat contain all three pathways of TAG synthesis in the order GP > DHAP
> MAG, but the existence of the MAG pathway
in the milk glands is in some doubt. The acylation
of 2-monoacylglycerol is not an unambiguous
indication of this pathway, because in fact MAG
can enter the GP pathway after hydrolysis to
glycerol and phosphorylation to glycerol-3phosphate. Even the low acylation of 2monoacylglycerol without prior hydrolysis,
observed in the milk glands of the goat, is no
indication for the existence of the MAG pathway,
as this can be explained by acyl transfer by lipases
[85]. The GP pathway of TAG synthesis is widely
found in plants and animals, and the MAG pathway has been clearly demonstrated in insects. In
the microsomes of the Mediterranean fruit fly
Ceratitis capitata, the acylation of glycerol-3phosphate (G-3-P) is inhibited by MAG,
whereas, conversely, G-3-P has no effect on
MAG acylation. In Periplaneta americana, the
MAG pathway appears to playa larger role in the
midgut than in the fat bodies; in Locusta migratoria, the MAG pathway serves primarily in the
formation of DAG as a transport form, and the
GP pathway serves for TAG storage [63].
15.2.2 Energy Substrates in Muscles
and Other Tissues
The greatest specialization in favour of particular
energy-yielding substrates is found in the Oight
muscles of the insects. Because of their tracheal
system, they have a purely aerobic metabolism in
which different species show preference for the
utilization of carbohydrate, fat or proline. The
muscles display corresponding characteristic differences in their complement of enzymes for
energy-yielding metabolism (Table 15.7a). The
flight muscles of hymenopterans and dipterans,
which usually fly only short distances, and the
thorax muscles of the non-flying cockroaches are
specialized to use carbohydrates. The glycogen
reserves of the flight muscles last for only a
limited time; the haemolymph trehalose, which is
made in the fat bodies from glycogen, is then
respired. In contrast, the flight muscles of longdistance fliers, such as many lepidopterans and
orthopterans can utilize fats. Several of these species also use mainly carbohydrates at the beginning of flight, but then gradually switch over to
the oxidation of fatty acids; this has been inves-
581
catabolized in the presence of sufficient oxygen.
As well as their major importance as energy reserves, fat stores can have other biological functions,
e.g. heat insulation in seals and whales, or the
improvement of buoyancy in the elasmobranchs,
which have no swim bladders [270], and also in
some teleosts, whales and aquatic invertebrates.
The lipid-rich forehead projection (melon) of dolphins and toothed whales presents a very unusual
function for lipids; it forms an ultrasound lens
with which to focus the sounds emitted in echo
navigation (p. 567; Table 15.5). Fats also serve as
energy supplies in the secretions intended for the
nutrition of young animals. The fat content of
mammalian milk is particularly high in species in
which the young have especially high energy
requirements (see Table 13.2, p. 467); this is the
case, for example, for young mice because of
their small body size, and for young whales
because of their unusual lifestyle. The nutritional
secretions discussed in the following sections
include the crop milk of pigeons, the stomach oils
of petrels and the secretion from the Dufour
glands of several hymenopterans.
15.2.1 Triacylglycerols (Triglycerides)
The most common reserve fats, the triacylglycerols (TAGs), have fatty acid compositions which
vary widely with the species and diet. In mammals, the main site of TAG production and storage is the fat-tissue cells (adipocytes), but the gut
mucosa, liver and milk glands are also capable of
intensive TAG synthesis. In the insects, the gut
wall and the fat-body cells are the main sites of
TAG synthesis and storage. The biosynthesis of
TAG can occur via one of three pathways. The
glycerol-3-phosphate (GP) pathway and the
dihydroxyacetone phosphate (DHAP) pathway
proceed via phosphatidic acids, which are also
the starting point for phospholipid synthesis
(Fig. 15.22, p.603). The phosphatidic acids are
dephosphorylated by an acidic phosphatase to
1,2-diacylglycerol (DAG), which can then be converted to TAG by acyl transfer. The third possibility, the monoacylglycerol (MAG) pathway,
leads from the product of lipase activity 2monoacylglycerol via DAG to TAG. The DHAP
pathway is of great importance in just a few mammalian organs, e.g. the brain. The MAG pathway
has been mainly investigated in the cells of the
small intestine, in which the MAG and fatty acids
extracted from the gut lumen are resynthesized to
give TAG and are given up into the lymph as chylomicrons after combination with phospholipids,
cholesterol and proteins (p. 194).
The fat tissues of the rat contain all three pathways of TAG synthesis in the order GP > DHAP
> MAG, but the existence of the MAG pathway
in the milk glands is in some doubt. The acylation
of 2-monoacylglycerol is not an unambiguous
indication of this pathway, because in fact MAG
can enter the GP pathway after hydrolysis to
glycerol and phosphorylation to glycerol-3phosphate. Even the low acylation of 2monoacylglycerol without prior hydrolysis,
observed in the milk glands of the goat, is no
indication for the existence of the MAG pathway,
as this can be explained by acyl transfer by lipases
[85]. The GP pathway of TAG synthesis is widely
found in plants and animals, and the MAG pathway has been clearly demonstrated in insects. In
the microsomes of the Mediterranean fruit fly
Ceratitis capitata, the acylation of glycerol-3phosphate (G-3-P) is inhibited by MAG,
whereas, conversely, G-3-P has no effect on
MAG acylation. In Periplaneta americana, the
MAG pathway appears to playa larger role in the
midgut than in the fat bodies; in Locusta migratoria, the MAG pathway serves primarily in the
formation of DAG as a transport form, and the
GP pathway serves for TAG storage [63].
15.2.2 Energy Substrates in Muscles
and Other Tissues
The greatest specialization in favour of particular
energy-yielding substrates is found in the Oight
muscles of the insects. Because of their tracheal
system, they have a purely aerobic metabolism in
which different species show preference for the
utilization of carbohydrate, fat or proline. The
muscles display corresponding characteristic differences in their complement of enzymes for
energy-yielding metabolism (Table 15.7a). The
flight muscles of hymenopterans and dipterans,
which usually fly only short distances, and the
thorax muscles of the non-flying cockroaches are
specialized to use carbohydrates. The glycogen
reserves of the flight muscles last for only a
limited time; the haemolymph trehalose, which is
made in the fat bodies from glycogen, is then
respired. In contrast, the flight muscles of longdistance fliers, such as many lepidopterans and
orthopterans can utilize fats. Several of these species also use mainly carbohydrates at the beginning of flight, but then gradually switch over to
the oxidation of fatty acids; this has been inves-