15.2.3 Wax Esters, Hydrocarbons and "Ether Glycerides"
583
strate of flight muscle metabolism in the stable fly
Glossina morsitans and the potato beetle Leptinotarsa decemlineata. The proline metabolism of
these species was described on p. 421. Proline additionally plays a significant role as an energy substrate in the gut wall of the migratory locust and
in the mantle muscles of the cephalopods [35].
Fkatty acids are also important energy substrates in many vertebrate muscles, including those of
humans. The various energy substrates of human
muscles differ, on the one hand, in the amounts of
energy stored and, on the other hand, in the maximal possible rates of energy production (see Table 14.4, p.536). An adult male has an energy
reserve of about 340 000 kJ in body fat and only
about 7500 kJ in the liver glycogen and muscle
glycogen. Thus, the 12000 kJ of energy required
for a marathon run can only be obtained by oxidation of fatty acids in the muscles. The proportion of total energy used that is derived from fatty
acids is limited by the rate at which these can be
made available. In contrast to some birds and
fish, human muscles contain only small amounts
of fatty acids in the form of TAG; the muscle
therefore oxidizes mainly exogenous fatty acids
obtained from the fat tissues and transported in
the blood as non-esterified fatty acids (NEFA)
bound to albumin. The blood NEFA concentration is regulated between 0.3 and 2.0 mmol/l,
with the highest values being found during persistent muscle activity, during hunger and in the
diabetic condition. The release of fatty acids from
the fat tissues (lipolysis) is hormone controlled
with cAMP acting as the second messenger to
activate a protein kinase and bring about the conversion of lipase b to active lipase a. The fact that
fatty acids can be so rapidly turned over, despite
their low concentration in the plasma, is due to
their short residence time in the plasma; they
have a half-life of 2 min.
The intensive utilization of fatty acids in
muscle metabolism is a feature of the breast
muscles of the pigeon and other long-distance
fliers amongst the birds, and of certain fish muscles. Vertebrate muscles are made up of different
types of fibres which differ not only in their functional and contractile characteristics but also in
their complement of enzymes for energy-yielding
metabolism. The extremes are represented by the
"fast-twitch white" type, which is specialized for
anaerobic glycolysis production of energy for
sudden, short-lived activity, and the "slow-twitch
red" type, which is adapted for persistent activity, has a high capacity for fatty acid and pyruvate
oxidation, and owes its red colour to its myoglobin content (Table 15. 7b). Most vertebrates possess intermediate types with many muscles composed of several types of fibre. However, most of
the body musculature of the primitive fish is
made up exclusively of white fibres which are
used only during fleeing or hunting movements;
the thin red lateral muscles serve during continuous swimming. The red muscles of the salmon
use mainly fatty acids at the start of spawning
migration, and later use amino acids, with alanine being much more important than proline
[178].
15.2.3 Wax Esters, Hydrocarbons
and "Ether Glycerides"
Certain animal groups contain l-alkyl-2,3-diacylglycerols (ether glycerides), wax esters and
hydrocarbons as storage lipids, either in addition
to or instead of triacylglycerols. Glycerolipids
with an alcohol (alkyl residue) instead of a fatty
acid (acyl residue) at position sn-l are widely
found in the phospholipids but occur much less in
the neutral fats. Small quantities of l-alkyl-2,3diacylglycerols are found in the liver of mammals
and amphibians; larger quantities are found in
the preputial glands of the mouse and rat; and the
largest quantities amongst the vertebrates are
present in the elasmobranch. In fact, the trivial
names chimyl, batyl and selachyl alcohols for
I-hexadecyl-, l-octadecyl- and l-octadec-9-enylglycerol come from the elasmobranch Chimaeridae, Batoidei and Selachii, in which they were
first discovered. Ether glycerides with alkyl residues with 8-19 C atoms make up about 20-69 %
of the total lipids in the hepatopancreas of several
Japanese brachyurans. Almost 6 % of the body
lipids of the cephalopod Octopus dofleini consist
of ether glycerides, with the highest concentrations again occurring in the hepatopancreas.
1(2' -Hydroxyalkyl)- and 1(2' -methoxyalkyl)-2,3diacylglycerols have also been found in some marine animals [104].
Traces of wax esters arising from the diet are to
be found in probably all animals. Larger
amounts, constituting approximately 10 to over
90 % of the total lipids, have been reported in
more than 120 species of marine animals from
seven phyla [133]; a selection of the data is shown
in Table 15.8. Wax esters are particularly widespread in the crustacean groups Copepoda,
Euphausiacea and Mysidacea, in the cartilaginous and bony fish, including the crossopterygian
Latimeria chalumnae, and in the toothed whales.
583
strate of flight muscle metabolism in the stable fly
Glossina morsitans and the potato beetle Leptinotarsa decemlineata. The proline metabolism of
these species was described on p. 421. Proline additionally plays a significant role as an energy substrate in the gut wall of the migratory locust and
in the mantle muscles of the cephalopods [35].
Fkatty acids are also important energy substrates in many vertebrate muscles, including those of
humans. The various energy substrates of human
muscles differ, on the one hand, in the amounts of
energy stored and, on the other hand, in the maximal possible rates of energy production (see Table 14.4, p.536). An adult male has an energy
reserve of about 340 000 kJ in body fat and only
about 7500 kJ in the liver glycogen and muscle
glycogen. Thus, the 12000 kJ of energy required
for a marathon run can only be obtained by oxidation of fatty acids in the muscles. The proportion of total energy used that is derived from fatty
acids is limited by the rate at which these can be
made available. In contrast to some birds and
fish, human muscles contain only small amounts
of fatty acids in the form of TAG; the muscle
therefore oxidizes mainly exogenous fatty acids
obtained from the fat tissues and transported in
the blood as non-esterified fatty acids (NEFA)
bound to albumin. The blood NEFA concentration is regulated between 0.3 and 2.0 mmol/l,
with the highest values being found during persistent muscle activity, during hunger and in the
diabetic condition. The release of fatty acids from
the fat tissues (lipolysis) is hormone controlled
with cAMP acting as the second messenger to
activate a protein kinase and bring about the conversion of lipase b to active lipase a. The fact that
fatty acids can be so rapidly turned over, despite
their low concentration in the plasma, is due to
their short residence time in the plasma; they
have a half-life of 2 min.
The intensive utilization of fatty acids in
muscle metabolism is a feature of the breast
muscles of the pigeon and other long-distance
fliers amongst the birds, and of certain fish muscles. Vertebrate muscles are made up of different
types of fibres which differ not only in their functional and contractile characteristics but also in
their complement of enzymes for energy-yielding
metabolism. The extremes are represented by the
"fast-twitch white" type, which is specialized for
anaerobic glycolysis production of energy for
sudden, short-lived activity, and the "slow-twitch
red" type, which is adapted for persistent activity, has a high capacity for fatty acid and pyruvate
oxidation, and owes its red colour to its myoglobin content (Table 15. 7b). Most vertebrates possess intermediate types with many muscles composed of several types of fibre. However, most of
the body musculature of the primitive fish is
made up exclusively of white fibres which are
used only during fleeing or hunting movements;
the thin red lateral muscles serve during continuous swimming. The red muscles of the salmon
use mainly fatty acids at the start of spawning
migration, and later use amino acids, with alanine being much more important than proline
[178].
15.2.3 Wax Esters, Hydrocarbons
and "Ether Glycerides"
Certain animal groups contain l-alkyl-2,3-diacylglycerols (ether glycerides), wax esters and
hydrocarbons as storage lipids, either in addition
to or instead of triacylglycerols. Glycerolipids
with an alcohol (alkyl residue) instead of a fatty
acid (acyl residue) at position sn-l are widely
found in the phospholipids but occur much less in
the neutral fats. Small quantities of l-alkyl-2,3diacylglycerols are found in the liver of mammals
and amphibians; larger quantities are found in
the preputial glands of the mouse and rat; and the
largest quantities amongst the vertebrates are
present in the elasmobranch. In fact, the trivial
names chimyl, batyl and selachyl alcohols for
I-hexadecyl-, l-octadecyl- and l-octadec-9-enylglycerol come from the elasmobranch Chimaeridae, Batoidei and Selachii, in which they were
first discovered. Ether glycerides with alkyl residues with 8-19 C atoms make up about 20-69 %
of the total lipids in the hepatopancreas of several
Japanese brachyurans. Almost 6 % of the body
lipids of the cephalopod Octopus dofleini consist
of ether glycerides, with the highest concentrations again occurring in the hepatopancreas.
1(2' -Hydroxyalkyl)- and 1(2' -methoxyalkyl)-2,3diacylglycerols have also been found in some marine animals [104].
Traces of wax esters arising from the diet are to
be found in probably all animals. Larger
amounts, constituting approximately 10 to over
90 % of the total lipids, have been reported in
more than 120 species of marine animals from
seven phyla [133]; a selection of the data is shown
in Table 15.8. Wax esters are particularly widespread in the crustacean groups Copepoda,
Euphausiacea and Mysidacea, in the cartilaginous and bony fish, including the crossopterygian
Latimeria chalumnae, and in the toothed whales.