11. Lipids in Marine and Freshwater Organisms
285
11.2.6. Ether Lipids
The "ether lipids" are based on glycerol in which one alcohol oxygen is not
esterified to a fatty acid but instead becomes an ether oxygen joining the glycerol
with a long chain similar to fatty acids such as hexadecanoic or octadecenoic (Fig.
11.6). They are found in one or more organs of most animals including marine and
freshwater life forms. The 1-0-alkyl diacyl glyceryl ethers (DAGE) of Figure
11.6 (II) have been long known because of their frequent occurrence in the liver
oils of sharks (Kang et aI., 1997; Bordier et aI., 1996; Urata and Takaishi, 1996;
Bakes and Nichols, 1995). This is not a species characteristic as depot fats of
deep-sea squid may also have DAGE (Hayashi and Kawasaki, 1985). There has
been much discussion of the reason for DAGE, as they coexist with triacylglycerols in marine depot fats, and there is even an entire book on the subject
(Mangold and Paltauf, 1983), because the ether lipids are found in mammals as
well. Buoyancy control is usually brought forward as the rationale for the coexistence of DAGE and triacylglycerols, but the evidence is not totally convincing
that this is the only reason for their existence (Bakes and Nichols, 1995). The bulk
of the work has been on marine animal lipids rather than on those of freshwater
organisms (Sargent, 1989). The DAGEs (Fig. 11.6, II), if present, are nonpolar
and are usually found in depot fats of aquatic organisms. The membrane phospholipids (often loosely called the "polar" lipids) are apt to include to 1-0-alkyl-acyl
phospholipids (Fig. 11.6, I), and may well be accompanied by the 1-0-alk-1'enacyl-alkyl phospholipid (Fig. 11.6, III). The latter structure, often called a
plasmalogen, is unstable in mineral acids, cleaving at the ether group to give an
aldehyde (Fig. 11.6, V) which in acid preparation of methyl esters becomes a
dimethyl acetal. As the dimethyl acetal, this material is sometimes confusing in
analyses of methyl esters by GLe, where it may coincide with isoacids (Brosche,
1985). High proportions of the latter with chain lengths corresponding to 16:0,
18:0, or 18: 1 are therefore suspect. The result of recent examinations of polar
classes of lipids by modem technology is that the proportions among ether phospholipids are often found to be very similar. This is illustrated by the lipids of four
different organisms listed in Table 11.6. Many aquatic invertebrates contain the
polar phospholipids II, III, and IV of Fig. 11.6 (Sargent, 1989).
The biochemistry of these ether lipids is not linked to any special animal group,
although muscle of several sharks has recently been shown to contain all the
alk-1' -enyl-acyl and 1-0-alk-l' -enyl-2-acyl-sn-glycero-3-phosphatidyl-ethanolamine and -choline classes, more so in the ethanolamine group of polar lipids than
in the choline group (Jeong et aI., 1996a). The I-O-alkyl chains of structure I (Fig.
11.6) were shown by this group to be heavily oriented to 16:0, 16: 1,18:0, and 18:1
structures. The same was true for the ether phospholipids of the ascidian Halocynthia roretzi and the sea urchin Strongylocentrotus intermedius (Jeong et aI.,
I996b). The acyl groups in these lipids are interesting in having nearly 50% of
20:5(03 and much less 22:6(03. This ratio probably reflects a phytoplankton and
macrophyte (epiphyte?) diet. The 2-position acyl chains were highly unsaturated,
285
11.2.6. Ether Lipids
The "ether lipids" are based on glycerol in which one alcohol oxygen is not
esterified to a fatty acid but instead becomes an ether oxygen joining the glycerol
with a long chain similar to fatty acids such as hexadecanoic or octadecenoic (Fig.
11.6). They are found in one or more organs of most animals including marine and
freshwater life forms. The 1-0-alkyl diacyl glyceryl ethers (DAGE) of Figure
11.6 (II) have been long known because of their frequent occurrence in the liver
oils of sharks (Kang et aI., 1997; Bordier et aI., 1996; Urata and Takaishi, 1996;
Bakes and Nichols, 1995). This is not a species characteristic as depot fats of
deep-sea squid may also have DAGE (Hayashi and Kawasaki, 1985). There has
been much discussion of the reason for DAGE, as they coexist with triacylglycerols in marine depot fats, and there is even an entire book on the subject
(Mangold and Paltauf, 1983), because the ether lipids are found in mammals as
well. Buoyancy control is usually brought forward as the rationale for the coexistence of DAGE and triacylglycerols, but the evidence is not totally convincing
that this is the only reason for their existence (Bakes and Nichols, 1995). The bulk
of the work has been on marine animal lipids rather than on those of freshwater
organisms (Sargent, 1989). The DAGEs (Fig. 11.6, II), if present, are nonpolar
and are usually found in depot fats of aquatic organisms. The membrane phospholipids (often loosely called the "polar" lipids) are apt to include to 1-0-alkyl-acyl
phospholipids (Fig. 11.6, I), and may well be accompanied by the 1-0-alk-1'enacyl-alkyl phospholipid (Fig. 11.6, III). The latter structure, often called a
plasmalogen, is unstable in mineral acids, cleaving at the ether group to give an
aldehyde (Fig. 11.6, V) which in acid preparation of methyl esters becomes a
dimethyl acetal. As the dimethyl acetal, this material is sometimes confusing in
analyses of methyl esters by GLe, where it may coincide with isoacids (Brosche,
1985). High proportions of the latter with chain lengths corresponding to 16:0,
18:0, or 18: 1 are therefore suspect. The result of recent examinations of polar
classes of lipids by modem technology is that the proportions among ether phospholipids are often found to be very similar. This is illustrated by the lipids of four
different organisms listed in Table 11.6. Many aquatic invertebrates contain the
polar phospholipids II, III, and IV of Fig. 11.6 (Sargent, 1989).
The biochemistry of these ether lipids is not linked to any special animal group,
although muscle of several sharks has recently been shown to contain all the
alk-1' -enyl-acyl and 1-0-alk-l' -enyl-2-acyl-sn-glycero-3-phosphatidyl-ethanolamine and -choline classes, more so in the ethanolamine group of polar lipids than
in the choline group (Jeong et aI., 1996a). The I-O-alkyl chains of structure I (Fig.
11.6) were shown by this group to be heavily oriented to 16:0, 16: 1,18:0, and 18:1
structures. The same was true for the ether phospholipids of the ascidian Halocynthia roretzi and the sea urchin Strongylocentrotus intermedius (Jeong et aI.,
I996b). The acyl groups in these lipids are interesting in having nearly 50% of
20:5(03 and much less 22:6(03. This ratio probably reflects a phytoplankton and
macrophyte (epiphyte?) diet. The 2-position acyl chains were highly unsaturated,