ETHER LIPIDS
Lipids with ether bonds to long-chain alkyl moieties in addition to having ester bonds to fatty acids are not important constituents of many lipids of commercial value, but they are very common in nature, especially as membrane constituents. Usually the ether bond is to position sn-1 of a glycerol moiety, which may be part of a non-polar lipid or more often a phospholipid in animal tissues or in anaerobic bacteria. At one time, they were considered to be little more than a biological novelty. However, findings of elevated levels of ether lipids in cancer tissues, followed by the discovery of distinctive ether lipids, such as platelet-activating factor (with its own web page), with important biological activities have greatly stimulated the interest in these compounds.
Two main types of glycerol ether bonds exist in natural lipids - ether and vinyl ether as illustrated. The double bond adjacent to the oxygen atom in the latter has the Z or cis configuration.
| The terms "plasmanyl-" and "plasmenyl-" lipids for alkyl and alkenyl ethers, respectively, are recommended by IUPAC-IUB, but they do not appear to have been widely taken up in the literature. Ether bonds are stable to both alkaline and acidic hydrolysis under most practical conditions, but vinyl ether bonds open readily under acidic conditions to form aldehydes. |
| For example, when acidic transesterification methods are used to prepare fatty acid methyl esters from lipid extracts that contain a proportion of vinyl ether bonds, free aldehydes are generated that are rapidly converted to dimethyl acetals and can be analysed in this form. |
In animal tissues, the alkyl and alkenyl moieties in both non-polar and phospholipids tend to be rather simple in composition with 16:0, 18:0 and 18:1 (double bond in position 9) predominating. Other alkyl groups may be present, but other than in fish lipids they are found at low levels only. The trivial names - chimyl, batyl and selachyl alcohols - are given to the glycerol ethers with 16:0, 18:0 and 18:1 alkyl groups, respectively, in the sn-1 position.
Pure ether lipids are rarely easy to separate from the fully acylated forms, and often their presence in tissues is inferred from isolation of their hydrolysis products or from spectroscopic methods.