Marine organisms can contain a very wide range of components containing atoms of arsenic, and these include lipids, although they are usually present at low levels. Some authors take a rather wide view of what constitutes an arsenolipid and thus include compounds such as trimethylarsine and its metabolites as lipids, simply on the basis of their solubility in organic solvents. Here, only those lipids that have been characterized sufficiently to be certain that they contain long alkyl-chains together with arsenic atoms somewhere in the molecule are considered.
1. Hydrocarbons and Fatty Acids Containing Arsenic Atoms
Long-chain hydrocarbons with a terminal dimethylarsinoyl moiety were first isolated and characterized from capelin, Mallotus villosus, as recently as 2008, and they have since been detected in other fish species and in brown algae. The main isomers were 1-dimethylarsinoylpentadecane, together with C17 and C19 analogues as well as one thought to be 1-dimethylarsinoyl all-cis-4,7,10,13,16,19-docosahexaene, if a biosynthetic relationship to docosahexaenoic acid is assumed. Traces of even-numbered species have been found in one organism, and one compound with seven double bonds has been detected in cod liver.
Similarly, a series of saturated fatty acids with a terminal dimethylarsinoyl moiety was detected initially in cod-liver oil with 15 to 19 carbon atoms, i.e. the dimethylarsinoyl group appears to replace the terminal methyl group of conventional even-numbered fatty acids. In addition, unsaturated fatty acids, which appear to be structurally related to oleic, docosapentaenoic and docosahexaenoic fatty acids, were detected. Only one fatty acid with an even number of carbon atoms was found, i.e. C24H38AsO3. However, an even wider range of such fatty acids has been detected in further studies, and more will no doubt be reported in future. In many species, these fatty acids were not linked to the complex lipids. The two fatty acids illustrated below are presumably related biosynthetically to hexadecanoic and docosapentaenoic acids.
All fish oils examined appear to contain a similar range of hydrocarbons and fatty acids of this type, with the proportions varying somewhat among species and at overall concentrations of 4.3 to 10.5 mg per kg. Gas chromatography and high-performance liquid chromatography linked to mass spectrometry are the preferred methods of analysis.
The mechanism for the biosynthesis of the compounds is a matter for speculation at present, and it has been suggested that dimethylarsinoylpropionic acid might be the primer molecule for a fatty acid synthase, for example. Similarly, it is only possible to speculate where these compounds arise in the marine food chain, but brown algae are certainly one source.
One further type of arsenolipid with a long alkyl chain to have been discovered consists of cationic trimethylarsenio fatty alcohols of which two molecular species have so far been detected in fish oils.
2. Glycerolipids containing Arsenic Atoms
The presence of a number of arsenic-containing lipids has been inferred from degradative studies of marine lipids in which arsenical compounds have been isolated from hydrolysates. These include analogues of phosphatidylcholine and sphingomyelin, but they are present at such low levels that it has not been possible to isolate them in pure form for definitive characterization. For example, based on the isolation and identification of glycerophosphorylarsenocholine after alkaline hydrolysis of lipid extracts, it appears certain that phosphatidylarsenocholine is a minor component of the lipids of mullet, lobster and other marine species.
Similarly, it has been known for some time from studies of lipid hydrolysates that a complex arsenic-containing glycophospholipid is present in fish and other marine organisms, i.e. diacylglycerophospho-2-hydroxypropyl-5-deoxy-5-(dimethylarsinoyl)-β-ribofuranoside, and this has now been definitively characterized in brown algae by modern mass spectrometric methods.
In the brown alga Saccharina latissima, the main fatty acids are saturated (C15 to C20) with 16:0 predominating, although small amounts of unsaturated fatty acids are also present (16:1 to 18:3). Much of the palmitic acid is in position sn-2 of this lipid. The more conventional phospholipids in the organism had very different fatty acid compositions.
The presence of these lipids in fish oils do not appear to raise toxicity problems when consumed by mammals. They are absorbed from the gastrointestinal tract of mice and humans, but the arsenic is rapidly excreted in the form of organic water-soluble metabolites, mainly compounds containing dimethylarsine oxide moieties and arsenobetaine.
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Updated February 17, 2014