Polyunsaturated fatty acids and eicosanoids in insects

Novel aspects of the metabolism of polyunsaturated fatty acids (PUFAs), precursors of prostaglandins and other eicosanoids, in insects are reviewed. A number of insect species are able to produce linoleic acid, 18:2 ($\text{n-6}$), de novo, a fatty acid that was previously believed to be essential for all animals. These insect species are unique among animals in that they possess a Δ12 desaturase which converts oleic acid, 18:1 ($\text{n-9}$) to 18:2 ($\text{n-6}$) and makes them nutritionally independent of plant derived polyunsaturated fatty acids (PUFA). The potential role of microorganisms in linoleate production was ruled out by studies using isolated tissue under axenic conditions from the house cricket, Acheta domesticus, an insect which does not contain intracellular microorganisms. The results showed that it is insect tissue that contains the Δ12 desaturase. This enzyme has been charactized from the house cricket and the American cockroach. In contrast to the plant Δ12 desaturase, which converts 18:1 esterified in a phospholipid as substrate to 18:2 ($\text{n-6}$), the insect Δ12 desaturase uses oleoyl-CoA as substrate. A number of insect species, including representatives of both those that do and do not produce linoleate, elongate and desaturate 18:2 ($\text{n-6}$) and 18:3 ($\text{n-3}$) to 20:4 ($\text{n-6}$) and 20:5 ($\text{n-3}$), respectively. A conspicuous exception to this are mosquitoes, which require 20:4 ($\text{n-6}$) or structurally related fatty acids in their diet. A number of insect species have been shown to metabolize 20:4 to prostaglandins and other eicosanoids.