O

vS-CoA

+ malonyl CoA 0

S-CoA

desaturase, - CoA-SH O

arachidonic acid

Figure 3.10 The biosynthetic relationship between oleic, linoleic, linolenic and arachidonic acids

3.1.3 Eicosanoids

A series of compounds called collectively eicosanoids, important in human physiology, are made from arachidonic acid using cyclo-oxygenase enzymes (Figure 3.11). They include prostaglandins, leucotrienes, thromboxanes and lipoxins. They are widely distributed in various tissues and exert hormone-like effects, and are effective at very low concentrations. One of the most important effects of aspirin (O-acetylsalicylic acid) is to block the cyclo-oxygenase enzyme and prevent the formation of prostaglandins. The subject of human eicosanoids is well covered in many biochemical texts.

What is important here is that prostaglandins and eicosanoids appear to be produced in all kinds of vertebrates and invertebrates. In insects they affect egg-laying, salt and water transport and cellular immune defenses. Prostaglandin GA2 (PGA2) (Figure 3.11) seems to be important in insect immune response.

Figure 3.11 The probable formation of PGA2 in insects, based upon the known reactions in vertebrates. The first step in the series is inhibited by aspirin. The prostaglandins PGG2 and PGF2a are intermediates in the sequence

methyl (R,Z)-14-methyl-8-hexadecenoate (R.Z)-14-methyl-8-hexadecenol

Figure 3.12 The origin of branched iso- and anteiso-fatty acids illustrated for the C16 and C17 acids respectively. Iso-acids have an even number of carbon atoms, anteiso-acids have an odd number. The two unsaturated anteiso-compounds are part of the sex pheromone of some Trogoderma species methyl (R,Z)-14-methyl-8-hexadecenoate (R.Z)-14-methyl-8-hexadecenol

Figure 3.12 The origin of branched iso- and anteiso-fatty acids illustrated for the C16 and C17 acids respectively. Iso-acids have an even number of carbon atoms, anteiso-acids have an odd number. The two unsaturated anteiso-compounds are part of the sex pheromone of some Trogoderma species

3.1.4 Branched Fatty Acids

Branched fatty acids, known as iso-acids and anteiso-acids, occur normally in small quantities in fats. Their synthesis begins with the amino-acids valine and isoleucine (Figure 3.12). This has been demonstrated with radio-labelled isotopes, by radio-active monitoring and with stable isotopes by 13C nuclear magnetic resonance spectroscopy or mass spectrometry. Both isobutyric acid and 2-methylbutyric acid are common defensive compounds among insects. Note that a chiral centre is introduced in 2-methylbutyric acid and anteiso acids.

The kapra beetle Trogoderma granarium and other species of Trogoderma use derivatives of the anteiso-unsaturated acid (R,Z)~ 14-methyl-8-hexadecenoic acid, such as the methyl ester and the corresponding alcohol (Figure 3.12), the aldehyde and derivatives with the £"-double bond, as part of the female-produced sex attractant.

l-Valine has been shown to be incorporated efficiently into meth-acrylic and isobutyric acids in the defensive secretion from the pygidial glands of the carabid beetle Scarites subterraneus (Attygalle, Meinwald and Eisner, Tetrahedron Letters, 1991, 32, 4849). The work was done by replacing all the hydrogens on carbon of valine with deuterium (Figure 3.13), and analyzing the methacrylic and isobutyric acids by mass spectrometry.

Females of the hemipteran bug Alydus eurinus release 2-methylbutyl butyrate and (£)-2-methylbutenyl butyrate as an attractant pheromone (Figure 3.14). Adults, when disturbed, also produce a defensive secretion of butyric and hexanoic acids from metathoracic glands.

Small amounts of odd-numbered fatty acids, e.g. C15, C17, and C19 are encountered in insects, as well as in vertebrate fat. These are biosynthe-sized starting from a propionic acid group to which are added acetate units in the usual way.

cd-c-c, -- cd-c-dt -- cd-c' -^ ^ .c-c, d3c' ^ oh d3c- oh d3c oh d2c oh deuterated valine deuterated deuterated isobutyric acid methacrylic acid

Figure 3.13 The biosynthesis of isobutyric acid and methacrylic acid in a beetle demonstrated by deuterium labelling

Figure 3.14 Small-molecule esters that form the sex pheromone of the bug Alydus eurinus

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