Hydroperoxides of linolenate

The cloning, characterisation and expression of many lipoxygenase (TOX) [17] and hydroperoxide lyase (HPL) [18] genes has led researchers to propose new processes for the production of green note flavours. HPT specifically produces the highly demanded compound ds-3-hexenal from the 13-hydroperoxide of linolenic acid and hexanal from the hydroperoxide of linoleic acid, both of which are formed by TOXs (Scheme 26.2). 

Hydroperoxides of linolenate decompose more readily than those of oleate and linoleate because active methylene groups are present. The active methylene groups are the ones located between a single double bond and a conjugated diene group. The hydrogen at this methylene group could readily be abstracted to form dihydroperoxides. The possibilities here for decomposition products are obviously more abundant than with oleate oxidation. 

The primary generated hydroperoxides of linolenic acid -9S-hydroperoxy-10E,12Z,16Z-octadecatrienoic acid (9S-HP0TE) and 16S-hydro-peroxy-9Z,12Z,14E-octadecatrienoic acid (16S-HP0TE) -... 

Figure 3. The formation of hydroperoxy cyclic peroxides and prostaglandin-like endoperoxides from the 13S-hydroperoxide of linolenic acid by peroxy radical rearrangement. Structures are...

Figure 2.11. Formation of bicyclic endoperoxides from 12- and 13-hydroperoxides of linolenate.

The large number of precursors of volatile decomposition products affecting the flavor of oils has been discussed in Chapter 4. Only qualitative information is available on the relative oxidative stability of hydroperoxides, aldehydes and secondary oxidation products. As observed with the unsaturated fatty ester precursors, the stability of hydroperoxides and unsaturated aldehydes decreases with higher unsaturation. Different hydroperoxides of unsaturated lipids, acting as precursors of volatile flavor compounds, decompose at different temperatures. Hydroperoxides of linolenate and long-chain n-3 PUFA decompose more readily and at lower temperatures than hydroperoxides of linoleate and oleate. Similarly, the alkadienals are less stable than alkenals, which in turn are less stable than alkanals. The short-chain fatty acids produced by oxidation of unsaturated aldehydes will further decrease the oxidative stability of polyunsaturated oils. For secondary products, dimers are less stable than dihydroperoxides, which are less stable than cyclic peroxides. 

In some cases the action of lipoxygenase leads to development of a characteristic aroma. Galliard et al. (1976) found that the main aroma compounds of cucumber, 2-trans hexenol and 2-trans, 6-cis-nonadienal, are produced by reaction of linolenic acid and lipoxygenase to form hydroperoxide... 

Figure 4. Formation of hydroperoxides and conjugated dienes (CD) as a function ofthe radiation dose (dose rate = 9.8 Gy. min ) in linolenate (18 3) aqueous micellar solutions atpH=l 0.5. Influence ofthe concentration of linolenate (25 mM and 7.5 mM)on the peroxidation process [16].

Elaboration of the discovery that 15-hydroperoxy-AA inhibits PGI2 synthetase s has revealed that 12-hydroperoxy-AA inhibits thromboxane synthetase, while u)-8 and 03-10 hydroperoxides of linoleic, dihomo-y-linolenic and AA inhibit prostacyclin synthetase.It seems possible that these hydroperoxides may perform a negative feedback role to inhibit excess thromboxane or PGI2 formation. The effects of the 5-hydroperoxy-AA formed by rabbit polymorphonuclear leucocytes have not been investigated. 

Likewise, LX)X occurs constitutively in tomato fruit and leaves (49,58,60,111-113). Both Type I and II LOX s are present in the tomato fruit which produce both lipid hydroperoxides (of linoleic and linolenic acids) and carbonyls (e.g., malondialdehyde)(49,58). The tomato variety "Castlemart" shows low levels of constitutive LOX type I activity, but much higher levels have been found in wild species of Lvcooersicon. particularly in L. hirsutum f. glabratum (99 unpubl. data). It has been suggested... 

Table 3.2 Headspace analysis of oxidized soya bean oil (peroxide value 9.5) by three methods direct injection, dynamic headspace gas chromatography (HSGC) and static HSGC. OlOOH = oleic acid hydroperoxide LoOOH = linoleic acid hydroperoxide LnOOH = linolenic acid hydroperoxide.

A novel enzyme system, so far only identified in potato tubers, converts the 9-hydroperoxides of linoleic and linolenic acids to divinyl ether derivatives (colneleic and colnelenic acids, respectively) in which an atom of oxygen is inserted into the fatty acid chain (Galliard and Phillips, 1972 Galliard... 

As shown in Fig. 3, the initial products from the 9- and 13-hydroperoxides of linoleic acid are the volatile aldehydes c/j-3-nonenal and hexanal and the corresponding C9 and C12 0x0 acid fragments. Analagous volatile products from linolenic acid are cw-3,c/s-6-nonadienal and cw-3-hexenal. However, in most plants, an isomerase enzyme converts the cis-3-enals to the trans-2 isomers (see Fig. 3). Such an enzyme cis-Z, trans-l-emA isomerase has been partially purified from cucumber fruits (Phillips et al., 1979). 

The significantly higher proportion of the 16-hydroperoxide relaHve to that of the 9-hydroperoxide of Hnolenate (Table 2.3) indicates that oxygen attack favors the terminal carbon-16 position of the 12,16-pentadienyl radical closest to the end of the fatty acid chain of linolenate. The same preference of oxygen... 

PGG and PGH, which promote platelet aggregation and can cause smooth muscle contraction and inflammation (Figure 2.14). Thromboxanes are important metabolites of PGG and PGH formed by ring opening (Figures 13.10, 13.11) they are also involved in platelet aggregation and implicated in thrombosis (see Chapter 13). The bicycloendoperoxides produced by autoxida-tion of pure 13-hydroperoxide of a-linolenate and 9-hydroperoxide of... 

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