Formation of conjugated diene and triene products in lipoxygenase oxidation of C18, C20, C22 PUFAs

The determination of conjugated diene formation revealed that the mol % conversions of allcis-6,9,12-octadecatrienoic acid γ-linolenic, 18:3(n-6)], allcis-5,8,11,14-eicosatetraenoic acid arachidonic, 20:4(n-6)], allcis-5,8,11,14,17-eicosapentaenoic acid 20:5(n-3)], and allcis-4,7,10,13,16,19-docosahexaenoic acid 22:6(n-3)] into conjugated diene products by soybean lipoxygenase-1 at pH 9.0 were 84, 86, 60 and 40% of that of allcis-9,12-octadecadienoic acid linoleic, 18:2(n-6)], respectively. On the other hand, the conversions of allcis-9,12,15-octadecatrienoic acid α-linolenic, 18:3(n-3)], allcis-5,9,12-octadecatrienoic acid (c5,c9,c12-18:3), andtrans-5,cis-9,cis-12-octadecatrienoic acid (t5,c9,c12-18:3) were equal to that of 18:2(n-6). The lowering of the conjugated diene formation in the oxidation of 18:3(n-6), 20:4(n-6), 20:5(n-3), and 22:6(n-3) by the lipoxygenase was thought to be caused by the further oxidation of conjugated diene monohydroperoxides to yield conjugated triene products. For this reason, the conventional lipoxygenase method gave erroneous values forcis,cis-methylene interrupted polyunsaturated fatty acids (PUFA) in oils containing a large amount of 20:5(n-3) and 22:6(n-3) such as fish oils. However, by changing the pH of reaction mixtures from 9.0 to 11.0, the secondary oxidation of conjugated diene monohydroperoxides was completely inhibited, and the PUFA values in fish oils obtained by this improved method were in good agreement with those obtained by a GLC method.