In addition, it has been shown for soybean LOX-1 that its positional specificity
against LA is affected by a pH-dependent mechanism (Gardner, 1989). By contrast,
the (13S)-hydroperoxy derivative is formed at all pH values, though in the presence
of Tween 20 (possibly by shielding the carboxylate group at neutral or acidic pH-
values), 9-lipoxygenation was not observed above pH 8.5 and it increased linearly
when the pH was shifted towards more acidic values. From these data it was con-
cluded that the (9S)-hydroperoxy derivative is formed only from the nonionized
carboxylic acid form of LA, and methyl esterification of LA blocked the formation
of the (9S)-hydroperoxy derivative by LOX-1, but not that of the (13S)-hydroper-
oxide. Hence it was suggested that 9-lipoxygenation occurs only when the fatty acid
substrate is aligned in a head-to-tail orientation compared to 13-lipoxygenation (see
15.3 The structural bases of the positional specificity of LOXs 331
Figure 11. Formation of oxylipins in plants by the LOX pathway.