Transformations of alpha-linolenic acid in leaves of corn (Zea mays L.)

Short incubation of [1-14C]alpha-linolenic acid with whole homogenate of leaves of corn (Zea mays L.) led to the formation of 4 major lipoxygenase products, i.e. 12-oxo-13-hydroxy-9(Z),15(Z)-octadecadienoic acid, 12-oxo-10,15(Z)-phytodienoic acid (12-oxo-PDA), 9(R,S)-hydroxy-12-oxo-10(E),15(Z)-octadecadienoic acid, and 13-oxo-9,11-tridecadienoic acid. Trapping experiments demonstrated transient appearance of the unstable allene oxide, 12,13(S)-epoxy-9(Z),11,15(Z)-octadecatrienoic acid, precursor of the three first mentioned compounds. The major lipoxygenase products formed from endogenous alpha-linolenic acid upon homogenization of corn leaves were identified as 12-oxo-13-hydroxy-9(Z),15(Z)-octadecadienoic acid (11.4 +/- 2.4 micrograms/g) and 12-oxo-PDA (5.8 +/- 1.2 micrograms/g). Steric analysis showed that 12-oxo-PDA formed from endogenous alpha-linolenic acid was largely the natural 9(S),13(S) enantiomer. Thus, corn leaves, like seeds of corn, contain hydroperoxide dehydrase and allene oxide cyclase. However, the lipoxygenase of corn leaves specifically catalyzed oxygenation of the 13 (omega 6) position of alpha-linolenic acid and thus differed from the previously recognized corn seed lipoxygenase which is specific for the 9 position.