Enzymic pathway to ethyl vinyl ketone and 2-pentenal in soybean preparations

Previous work. by this laboratory showed that under anaerobic conditions and the presence of a polyunsaturated fatty acid, soybean (Glycine max L.) lipoxygenase isoenzymes converted a lipoxygenase-catalyzed oxidation product of Linolenic acid, 13(S)-hydroperoxy-9(Z),11(E),15(Z)-octadecatrienoic acid, into 1-penten-3-ol, 2(Z)-penten-1-ol, and 13-oxo-9(Z),11(E)-tridecadienoic acid. It seemed plausible that the ''raw bean odor'', ethyl vinyl ketone, previously isolated from soybean homogenates by other workers could arise from oxidation of 1-penten-3-ol by alcohol dehydrogenase. It is shown here that both ethyl vinyl ketone and 2-pentenal are produced by a soybean preparation after anaerobic incubation with 13(S)-hydroperoxy-9(Z),11(E),15(Z)-octadecatrienoic acid and linolenic acid and that NAD(+) stimulated the formation of 2-pentenal. In the presence of NAD+, two separable isoenzymes of soybean alcohol dehydrogenase were capable of utilizing as substrates both 1-penten-3-ol and 2(Z)-penten-1-ol, as well as (2E)-hexen-1-ol. In terms of substrate preference indicated by K-m, the order was 2(E)-hexen-1-ol > 2(Z)-penten-1-ol > 1-penten 3-ol. Because ethyl vinyl ketone formed in the presence of only 13(S)-hydroperoxy-9(Z),11(E),15(Z)-octadecatrienoic acid and linolenic acid in the absence of NAD(+), another pathway also seemed possible.