Enzymic Pathway to Ethyl Vinyl Ketone and 2-Pentenal in Soybean Preparations

Abstract

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 Km, 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. Keywords: Lipoxygenase; 1-penten-3-ol; 2(Z)-penten-1-ol; alcohol dehydrogenase; ethyl vinyl ketone; 2-pentenal; linolenic acid; hydroperoxide; flavor