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Title: OXYGENATION OF (3Z)-ALKENAL TO (2E)-4-HYDROXY-2-ALKENAL IN SOYBEAN SEED (GLYCINE MAX L.)

Author
item TAKAMURA, HITOSHI - NARA WOMEN'S UNIV, JAPAN
item Gardner, Harold

Submitted to: Journal of Biological Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/14/1996
Publication Date: N/A
Citation: N/A

Interpretive Summary: Plants produce characteristic volatile odors from polyunsaturated fatty acids. Previous work showed that two of these odors impart "grassy" or "cucumber" notes and that these volatile compounds are one strategy plants use to protect themselves from disease-causing fungi. In soybeans, these odors are produced, but they have a fleeting existence. It was found in this study that the odors are biologically degraded by soybeans into two pathways. One transforms grassy and cucumber odors into less intense "spicy-green" and "shoe-leather" odors, respectively. The other pathway gives compounds with no odor. Both pathways lead to end-products that are equally or more toxic to fungi as shown by previous work.

Technical Abstract: (3Z)-Alkenals, such as (3Z)-hexenal and (3Z)-nonenal, are produced from polyunsaturated fatty acids via lipoxygenase and hydroperoxide lyase catalysis, but in soybeans (Glycine max L.), (3Z)-alkenals have a fleeting existence. In this study, it was shown that soybeans possess two pathways that metabolize (3-Z)-alkenals. One is a soluble (3Z):(2E)-enal isomerase that transformed (3Z)-hexenal and (3Z)-nonenal into the corresponding (2E) alkenals. The other was a membrane-bound system that converted (3Z)- hexenal and (3Z)-nonenal into (2E)-4-hydroxy-2-hexenal and (2E)-4-hydroxy- 2-nonenal, respectively. The latter conversion was shown to absorb O with a pH optimum of 9.5. Little effects observed with lipoxygenase inhibitors ruled out oxidation catalyzed by lipoxygenase. Instead a specific (3Z)- alkenal, oxygenase was implicated. The putative 4-hydroperoxy-2-alkenals formed were presumably reduced by hydroperoxide-dependent peroxygenase (epoxygenase) known to be present in soybean membranes. A greater inhibition of 4-hydroxy-2-alkenal formation by preincubation with 5mM H2O2 versus simulataneous addition of H2O2 implicated peroxygenase since H2O2 preincubation is known to inactivate hydroperoxide-dependent peroxygenasees. Because 4-hydroxy-2-alkenals are potent toxins, the compounds may be produced as nonvolatile plant defensive substances.