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Leucine-Derived Cyano Glucosides in Barley1
Barley (Hordeum vulgare) seedlings contain five cyano glucosides derived from the amino acid L-leucine (Leu). The chemical structure and the relative abundance of the cyano glucosides were investigated by liquid chromatography-mass spectrometry and nuclear magnetic resonance analyses using spring barley cultivars with high, medium, and low cyanide potential. The barley cultivars showed a 10-fold difference in their cyano glucoside content, but the relative content of the individual cyano glucosides remained constant. Epiheterodendrin, the only cyanogenic glucoside present, comprised 12% to 18% of the total content of cyano glucosides. It is proposed that the aglycones of all five cyano glucosides are formed by the initial action of a cytochrome P450 enzyme of the CYP79 family converting L-Leu into Z-3-methylbutanal oxime and subsequent action of a less specific CYP71E enzyme converting the oxime into 3-methylbutyro nitrile and mediating subsequent hydroxylations at the alpha-, as well as beta- and gamma-, carbon atoms. Presence of cyano glucosides in the barley seedlings was restricted to leaf tissue, with 99% confined to the epidermis cell layers of the leaf blade. Microsomal preparations from epidermal cells were not able to convert L-[^sup 14^C]Leu into the biosynthetic intermediate, Z-3-methylbutanal-oxime. This was only achieved using microsomal preparations from other cell types in the basal leaf segment, demonstrating translocation of the cyano glucosides to the epidermal cell layers after biosynthesis. A beta-glucosidase able to degrade epiheterodendrin was detected exclusively in yet a third compartment, the endosperm of the germinating seed. Therefore, in barley, a putative function of cyano glucosides in plant defense is not linked to cyanide release.
Cyanogenesis, i.e. the ability of living cells to release cyanide under certain biotic and/or abiotic stress conditions, is an old trait (Lechtenberg and Nahrstedt, 1999). In most cases, cyanide release reflects cleavage of a cyanogenic glucoside into the corresponding cyanohydrin and Glc by the initial action of a beta-glucosidase. Subsequently, the cyanohydrin is cleaved into a ketone or aldehyde and hydrogen cyanide, either catalyzed by an a-hydroxy nitrilase or nonenzymatically. Cyanogenic glucosides have been found in over 3,000 plant species and may exert a role in plant defense reactions (Tattersall et al., 2001).
Cyanogenic glucosides are derived from the amino acids L-Val, L-Ile, L-Leu, L-Phe, or L-Tyr and the...