Leucine-derived cyano glucosides in barley.
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-[(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.
Plant Biochemistry Laboratory, Department of Plant Biology, Royal Veterinary and Agricultural University, Frederiksberg, Copenhagen, Denmark. kani@kvl.dk
Kirsten Annette
KA
Nielsen
Carl Erik
CE
Olsen
Katrine
K
Pontoppidan
Birger Lindberg
BL
Møller
D000900:Anti-Bacterial Agents / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism; D001430:Bacteriocins; D002250:Carbon Radioisotopes; D002851:Chromatography, High Pressure Liquid; D005658:Fungi / Q000254:growth & development; D005960:Glucosides / Q000737:chemistry / Q000302:isolation & purification* / Q000378:metabolism; D001467:Hordeum / Q000378:metabolism* / Q000382:microbiology; D007930:Leucine / Q000378:metabolism*; D013058:Mass Spectrometry; D009570:Nitriles / Q000378:metabolism*; D010455:Peptides; D018515:Plant Leaves / Q000166:cytology / Q000378:metabolism / Q000382:microbiology; D013379:Substrate Specificity; D001617:beta-Glucosidase / Q000378:metabolism
D000900:Anti-Bacterial Agents; D001430:Bacteriocins; D002250:Carbon Radioisotopes; D005960:Glucosides; D009570:Nitriles; D010455:Peptides; C050558:epidermin; D001617:beta-Glucosidase; D007930:Leucine