PLANT PHYSIOLOGY , Vol 114, Issue 4 1283-1291, Copyright © 1997 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
Involvement of Cytochrome P450 in Glucosinolate Biosynthesis in White Mustard (A Biochemical Anomaly)
R. N. Bennett, G. Kiddle and R. M. Wallsgrove
Biochemistry and Physiology Department, IACR-Rothamsted, Harpenden AL5 2JQ, United Kingdom
One of the first steps in glucosinolate biosynthesis is the conversion of
amino acids to their aldoximes. The biochemistry of this process is
controversial, and several very different enzyme systems have been
described. The major glucosinolate in white mustard (Sinapis alba) is
sinalbin, which is derived from tyrosine via its aldoxime, and this
conversion is catalyzed by a cytochrome P450 (Cyt P450) monooxygenase.
Phenylethyl- and alkenylglucosinolates are also present in white mustard
leaves, as are the enzymes catalyzing the relevant aldoxime formation from
homophenylalanine and methionine homologs, respectively. These enzymes are
similar to those found in Brassica sp. and are distinct from the
tyrosine-dependent enzyme in that they contain no heme and are unaffected
by Cyt P450 inhibitors. They are instead inhibited by the flavoprotein
inhibitor diphenylene iodonium and by Cu2+. In both white mustard and
oilseed rape (Brassica napus) methyl jasmonate specifically stimulates
indolylglucosinolate biosynthesis and yet has no effect on sinalbin
accumulation in either cotyledons or leaves of white mustard. White mustard
appears to be unique among crucifers in having a Cyt P450 aldoxime-forming
enzyme for biosynthesis of one glucosinolate, although it also contains all
of the non-Cyt P450 enzyme systems found in other members of the family.
Sinalbin biosynthesis in white mustard is therefore an inappropriate model
system for the synthesis of other glucosinolates in crucifers, including
canola and oilseed rape.