PLANT PHYSIOLOGY , Vol 109, Issue 1 299-305, Copyright © 1995 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
Glucosinolate Biosynthesis (Further Characterization of the Aldoxime-Forming Microsomal Monooxygenases in Oilseed Rape Leaves)
R. N. Bennett, A. J. Hick, G. W. Dawson and R. M. Wallsgrove
Biochemistry and Physiology Department (R.N.B., R.M.W.) and Biological and Ecological Chemistry Department (A.J.H., G.W.D.), IACR Rothamsted, Harpenden AL5 2JQ, United Kingdom
The initial steps in glucosinolate biosynthesis are thought to proceed from
amino acids, via N-hydroxy amino acids, to aldoximes. We showed previously
that microsomes from green leaves of oilseed rape (Brassica napus cv
Bienvenu) contain two distinct monooxygenases that catalyze the conversion
of homophenylalanine and dihomomethionine to their respective aldoximes.
Further characterization of these enzymes has now demonstrated that the
latter enzyme catalyzes the NADPH-dependent oxidative decarboxylation of
two higher homologs of methionine, in addition to dihomomethionine. No
activity was found for either enzyme with L-methionine, DL-homomethionine,
L-phenylalanine, L-tyrosine, or L-tryptophan. Both of these rape
monooxygenase activities are dependent on O2, not requiring any other O2
species or radical. The presence of an unoxidized sulfur atom and its
relative position in the side chain of the aliphatic substrates are
important for binding to the active site of the methionine-homolog enzyme.
Neither enzyme has any characteristics of a cytochrome P450-type enzyme,
and antiserum raised against cytochrome P450 reductase did not
significantly inhibit monooxygenase activity.
