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PLANT PHYSIOLOGY , Vol 115, Issue 4 1661-1670, Copyright © 1997 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
Isolation and Reconstitution of Cytochrome P450ox and in Vitro Reconstitution of the Entire Biosynthetic Pathway of the Cyanogenic Glucoside Dhurrin from Sorghum
R. A. Kahn, S. Bak, I. Svendsen, B. A. Halkier and B. L. Moller
Plant Biochemistry Laboratory, Department of Plant Biology, The Royal Veterinary and Agricultural University, 40 Thorvaldsensvej, DK-1871 Frederiksberg C, Copenhagen, Denmark (R.A.K., S.B., B.A.H., B.L.M.)
A cytochrome P450, designated P450ox, that catalyzes the conversion of
(Z)-p-hydroxyphenylacetaldoxime (oxime) to p-hydroxymandelonitrile in the
biosynthesis of the cyanogenic glucoside
[beta]-D-glucopyranosyloxy-(S)-p-hydroxymandelonitrile (dhurrin), has been
isolated from microsomes prepared from etiolated seedlings of sorghum
(Sorghum bicolor L. Moench). P450ox was solubilized using nonionic
detergents, and isolated by ion-exchange chromatography, Triton X-114 phase
partitioning, and dye-column chromatography. P450ox has an apparent
molecular mass of 55 kD, its N-terminal amino acid sequence is
-ATTATPQLLGGSVP, and it contains the internal sequence MDRLVADLDRAAA.
Reconstitution of P450ox with NADPH-P450 oxidoreductase in micelles of
L-[alpha]-dilauroyl phosphatidylcholine identified P450ox as a
multifunctional P450 catalyzing dehydration of (Z)-oxime to
p-hydroxyphenylaceto-nitrile (nitrile) and C-hydroxylation of
p-hydroxyphenylacetonitrile to nitrile. P450ox is extremely labile compared
with the P450s previously isolated from sorghum. When P450ox is
reconstituted in the presence of a soluble uridine diphosphate glucose
glucosyltransferase, oxime is converted to dhurrin. In vitro reconstitution
of the entire dhurrin biosynthetic pathway from tyrosine was accomplished
by the insertion of CYP79 (tyrosine N-hydroxylase), P450ox, and NADPH-P450
oxidoreductase in lipid micelles in the presence of uridine diphosphate
glucose glucosyltransferase. The catalysis of the conversion of Tyr into
nitrile by two multifunctional P450s explains why all intermediates in this
pathway except (Z)-oxime are channeled.
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