Chemistry Reference
In-Depth Information
to yield neopinione, which exists in equilibrium with its tautomer codeinone. The
NADPH-dependent codeinone reductase catalyzes the reduction of codeinone to
codeine and has been cloned (50, 51). Finally, codeine is demethylated by an
uncharacterized enzyme to yield morphine.
The localization of isoquinoline biosynthesis has been investigated at the
cellular level in intact poppy plants by using
in situ
RNA hybridization and
immunoflouresence microscopy. The localization of 4
-OMT (reticuline biosyn-
thesis), berberine bridge enzyme (saguinarine biosynthesis), salutaridinol acetyl-
transferase (morphine biosynthesis), and codeinone reductase (morphine biosyn-
thesis) has been probed. 4
-OMT and salutaridinol acetyltransferase are localized
to parenchyma cells, whereas codeinone reductase is localized to laticifer cells in
sections of capsule (fruit) and stem from poppy plants. Berberine bridge enzyme
is found in parenchyma cells in roots. Therefore, this study suggests that two
cell types are involved in isoquinoline biosynthesis in poppy and that intercel-
lular transport is required for isoquinoline alkaloid biosynthesis (52). Another
study, however, implicates a single cell type (sieve elements and their compan-
ion cells) in isoquinoline alkaloid biosynthesis (53, 54). Therefore, it is not clear
whether transport of pathway intermediates is required for alkaloid biosynthesis
or whether the entire pathway can be performed in one cell type. Localization of
enzymes in alkaloid biosynthesis is difficult, and, undoubtedly, future studies will
provide more insight into the trafficking involved in plant secondary metabolism.
9.2.5 Metabolic Engineering of Morphine Biosynthesis
In attempts to accumulate thebaine and decrease production of morphine (a pre-
cursor to the recreational drug heroine), codeinone reductase in opium poppy
plant was downregulated by using RNAi (8). Silencing of codeinone reductase
results in the accumulation of (S)-reticuline but not the substrate codeinone or
other compounds on the pathway from (S)-reticuline to codeine. However, the
overexpression of codeinone reductase in opium poppy plants did result, in fact,
in an increase in morphine and other morphinan alkaloids, such as morphine,
codeine, and thebaine, compared with control plants (55). Gene expression levels
in low morphine-producing poppy plants have been analyzed also (56). Silencing
of berberine bridge enzyme in opium poppy plants also resulted in a change in
alkaloid profile in the plant latex (57).
The cytochrome P450 responsible for the oxidation of (S)-N-methylcoclaurine
to (S)-3
-hydroxy-N-methylcocluarine has been overexpressed in opium poppy
plants, and morphinan alkaloid production in the latex is increased subsequently
to 4.5 times the level in wild-type plants (58). Additionally, suppression of this
enzyme resulted in a decrease in morphinan alkaloids to 16% of the wild-type
level. Notably, analysis of a variety of biosynthetic gene transcript levels in these
experiments supports the hypothesis that this P450 enzyme plays a regulatory role
in the biosynthesis of benzylisoquinoline alkaloids. Collectively, these studies
highlight that the complex metabolic networks found in plants are not redirected
easily or predictably in all cases.
