Ortiz de Montellano Paul R.(Ed.) Cytochrome P450. Structure, Mechanism, and Biochemistry
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Cytochrome P450 and the Metabolism of AA and Eicosanoids
545
dissected rat renal microvessels, the target organ
for most of the prohypertensive effects of 20-
HETE, possess an androgen-regulated CYP4A8
20-HETE synthase^^^.
The mechanism(s) by which Cyp4al4 gene
product(s) control plasma androgen levels are yet
to be defined; however, ample precedent supports
a role for the kidney androgen receptor in regu-
lating renal Cyp 4A and 2C expression^^^"^^"^.
Furthermore, in male and female rats, the androgen-
mediated increases in systemic blood pressure,
and in the levels of kidney CYP4A8 transcripts ^^^,
are accompanied by a marked decrease in the
levels of renal CYP2C23 epoxygenase protein
and diminished microsomal EET biosynthesis^^^.
The androgen-mediated counter-regulation of
renal CYP4A w-hydroxylases and 2C epoxy-
genases^^^ suggests that its effects on blood
pressure results from coordinated, nephron site-
specific, increases in the biosynthesis of prohy-
pertensive 20-HETE, and decreases in
antihypertensive EET formation. Finally, in mice,
activ^ation of PPARa upregulates the expression of
Cyps 4al0 and 4al4 (but not 4al2)i3^ and the
counter-regulation of rat Cyps 4A and 2C iso-
forms by PPARa ligands is published^ ^^.
Nevertheless, the pressure effects of PPAR ligands
in mice have yet to be fully defined, although, they
have been characterized as antihypertensive in
rats^^'
140,141
jjjg studies summarized suggest that
blood pressure regulation by renal P450s involves
combinations of regulatory (transcriptional) and
functional (tubular and hemodynamic) compo-
nents,
and that the organ balance of
pro-
and anti-
hypertensive mediators and thus, its functional
status,
is dictated by: (a) the nephron segment-
specific expression and regulation of the corre-
sponding genes, (b) the enzymatic properties of
the encoded proteins, and (c) the expression and
activities of ancillary enzymes responsible for
EET and/or HETE metabolism, disposition or
activation.
4. Conclusion
The studies of the P450 AA monooxygenase
have uncovered new and important roles for P450
in the metabolism of endogenous substrates, and
added P450 to the list of enzymes that participate
in the metabolism of
AA,
a fatty acid that serves
as the precursor for the biosynthesis of several
physiologically important lipid mediators. The
functional relevance of this metabolic pathway is
suggested by the many important biological activ-
ities attributed to its products. These studies, as
well as the documented endogenous roles of
P450s in cholesterol, steroid, and vitamin metabo-
lism are contributing to establish this enzyme
system as a major participant in the regulation of
cell, organ, and body physiology. Among these,
the phenotypic analysis of mice carrying dis-
rupted copies of the CYP4al4 gene unveiled new
and important roles for the P450 enzymes in car-
diovascular physiology and the control of systemic
blood pressures, and suggested the human
homologs of the rodent CYP 2C and 4A AA
epoxygenases and ca-hydroxylases as candidate
genes for the study of their role in the pathophysi-
ology of hypertension, and cardiovascular disease.
Acknowledgments
None of the studies carried out in the authors'
laboratory could have been possible without the
continuous and generous grant support from
NIGMS and NIDDK.
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12
Cytochrome P450s in Plants
Kirsten Annette Nielsen and Birger Lindberg Meller
1.
Introduction
Plants are sessile organisms that cannot avoid
exposure to adverse climatic conditions or attack
from herbivores and pests by escaping. To survive
and protect themselves, they are dependent on the
ability to (a) redirect their overall metabolism to
meet environmental constraints, (b) construct
physical barriers that are difficult to penetrate,
(c) produce chemicals that make the plant toxic to
pests and herbivores, and (d) communicate with
the environment, for example, to attract pollinators.
Cytochrome P450 enzymes (P450s) play a key role
in enabling plants to achieve these main goals.
1.1.
Natural Products
Plants are the best organic chemists in nature as
evidenced by their ability to synthesize all neces-
sary carbon compounds with carbon dioxide as the
sole carbon source and by their ability to synthe-
size a vast number of natural products. Currently,
structures for more than 100,000 different natural
products isolated from plants are known^~^, and
with time this number will increase into millions.
Natural products are classified as phytoanticipins,
phytoalexins, and/or attractants. In the last decade,
the majority of the biosynthetic pathways responsi-
ble for natural product synthesis have been shown
to include P450s as key enzymes. Such pathways
include the biosynthetic pathways for cyanogenic
glucosides, glucosinolates, isoflavonoids, and
alkaloids. In addition, a number of plant P450s
have been shown to catalyze detoxification of
harmfiil agents including herbicides'^.
1.2. Chemical Warfare
The chemical warfare between plants and her-
bivores and pests is complex and takes place at
many trophic levels. The plant Apium graveolens
(celery) is known to combat Helicoverpa zea (com
earworm) by producing allelochemicals including
fiiranocoumarins in a P450-dependent series of
reactions^. The herbivore, however, is able to
detoxify the friranocoumarins. The detoxification
pathway is induced by jasmonate, a wound-induced
plant signal compound. Jasmonate activates the
transcription at least of four herbivore P450 genes^.
The continuous chemical warfare between plants
on one side and herbivores and pests on the other
may enforce plants to constantly evolve new natu-
ral products and insects to find means to detoxify
these. This requires recruitment of enzymes with
altered biological functions probably mediated by
modifications and duplications of existing genes.
P450s are key players in securing recruitment of
these new functions as exemplified by the recruit-
ment of an allele specific P450 in Drosophila to
acquire resistance to chemical insecticides^.
1.3. Chemical Communication
Plants are dependent on intense communi-
cation with their surroundings via biochemical
Kirsten Annette Nielsen and Birger Lindberg IVIoller • Plant Biochemistry Laboratory, Royal Veterinary
and Agricultural University, 40, Thorvaldsensvej, DK-1871 Frederiksberg C, Copenhagen, Denmark.
Cytochrome P450: Structure, Mechanism, and Biochemistry, 3e, edited by Paul R. Ortiz de Montellano
Kluwer Academic / Plenum Publishers, New
York,
2005.
553
554
Kirsten A. Nielsen and Birger L. Moller
signalling, for example, to mediate pollination and
seed spreading ^ In cases with insect-mediated
pollination, plants synthesize insect attractants. A
special group of secondary metabolites, glucosi-
nolates, is produced within the taxonomic order
Capparales. Glucosinolates have dual roles acting
both as attractants for specialized insects and as
deterrents for generalist herbivores. P450s are key
enzymes in the biosynthesis of glucosinolates^' ^.
Nicotiana tabacum (tobacco) from the taxonomic
order Solanes produces cembranoid-type terpenes
as insect attractants. The cembranoid terpene gland
exudates contain a- and p-epimers of cembra-
2,7,1 l-triene-4,6-diol and these attract different
pollinating insects. Unfortunately, they also
enhance oviposition of the unwanted insect Myzus
nicotiana (red aphid)^^.
1.4. Medicinal Agents
Humans take advantage of plant natural prod-
ucts as drugs or lead compounds in medicine, for
example, as anesthetics and anticarcinogens.
Alkaloid-containing plants have been used in
human medicine for thousands of
years.
One very
large and structurally diverse group of alkaloids
are the tetrahydrobenzylisoquinoline alkaloids.
Papaver somniferum (opium poppy) from the tax-
onomic order Ranunculales produces more than
100 such L-tyrosine derived alkaloids including
the potent anesthetic morphine"' '^. The biosyn-
thesis of tetrahydrobenzylisoquinoline alkaloids
involves P450s with unique catalytic properties'^.
The nutraceuticals daidzein and genistein
belong to the isoflavonoids and are phytoestrogens
preventing breast and prostate cancers'"*. The
dietary compounds are synthesized especially
in Leguminosae belonging to the Fabales order.
Isoflavonoid biosynthesis also requires a unique
P450 enzyme that catalyzes aryl group migra-
tion^^. Alkaloids and isoflavonoids play important
roles in plant defense and their biosynthesis are
tightly regulated and inducible processes^' \
The purpose of this review is to highlight
recent key findings on plant P450s. The genome
sequencing programs have identified the P450s as
the largest superfamily in plants. The catalytic
properties of most of these P450s remain elusive.
We focus on P450s involved in the synthesis of
natural products belonging to the groups of
cyanogenic glucosides, glucosinolates, alkaloids.
and isoflavonoids. Special emphasis is on the
plant model Arabidopsis thaliana for which the
complete genome sequence is available'^ and
which is easily amenable to genetical modifica-
tions'^
and provides an excellent model plant for
metabolic engineering. Exploitation of P450s to
reach increased production levels for desired
natural compounds and transfer of entire biosyn-
thetic pathways into other plant species will be
discussed. The first part is a short presentation of
different tools used to achieve gene—^to function
relationship, the bottleneck in P450 functional
genomics
2.
The P450 Superfamily
in Plants
So far genomic and expressed sequence tag
(EST) sequencing projects have revealed a total of
1,059 plant P450 sequences'^. Phylogenetic
analyses based on translated raw DNA sequence
data have spaced the P450s into 10 clans that
include 59 families and an extensive number of
subfamilies'^'
^^.
In the A. thaliana genome alone,
a superfamily of 272 cytochrome P450 genes
including 26 pseudo genes were annotated and
named'^' ^'. These genes represent members of
45 out of the 59 currently assigned plant P450
families^^'
^^.
P450s constitute the largest and con-
tinuously expanding superfamily in plants. From
the Oryza sativa cvs japonica and indica (rice
cultivars)^^'
'^^
genome sequence projects, as many
as 458 predicted P450 genes were annotated by
the end of September 2002^^.
2.1.
Nomenclature
The large number of P450 enzymes found in
the Plant Kingdom are named and categorized
based on protein sequence identity and phyloge-
netic relationships^^. P450s assigned to the same
family share more than 40% sequence identity at
the amino acid level. Correspondingly, P450s
assigned to the same subfamily share more than
55%
sequence identity.
In plants, the identity rule has some exceptions
due to gene duplications and shuffling as
pointed out in Werck-Reichart et al (2002)^2. The
plant P450s are categorized into the following
Cytochrome P450s in Plants
555
families:
CYP51,
CYP71-99, CYP701-727, and
CYP73620. The CYP51 family is unique because
the sequence identity of the P450s belonging to
this family is well-enough conserved across phyla
to contain plant, fungal, bacterial as well as animal
sequences^^. The precise structure of the sterols
that serve as substrates for CYPSls varies among
different eucaryotes. This variability has been sug-
gested to represent adaptation to the availability of
different sterol precursors in different Kingdoms.
Plant P450s are membrane-bound proteins.
They are classified into the A-type and the non-A-
type P450s^^' ^^. It has been proposed that the
plant-specific A-type P450s originate fi*om a sin-
gle ancestral P450^^. A-type P450s share a simple
gene organization with a single phase 0 intron
with a highly conversed position^^. Typically, the
P450 genes are found to cluster with close rela-
tives on short stretches of all five^. thaliana chro-
mosomes indicating recent duplication events^^.
P450s involved in the biosynthesis of plant natural
products belong to the A-type. In this review, we
focus on A-type P450s belonging to the
CYP71,
CYP79, CYP80, CYP83, and CYP93 families and
their respective involvement in the biosynthesis of
cyanogenic glucosides, glucosinolates, alkaloids,
and isoflavonoids. A short description of the
biological function of non-A-type plant P450s is
confined to members of the CYP85 and CYP90
families that are involved in the production of
polyhydroxylated steroidal molecules.
3. Tools Available to Identify
Biological Functions
Only very few of the 246 predicted
P450 enzymes present in A. thaliana have had a
biological function assigned. Functional assign-
ments of the A. thaliana P450s are restricted to
23 enzymes belonging to 14 of
the
45 plant fami-
lies represented in the A. thaliana genome. These
identified enzymatic activities are: CYP51,
obtusifoliol 14a-demethylase^^; C7P725i,brassi-
nolide 26-hydroxylase^^; CYP73A5, cinnamate-4-
hydroxylase^^;
CYP74A1,
allene oxide synthase^^'^'^;
CYP74B2, hydroperoxide lyase^^; CYP75B1,
flavonoid 3'-hydroxylase^^; CYP79A2, phenyl-
alanine iV-hydroxylase^^; CYP79B2 and CYP79B3,
tryptophan A/-hydroxylases^^; CYP79F1 and
CYP79F2, methionine TV-hydroxylases^^-^^;
CYP83A1 and CYP83B1 enzymes converting
indole-3-acetaldoxime, p-hydroxyphenylacetal-
doxime, and phenylacetaldoxime into the
corresponding ^S-alky l-thiohydroximates"^^"^"^;
CYP84A1,
ferulic acid hydroxylase^^; CYP85A1,
steroid C-6-hydroxylase^^; CYP86A1 and
CYP86A8, fatty acid G5-hydroxylases^^' 4^;
CYP88A3 and CYP88A4 enzymes converting ent-
kaurenoic acid to GA^/^; CYP90A1, steroid C-23
hydroxylase^^; CYP90B1, steroid C-22 hydroxy-
lase^^;
CYP98A3, 3'-hydroxylase of phenolic
esters^^;
and CYP701A3, ^w^kaurene oxidase^^.
3.1.
Phylogenetic Relationships
The categorization of P450s from different
plant species into families and subfamilies based
on sequence identity and phylogenetic relation-
ships as discussed above typically does not con-
comitantly lead to an assignment of biological
and/or enzymatic function. In some families, the
P450s all appear to catalyze the same enzymatic
reaction. In other families, members of the same
family clearly catalyze very different enzymatic
reactions. These differences are illustrated with
the following examples. The CYP73 family is
composed of one subfamily, CYP73A with 37
members. CYP73As from Helianthus tuberus
(artichoke)^"^, Phaseolus aureus (mung bean)^^,
Medicago sativa (alfalfa)^^, Petroselium crispum
(parsley)^^, Popupus tremuloides (querken
aspen)^^'
^^, A. thaliana^^, Triticum aestivum
(wheat)^^, Cicer arietinum (chickpea)^ ^ have all
been demonstrated to be cinnamate 4-hydroxy-
lases.
The rest of the members of the CYP73A
subfamily are therefore with great confidence
assigned as cinnamate 4-hydroxylases solely
based on their amino acid sequence identity.
Members of the CYP74A subfamily have been
characterized as allene oxide synthases in
A.
thaliana^^'
^^, Linum usitatissium (flaxseed)^^,
Hordeum vulgare (barley)^^, and Lycopersicon
esculentum (tomato)^"^. However, members of the
closely related CYP74B subfamily possess fatty
acid hydroperoxide lyase activity as demonstrated
in A. thaliana and L. esculentum (tomato)^^' ^^.
Members of a single subfamily may also catalyze
different and consecutive steps in a biosynthetic
pathway as reported for members of CYP90A and
CYP90B (see Section 4.1). The different steps of