An organism’s normal functioning requires
the detoxification of hormones and ingested xenobiotic compounds. In the three domains of life, more than 2000
cytochrome P450 enzymes are known which have been classified into more
than 235 families. These proteins are present in virtually all
eukaryotic cells where they are typically attached to membranes (such
as those of the ER or mitochondria) (Williams, 2004; Jacobs, 2003).
Prokaryotes
possess P450 cytochromes but they are soluble in the cytosol (Boddpaulli,
1992). Some bacteria possess as many as 20 P450 enzymes while
others lack them. In bacteria,
they function in a number of metabolic pathways (including the metabolism
of pollutants) and drug resistance (De Mot, 2002; Rowley, 2003). Bacterial CYP102A1, A2, and A3 are fusion proteins
which combine heme and reductase domains. They hydroxylate fatty acids and mutations in
these genes produce enzymes which can hydroxylate a diversity of unrelated
substrates (Lentz, 2004). Eubacteria
and archaea are pictured in the following images.
Plants (such as Arabidopsis) can possess
about 275 different genes in this family, where they can function in biosynthesis
and herbicide resistance. In insects
cytochromes function in development and resistance to toxins in pesticides
and ingested plants. About 90 are
known in fruit flies (De Mot, 2002).
The
iron present in these hemoproteins (which provides pigment) and the peak
optical absorption of 450 nm are referred to in the name of these enzymes. P450 enzymes are involved in sterol biosynthesis
in fungi, plants, and animals. There
are two subfamilies of P450 cytochromes: class I are iron-sulfur proteins
on mitochondrial membranes and class II are flavoproteins of endoplasmic
reticulum (Boddpaulli, 1992). Seventeen
of the eighteen families of P450 enzymes present in mammals are also represented
in fish (the only exception being the CYP39 family). Of the eighteen families, duplications of the
CYP2 family members have produced the greatest diversity (Nelson, 2003). Fifty-seven P450 enzymes are known in humans
(Cauffiez, 2004; Du, 2004). Human
P450 enzymes break down fatty acids, sterols, steroids, vitamins A and
D (Du, 2004).
Arachidonic acid is transformed by P450 enzymes
to a number local hormones which control calcium concentrations, the release
of hormones, platelet aggregation, and other roles. The CYP2J, CYP2N, and CYP2P subfamilies of P450
enzymes function in arachidonic acid metabolism and are members of a clade
of P450 enzymes shared between fish and mammals (Oleksiak, 2003).
Capsaicin
can be broken down by members of the C, D, and E families. Nicotine can
be broken down by members of the B, C, D, and E families (Du,
2004).
CYP1A1 metabolizes polycyclic
aromatic hydrocarbons. One allele
(present in 10% Caucasians) increases the risk of lung cancer in smokers
and pregnant women possessing this allele who smoke increase the risk
of lower birthweight in the child. Liver
cells are depicted below.
CYP1A2 metabolizes aflatoxin,
acetaminophen, caffeine, and many arylamines. There is significant variation in the levels
of human expression and this may contribute to the susceptibility to environmental
toxins and cancer caused by environmental agents. This enzyme is known to metabolize more than
20 drugs and may compose about 15% of the P450 cytochromes produced by
the liver. Expression levels are
higher in men than women and are lowered by the use of oral contraceptives. Evolutionary comparisons of CYP1A2 gene sequences
support an African origin for modern humans.
CYP2A enzyme removes
a side chain from cholesterol molecules and is expressed in tissues which
synthesize steroid hormones (such as the adrenal cortex, the gonads, and
the placenta). In cells, it occurs
on the inner mitochondrial membrane. Mutations
in this gene cause lipid hyperplasia.
CYP2A6 metabolizes nicotine
to cotinine. Individuals whose
mutations result in the absence of functional enzyme are less likely to
smoke (or to smoke less if they do smoke) and are protected from some
of the carcinogenic aspects of smoking since CYP2A6 also metabolizes the
nitrosamines of smoke, producing carcinogenic products.
CYP2B is induced by
phenobarbital. Its expression is
increased by beta-carotene and may explain the co-carcinogenic effect
of beta-carotene observed in smokers—in metabolizing compounds in cigarette
smoke, this enzyme may create more reactive molecules which are linked
to cancer.
CYP2B6 metabolizes a
number of drugs and its expression affects the success of drugs for cardiovascular
disorders. CYP2B6 metabolizes estrogen, estrone,
testosterone, retinoic acid, and ingested compounds such as nicotine,
coumarin, and ochratoxin A (Du, 2004).
Liver cells are depicted below.
CYP2C9 metabolizes tolbutamide
used in the treatment of diabetes and mutations in the gene result in
poor metabolism of the drug.
CYP2C8 influences the
effect of certain substances on endothelial vasodilation.
CYP2C18 is one of several
cytochromes which are expressed more on one side of the heart than the
other and have an effect on the efficacy of certain drugs for the heart.
CYP2C19 is also known
as aromatase and estrogen sythetase. It
converts androgens into estrogens and has a role in producing gender differences
in adipose distribution (by being activated in some regions in women but
not men). Both males and females
require the action of estrogen in
the development of normal skeletal morphology. Not only is it expressed where steroids are
synthesized (such as the adrenal gland in the following image) but also
in muscle, hair, adipose, the brain, and the liver. Mutations cause abnormalities in sexual differentiation
and maturation and, in women, can cause amenorrhea, polycystic ovaries,
and possibly virilization as well.
CYP2D6 is expressed
in the dermis.
CYP2E is expressed on
hepatic microsomes where it binds carbon monoxide. It is induced by ethanol. One polymorphism is associated with increased
alcohol consumption while another allele increases the risk of lymphoid
malignancy.
CYP2F1 metabolizes ethoxycoumarin.
CYP2G1 and CYP2G2 are
expressed in the mammalian olfactory mucosa although in humans the alleles
of these genes are usually nonfunctional.
CYP2J2 is primarily
expressed in the heart, liver, intestines, and kidneys. It is also called arachidonic acid epoxygenase.
CYP2R1 is expressed
in the epidermis. The human epidermis
is depicted in the following image.
CYP2U1 is expressed
in the epidermis.
CYP2W1 is expressed
in the epidermis.
CYP2S1 is expressed
in the epithelia of the respiratory and digestive tracts (such as the
intestinal lining in the following image) and its expression is induced
by dioxin.
Two CYP2T pseuodogenes
are located on chromosome 19.
CYP3A subfamily is the
major subfamily involved in the metabolism of over 500 clinical drugs. These enzymes are the most abundant P450 enzymes
produced by the liver.
CYP3A5 metabolizes drugs
and sex hormones. There are allele
differences across different ethnic groups.
CYP3A4
in the human liver is the most common P450 enzyme and it metabolized drugs
and estrogen (Williams, 2004).
CYP3A43 is expressed
most in the liver, gastrointestinal tract, and the kidney.
CYP4A11 is involved
in the metabolism of fatty acids and prostaglandins.
CYP4B1 is known to be
expressed in the lungs and mutations may be involved in bladder cancer.
The
subfamily CYP4F is involved in the synthesis and breakdown of eicosinoids,
some of which are involved in inflammation (Cauffiez, 2004).
CYP4F2 metabolizes a
leukotriene involved in inflammation.
CYP4F3 metabolizes leukotrienes.
CYP7A1 functions in
the first step of bile acid synthesis in liver microsomes. The concentration of this enzyme affects LDL
levels. The following image is
of a liver lobule from a pig liver.
CYP8B1 is involved in
bile acid biosynthesis.
CYP11B2 is involved
in aldosterone biosynthesis. Mutations
can cause hypoaldosteronism, hypertension, and susceptibility to low renin
levels.
CYPB1 acts as a monoxygnease
in the metabolism of dioxin and other halogenated aromatic compounds (some
of which can bind to helix-turn-helix transcription factors).
Mutations can cause glaucoma.
CYPD genes are located in a cluster on chromosome 22 (CYPD6 is functional and several pseudogenes are present as well). Increased CYPD expression is linked to cancer of the lung, bladder, liver, pharynx, and stomach, especially in smokers. CYPD metabolizes about 14 drugs and detoxifies a number of neurotoxins and xenobiotics. Some of the products of these reactions may be more reactive than the substrate and be potential carcinogens. One common polymorphism (which can reflect ethnic group) can determine who well certain drugs are metabolized. One mutation results in the inability to metabolize debrisoquine—it is said that one head of pharmacy died due to the drug and was the first known case of a polymorphism which affected drug metabolism (OMIM).
CYP24 metabolizes vitamin
D. Mutations can increase the risk
of atherosclerosis.
CYP26A1 and CYP26 A2
are involved in the metabolism of retinoic acid (which is an important
signal in embryological development).
These enzymes could limit the areas where retinoic acid is active. In mice, mutations cause spinal abnormalities.
CYP27A1 functions in
mitochondria in the metabolism of sterols.
Mutations can cause cerebrotendiosus xanthomatosus.
CYP46 is involved in
cholesterol metabolism.
CYP51 is the only P450
gene family found in all eukaryotic phyla and serves as a “housekeeping
gene”.
Thromboxane synthase
is a P450 cytochrome which converts the prostaglandin endoperoxide to
thromboxane. Mutations result in
clotting difficulties.
P450 ENZYMES AND STEROID
HORMONES
Steroids are universally found in organisms, and function as components of the cell membrane, hormones, vitamins, and cytotoxins. They were probably among the original molecules which existed in primitive organisms (Agarwal, 1993). Some bacteria and plants convert cholesterol into hormones which can be active in animals and plants. All P450 synthesis of steroids begins with cholesterol. Plants have been shown to produce cortisol, mineralcorticoids, progestins, testosterone, estrogens, and ecdysone (Agarwal, 1993).
The
genome duplications which occurred early in vertebrate history amplified
the families of steroid hormone receptors and the P450 enzymes and hydroxysteroid
dehydrogenases involved in the synthesis of steroids (Baker, 2004).
The following diagram depicts the importance
of P450 enzymes in catalyzing steps involved in the production of a variety
of steroid hormones from cholesterol.
