Very early in the history of life, organisms began to use cholesterol.  Among other roles, it is an essential component of the cell membrane which prevents the membrane from being too fluid. MEMBRANE
     Early in the history of life, enzymes which metabolize cholesterol evolved, such as enzymes of the cytochrome P450 superfamily.  The cytochrome P450 superfamily of enzymes is involved in the metabolism of steroids, drugs, and xenobiotics in the liver (such as the image of cells of the pig liver below), and other sites such as the skin (Du, 2004).

     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.




     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.

     P450 enzyme polymorphisms can influence the effect of drugs and the risk of cancer (Dally, 2004).  It seems that some of the carcinogenic properties of estrogen result from the intermediates which are produced by P450 metabolism of estrogen (Russo, 2003).  The following image is of the cells of a uterine cancer.

St. Johns wort (pictured below) increases expression of multidrug resistence protein 2 (MRP2) and two P450 enzymes (Shibayama, 2004).