Intro to Pharm and Tox Topics   

Cytochrome P450 Enzymes

Phase 1 reactions include oxidation reactions by Cytochrome P450 (CYP). CYP is a large group of drug-metabolizing enzymes in humans, composed of a hemoprotein and a mono-oxygenase. P450 refer to the ability of the reduced (ferrous) form of the hemoprotein to react with carbon monoxide yielding a complex with an absorption at 450 nm.

CYP is a complex of a hemoprotein and a monooxigenase. Ir requires energy, in the form of NADPH, and oxygen. One molecule of oxygen is consumed for each substrate oxidized. One oxygen atom goes to the subtrate, another to water:

What is this reaction suposed to be? Click to enlarge ==>

P450 catalyzes many oxidation reactions: aliphatic hydroxylation, aromatic hydroxylation, N-dealkylation, O-dealkylation, S-demethylation, epoxidation, deamination, sulfoxide formation, N-oxidation, and dealogenation.

The products of aliphatic side chain oxidation are usually alcohols with the hydroxyl at the next-to-last carbon, although carboxylic acids may occur as minor metabolites:

RCH2CH3     RCHOHCH3

Oxidation to a ketone may also occur away from the terminal carbon:

RCH2R'     RCHOR'

Introduction of a hydroxyl group into an aromatic ring b P450 involves a transient epoxidation of the ring, including migration of a hydride to the carbocation and tautomerization of the ketone product. P450 catalyzes the epoxide formation, which then spontaneusly forms the corresponding aromatic hydroxide:

N-methyl, N-ethyl and N-alkyl groups can be removed oxidatively and converted to aldehydes. The inital oxidative step forms a hydroxyalkyl congener, which spontaneusly breaks down to form the correspomding nonalkyl compound and an aldehyde:

R"         RNR'CH3

R"                                                     R"                                                                RNR'CH2OH       RNH   +   R'CHO

Aromatic ethers and certain thioethers are cleaved in a reaction similar to N-dealkylation:

ROR'CH3     ROR'CH2OH       ROH   +   R'CHO

RSR'CH3     RSR'CH2OH       RSH   +   R'CHO

Many aromatic or olefinic compounds are metabolized to arene or alkene oxides. For example, the carcinogenic polycyclic hydrocarbon benzo[a]pyrene is metabolized to several epoxides (only two are shown below):

In an oxidative deamination,P450 catalyzes the formation of an acetone:

R'         RCHNH2

R'         RCHO    +   NH3

Thioethers in general are oxidized to sulfoxides:

RSR'     RSOR'

N-oxidation and N-hydroxilation of secondary, tertiary and aryl amines may be catalyzed by P450 as well as other microsomal enzymes:

Halogen atoms may be removed from xenobiotics by oxidative or reductive catalysis of P450, sometimes to yield a ketone, alcohol or aldehyde (?):

R'         RCHX                 R"

R'         RCOH    +    HX                 R"

R'         RCHX  +  e-                 R"

R'         RC •    +    X -                 R"

The P450 superfamily is composed of families and subfamilies of enzymes that are defined solely on the basis of their amino acid sequence similarities. P450 protein sequence from one family exhibits 40% resemblance to P450 from another family. Proteins within a single subfamily always share more than 55% sequence similarity. Each protein is named using the root CYP followed by an arabic number, then a letter, then another arabic number, designating gene family, subfamily and P450 form, respectively. For example: CYP2 is a family, CYP2D is a subfamily, CYP2D6 is a protein form.

CYP3A is the most abundant CYP in human liver, and is also found in gut and kidneys. More than 50% of all drugs are metabolized at least in part by CYP3A. CYP3A4 is found in every liver (?). CYP3A7 is found only in fetal liver. CYP3A catalyzes hydroxylations, N-demetylations and hydroxylation. Drug interactions related to CYP3A may be beneficial (ex. cyclosporin metabolized by CYP3A, plus diltiazem blocking 3A) or harmful (ex. terfenadine plus ketoconazole ??). CYP3A in the gut may be inhibited using time dosing so as to increase biavailability of the drug of interest (ex. itraconazole inactivates CYP3a and is given to decrease triazolam biotransformation; rotonavir blocks CYP3A and is given with protease inhibitors to treat HIV at a lower cost).

CYP2C19 metabolizes miscelaneous groups of compounds by hydroxylation and N-demethylation reactions.More important CYP, an example substrate and their main activity are listed below:

Continue to "Phase 2 Reactions" or take a quiz: [Q1].

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1- What is cytchrome P450?
Large group of drug metabolizing enzymes composed of a hemoprotein and a monooxygenase. 450 refers to the ability of the reduced (ferrous) form of the hemoprotein to react with carbon monoxide yielding a complex with absoeption at 450 nm.

2- What are the cofactors and reactants of P450, other than the xenobiotic being metabolized?
P450 requires NADPH and oxygen. One oxygen atm goes to substrate th other to water.

3- Describe the cycle of P450 catalysis.
- drug forms complex with oxidized (Fe3+) CYTP450
- complex (Fe2+) is reduced by CYP450 reductase at the same time that the flavoprotein is oxidized using NADPH (couple reaction)
- an oxygen nolecule binds to the reduced complex
- complex is oxidized (back to Fe2+) to an alcohol, one oxygen atom forms water (H+ is reduced)
- oxidized drug is released from the complex as an alcohol, enzyme is ready for another cycle

4- List 10 reactions catalyzed by P450
aliphatic hydroxylation
aromatic hydroxylation
N-dealkylation
O-dealkylation
S-demethylation
epoxidation
deamination
sulkfoxide formation
N-oxidation
dealogenation

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