Angela B. Telesforo

OBJECTIVES:

1. To discuss the biotransformation of xenobiotics.

2. To explain the role of enzymes in the biotransformation of xenobiotics.

3. To differentiate Phase 1 from Phase 2 reactions.

4. To compare the different reactions involved in Phase 1 and Phase 2 reactions.

What is Biotransformation? It is the conversion of chemicals to a more

water soluble compounds.

Xenobiotic – a chemical compound ( drug, pesticide, carcinogen ) that is foreign to a living organism.

Endogenous – chemical growing or originating from within.

Substrate – substance to be catalyzed

Substrate enzyme transformed product

co-enzyme

Ex.

ethyl alcohol alcohol acetaldehyde

(CH3CH2OH) dehydrogenase (CH3CHO)

Enzymes play a vital role in biotransformation

Transformation of Xenobiotics – either be beneficial or harmful

Depending on the dose and circumstances

Phase 1 - addition of a functional group.

Phase 2 - conjugation of the modified xenobiotic with another substance.

Conjugated Products

larger molecule than substrate

Generally polar in nature ( water soluble )

Have poor ability to cross cell membranes

Phase 1 Reactions

HYDROLYSIS

- reaction with the addition of water ( OH + H)

( esters, amines, hydrazines, carbamates )

ex.

procaine p-aminobenzoic acid +

diethylaminoethanol

Enzymes involved in Hydrolysis Carboxylesterases ( serum & tissues )

- hydrolyze endogenous lipid compounds

- generate pharmacologically active metabolites

Cholinesterases

- limit the toxicity of organophosphates

Epoxide hydrolase

- detoxify electrophilic epoxides ( cause cellular toxicity and genetic mutations )

REDUCTION

- substrate gains electrons

- occur with xenobiotics in which oxygen content is low

- reduction reactions frequently result in activation of a xenobiotic than detoxification

Ex.

Azo reduction – nitrogen-nitrogen double bonds

Nitro reduction – NO2

catalyzed by:

* CYP450

* NADPH-quinone oxidoreductase

ex.

nitrobenzene + H2 aniline + O2

• OXIDATION

- reactions in which substrate loses electrons

* oxygenation

*dehydrogenation

*electron transfer

Enzymes involved in Oxidation Alcohol dehydrogenase

primary alcohols aldehydes

secondary alcohols ketones

Aldehyde dehydrogenase

aldehydes carboxylic acids

( NAD – cofactor )

Monoamine oxidase ( MAO )

oxidative deamination of primary, secondary, and tertiary amines, including serotonin and some xenobiotics.

Prostaglandin H synthetase

( cyclooxygenase )

arachidonic acid prostaglandins

Cytochrome P450 ( CYP )

- found in hepatic ER microsomes

- heme containing

- classified into subfamilies based on amino acid sequence identity

- named in a species-specific manner

Factors that contribute to Decreased CYP enzyme activity

1. A genetic mutation – gives rise to the poor and intermediate metabolizer genotypes

2. Exposure to an environmental factor ( infectious disease or an inflammatory process ) - suppresses CYP enzyme expression

3. Exposure to a xenobiotic - inhibits or inactivates a preexisting CYP enzyme

By inhibiting cytochrome P450, one drug can impair the biotransformation of another – leading to an exaggerated pharmacologic or toxicologic response to the second drug

Factors that contribute to Increased enzyme activity

1. Gene duplication leading to over-expression of a CYP enzyme

2. Exposure to drugs and other xenobiotics that induce the synthesis of cytochrome P450

3. Stimulation of preexisting enzyme by a xenobiotic

Induction of cytochrome P450 by xenobiotics increases CYP enzyme activity

By inducing cytochrome P450, one drug can stimulate the metabolism of a second drug and thereby decrease or ameliorate its therapeutic effect.

Environmental Factors known to affect CYP levels

Medications

Foods

Social habits ( alcohol consumption, cigarette smoking )

Disease status ( diabetes, inflammation, viral & bacterial infection, hyperthyroidism, hypothyroidism )

It is possible that two or more CYP enzymes can contribute to the metabolism of a single compound.

Information on which human CYP enzyme metabolizes a drug can help predict or explain drug interactions

Inducers of cytochrome P450 increase the rate of xenobiotic biotransformation

P450 induction lowers blood levels, which compromises the therapeutic goal of drug therapy but does not cause an exaggerated response to the drug

P450 induction can cause pharmacokinetic tolerance whereby larger drug doses must be administered to achieve therapeutic blood levels due to increased drug biotransformation

Phase II Reactions

CONJUGATION

Conjugations result in a large increase in xenobiotic hydrop0hilicity – greatly facilitates excretion of foreign chemicals. ( except methylation & acetylation)

Most conjugation enzymes are mainly located in the cytosol.

Glucuronidation

Requires the cosubstrate uridine diphosphate-glucuronic acid ( UDP-glucuronic acid )

Reaction is catalyzed by UDP-glucuronosyltransferases ( UGTs )

Endogenous substrates include bilirubin, steroid hormones, and thyroid hormones

Conjugates of are polar, water-soluble metabolites

Excreted from the body in bile or urine

Cofactor availability can limit the rate of glucuronidation of drugs that are administered in high doses and are conjugated extensively, such as aspirin and acetaminophen

Sulfonation ( sulfate conjugation )

Catalyzed by sulfotransferases which produces a highly water-soluble sulfuric acid ester

The cosubstrate for the reaction is 3’-phosphoadenosine-5’-phosphosulfate (PAPS) which is synthesized from inorganic sulfate

Involves the transfer of sulfonate from PAPS to the xenobiotic

Conjugates are excreted mainly in urine

Sulfonation is an effective means of decreasing the pharmacologic and toxicologic activity of xenobiotics

Methylation

Minor pathway of biotransformation

Decreases the water solubility of xenobiotics

Masks functional groups that might otherwise be conjugated by other enzymes

The cosubstrate for methylation is S-adenosylmethionine ( SAM )

Methylation can also lead to increased toxicity

O-Methylation, N-Methylation, S-Methylation

Acetylation

N-acetylation is a major route of biotransformation for xenobiotics

aromatic amine aromatic amide

hydrazine hydrazide

N-acetylation of certain xenobiotics, such as isoniazid, facilitates their urinary excretion

N-acetylation is catalyzed by cytosolic N-acetyltransferases ( NAT ) requiring the cosubstrate acetyl-coenzyme A ( acetyl-CoA )

NAT1 and NAT2 ( acetyltransferases in humans )

Slow NAT2 acetylators are predisposed to drug toxicities

Drug toxicities

Excessive hypotension from hydralazine

Peripheral neuropathy from isoniazid and dapsone

Systemic lupus erythematosus from hydralazine and procainamide

Toxic effects of coadministration of anticonvulsant phenytoin with isoniazid

Amino Acid Conjugation

2 pathways

conjugation of xenobiotics containing:

Carboxylic acid group with the amino group of amino acids glycine, glutamine and taurine

Aromatic hydroxylamine with the carboxylic acid group of amino acids serine and proline

Amino acid conjugates of xenobiotics are eliminated primarily in urine

Conjugation of hydroxylamines with amino acids is catalyzed by cytosolic aminoacyl-tRNA synthetases and requires ATP

Glutathione conjugation

Tripeptide glutathione comprises of glycine, cysteine, and glutamic acid

Catalyzed by a family of glutathione S-transferases that are present in most tissues

Types of conjugation reactions

1. Displacement reactions – glutathione displaces an electron-withdrawing group

2. Addition reactions - glutathione is added to an activated double bond or strained ring system

Glutathione conjugates formed in the liver can be effluxed into bile and blood, and they can be converted to mercapturic acids in the kidney and excreted in urine

Conjugation with glutathione represents an important detoxication reaction :

- because electrophiles are potentially toxic species that can bind to critical nucleophiles ( proteins & nucleic acids ) causing cellular damage and genetic mutations.

Glutathione is a cofactor for glutathione peroxidase, important in protecting cells against lipid and hemoglobin peroxidation

Conjugation with glutathione enhances the toxicity of a xenobiotic by:

1. releasing a toxic metabolite

2. Being inherently toxic

3. Being degraded to a toxic metabolite