2- Biotransformation ( Edited ) - 2015
-
Upload
reg-arbotante -
Category
Documents
-
view
243 -
download
1
description
Transcript of 2- Biotransformation ( Edited ) - 2015
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.