nonmicrosomal enzymes

7/30/2019 Nonmicrosomal Enzymes

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Non microsomal enZymes


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ENZYMES

Phase 1 reaction. (Non synthetic

phase).

Oxidation, reduction or

hydrolysis.

Oxidation- MICROSOMAL

ENZYMES- located in the

Smooth endoplasmic

reticulum in Liver and kidney,

intestinal mucosa, lung

Eg:Monooxygenases, Cyp450.

Glucoronyl transferases

Inducible by drugs, diet and

other factors

Lesser polymorphism

Phase II reaction. (Synthetic

phase)

NON MICROSOMAL

Enzymes- in cytoplasm and

mitochondria of hepatic cells

and other tissues ( plasma)

Eg: flavoprotein oxidases,

esterases, amidases,

conjugases, all conjugations (

except glucoronidation)(some

,reduction enzymes) Not inducible

Genetic polymorphism

(acetyltransferase,pseidocholi

nesterase)


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NON MICROSOMAL : ENZYMES IN CYTOSOL

(the soluble fraction of the cytoplasm):

(a) Phase I: alcohol dehydrogenase, aldehyde

reductase, aldehyde dehydrogenase, epoxide

hydrolase, esterase.

(b) Phase II: sulfotransferase, glutathione S-

transferase, N-acetyl transferase, catechol 0-methyl transferase, amino acid conjugating

enzymes.


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MITOCHONDRIA.

(a) Phase I: monoamine oxidase, aldehyde

dehydrogenase, cytochrome P450. (b) Phase II: N-acetyl transferase, amino acid

conjugating enzymes.

LYSOSOMES.

Phase I: peptidase.

NUCLEUS.

Phase II: uridine diphosphate


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ESTERASES

Esters, Amides, Hydrazides, and carbamates are hydrolyzed

Hydrolysis in plasma mainly by cholinesterase (nonspecific

acetylcholine esterases, pseudocholine esterases, and other

esterases)

In the liver by specific esterases for particular groups of

compounds.

Rate of Enzymatic hydrolysis of esters and amide: Role in the

onset of pharmacological activity and its duration

SUBCELLULAR LOCALIZATION. Endoplasmic reticulum and

cytosol.

TISSUE DISTRIBUTION. Ubiquitous, liver (centrilobular region),

kidney (proximal tubules), testis, intestine, lung, plasma, and red

blood cells.


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ESTERASES

SUBSTRATES. Esters and amides.

REACTIONTYPE.Hydrolysis:

(a) Hydrolysis of esters:

R1COOR2 R1COOH + R2OH (b) Hydrolysis of amides:

R1CONH-R2 R1COOH + R2NH2

Hydrolysis of amides can occur by amidases in the

liver and in general, enzymatic hydrolysis of amides

is slower than that of esters.

Amides , also hydrolyzed by esterases with a much

slower rate than the corresponding esters.


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Carboxylesterases and cholinesterases

Serine esterases

The catalytic site contains a nucleophilic serine residue- participates

in hydrolysis of various xenobiotic, endobiotic substrates.

Carboxylesterases : serum, liver, intestine, and other tissues

Cholinesterases in blood (and muscles depending on the route of

xenobiotic exposure)

Collectively determine the duration and site of action of certain

drugs.

Eg: Procaine( esteris rapidly hydrolyzed, a local anesthetic.)

Procainamide, the amide analog of procaine, ( hydrolyzed slowly;cardiac arrhythmia.)

The hydrolysis of xenobiotics by carboxylesterases , other hydrolytic

enzymes is not always a detoxication process


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CARBOXYLESTERASES

60 kDa glycoproteins

Wide variety of tissues, including serum. ( in liver is associated with

the endoplasmic reticulum, also in lysosomes and cytosol. ) The hydrolysis of xenobiotic esters and amides : largely catalyzed by

just two carboxylesterases called hCE1 and hCE2.

Human plasma does not contain carboxylesterases

Butyrylcholinesterases and Paraoxonases : responsible for thehydrolysis of amide and ester-containing compounds in the plasma

Also hydrolyze Endogenous compounds : palmitoyl-CoA,

monoacylglycerol, diacylglycerol, retinyl ester, platelet- activating

factor, and the synthesis of fatty acid ethyl ester ther esterified

lipids.

Mechanism : analogous serine-proteases.

A charge relay among : a.a. residue : glutamate ,histidine,

nucleophilic residue :serine


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Aldridge: Basis of their interaction with OP compounds,

A-esterases : hydrolyze OP compounds

B-esterases: inhibited by OP compounds

C-esterases : do not interact with OP compounds as. A

Confusing :

because it divides the paraoxonases into the A- and C esterase class

The human paraoxonase hPNO1 hydrolyzes OP compounds and so

can be classified as an A-esterase hPON2 and hPON3 can be classified as C-esterases because they do

not

hydrolyze OP compounds, nor are they inhibited by them)

Furthermore, carboxylesterases and cholinesterases, two distinctclasses

of hydrolytic enzymes, are both B-esterases according to Aldridge

because both are inhibited by OP compounds


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ESTERASES


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CHOLINESTERASES (AChE and BChE)

Acetylcholinesterase (AChE) : High activity toward acetylcholine

Butyrylcholinesterase (BChE, /Pseudocholinesterase): High activity

toward acetylcholine and butyrylcholine (and propionylcholine) BChE can also hydrolyze: bambuterol, chlorpropaine, cocaine,

methylprednisolone acetate, heroin, isosorbide diaspirinate,

mivacurium, procaine, succinylcholine, tetracaine

Eserine is an inhibitor of both enzymes

BW84C51 is a selective inhibitor of AChE

Iso-OMPA is a selective inhibitor of BChE

Exist in six different forms with differing solubility in in three

states: soluble (hydrophilic), immobilized (asymmetric), and

amphiphilic globular (membrane-bound through attachment to

the phospholipid bilayer)

Six forms : monomer (G1), dimer (G2), tetramer (G4), tailed

tetramers (A4), double tetramers (A8), and triple tetramers

(A12).


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All forms are expressed in muscle.

AChE, major form in brain: tetramer G4 (anchored with a 20-

kDa side chain containing fatty acids)

The major form in erythrocytes : the dimer G2 (anchored

with a glycolipid-phosphatidylinositol side chain).

BChE: major form in serum is the tetramer G4 (a

glycoprotein with Mr 342 kDa).

In both AChE and BChE, the esteratic site (containing the

active site serine residue) is adjacent to an anionic (negatively

charged) site that interacts with the positively chargednitrogen on acetylcholine and butyrylcholine.


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Carboxylesterases and cholinesterases

In blood and tissues play an important role in

limiting the amount of OP compounds thatreaches AChE in the brain

ChE inhibition : is the mechanism of toxicity

of OP and carbamate insecticide 70-90% loss of AChE activity is lethal to

mammals, insects, and nematodes


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An inverse relationship between serine esterase

activity and susceptibility to the toxic effect of OP

compounds

Factors that decrease serine esterase activity

potentiate the toxic effects of OP compounds

Eg-susceptibility of animals to the toxicity ofparathion, malathion, and

diisopropylfluorophosphate (DFP) is inversely related

to the level of serum esterase activity (which reflects

both carboxylesterase and BChE activity).


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Esterases are not the only enzymes involved in the

detoxication of OP pesticides.

Certain OP compounds are detoxified by cytochrome

P450, flavin monooxygenases,and glutathione

transferases.

Paraoxonases, enzymes that catalyze the hydrolysis

of certain OP compounds, appear to play only a

minor role in determining susceptibility to OP


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ESTERASES

POLYMORPHISM.

Approximately 2% of Caucasians have defective serum

cholinesterase activity

SPECIES DIFFERENCES

Activity is higher in small laboratory animals such as the rat

and mouse than in humans.


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PARAOXONASES (LACTONASES)

.Calcium-dependent enzymes containing a

critical sulfhydryl (-SH) group; as such they areinhibited by EDTA, metal ions (Cu andBa), and

various mercurials such as phenylmercuric

acetate (PMA) Catalyze the hydrolysis of a broad range of

organophosphates, organophosphinites,

aromatic carboxylic acid esters, cyclic

carbonates, and lactones


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Three paraoxonases : hPON1, hPON2, hPON3.

hPON1 :

Liver microsomes and plasma, where it is associated exclusively

with high-density lipoprotein (HDL)

protects against atherosclerosis by hydrolyzing specific derivatives

of oxidized cholesterol and/or phospholipids in atherosclerotic

lesions

Appreciable arylesterase activity and the ability to hydrolyze the

toxic oxon metabolites of OPC insecticides

hPON2 : several tissues, not in plasma

hPON3: serum and liver and kidney microsomes


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ALKALINE PHOSPHATASE

Luminal surface of the enterocytes lining the wall of the small

intestine.

Hydrolysis of the prodrugs releasing the active drug at the

surface of the enterocytes, where it can be readily absorbed.

Clinical applications in the treatment of certain cancers.

Eg: to activate prodrugs in vivo and thereby generate potent

anticancer agents in highly selected target sites (e.g., at the

surface of tumor cells, or inside the tumor cells themselves)

Eg: prodrugs, such as fosphenytoin and fosamprenavir

The hydrolysis ofvalacyclovir to the antiviral drug acyclovir is

catalyzed by a human enzyme named valacyclovirase

(genesymbol: BPHL)

PEPTIDASE


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PEPTIDASE

Recombinant peptide hormones, growth factors, cytokines,

soluble receptors, and humanized monoclonal antibodies :

administered parenterally are hydrolyzed in the blood,Lysosomes and tissues by a variety of peptidases:

Aminopeptidases and Carboxypeptidases

Hydrolyze amino acids at the N- and C-terminus, respectively

Endopeptidases, which cleave peptides at specific internalsites (trypsin, for example, cleaves peptides on the C-terminal

side of arginine or lysine residues)

Peptidases cleave the amide linkage between adjacent amino

acids, function as amidases. ,The active site of peptidases : serine or cysteine residue,

which initiates a nucleophilic attack on the carbonyl moiety of

the amide bond.( like carboxylesterases)


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NAD(P)H-dependent reductases

Aldoketo reductases (AKRs): Cytosolic enzymes that reduce both

xenobiotic and endobiotic compounds, Function as dihydrodioldehydrogenases and oxidize the trans-dihydrodiols of various

polycyclic aromatic hydrocarbon oxiranes (formed by epoxide

hydrolase)


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Medium chain dehydrogenases/reductases (MDRs) :

convert alcohols to aldehydes

Eg: alcohol dehydrogenases


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Short chainDehydrogenases/reductases (SDRs):

Erythrocytic.cytosolic and microsomal

carbonyl reductase, reduction of a wide variety of carbonyl-

containing xenobiotics (other species express

more than two carbonyl reductases).


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DISULFIDE REDUCTASE

Cytosol

reduction


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SULFOXIDE REDUCTASE Thioredoxin-dependent enzymes in liver and kidney cytosol

Sulfoxide and N-Oxide Reduction: Reduce sulfoxides, whichthemselves may be formed by cytochrome P450 or flavin

monooxygenases

Eg: Sulindac is a sulfoxide that undergoes reduction to a sulfide,

which is excreted in bile and reabsorbed from the intestine.

Reduction may also occur nonenzymatically at an appreciable rate,

as in the case of the proton pump inhibitor rabeprazole

DIHYDROPYRIMIDINE DEHYDROGENASE


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DIHYDROPYRIMIDINE DEHYDROGENASE

1953-Japan: The mechanism of lethal interaction b/w Sorivudine

& 5-fluorouracil- involved inhibition of DPD 15 deaths

An NADPH-requiring, homodimeric protein (Mr 210 kDa)containing FMN/FAD an liver cytosold an ironsulfur cluster in each

subunit.

Location:, where it catalyzes the reduction of 5-fluorouracil and

related pyrimidines.

Sorivudine is converted in part by gut flora to (E)-5-(2-bromovinyl)

uracil (BVU), which lacks antiviral activity but which is converted by

DPD to a metabolite that binds covalently to the enzyme. Resulting

in irreversible inactivation(suicidal inactivation) of DPD

Marked inhibition of 5-fluorouracil metabolism, which increasesblood levels of 5- fluorouracil to toxic lethal levels (\

Genetic polymorphisms that result in a partial or complete loss of

DPD activity


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ALCOHOL DEHYDROGENASE (ADH)

SUBCELLULAR LOCATION: Cytosol

Major enzyme responsible for oxidation of alcohol (ethanol) to

aldehyde (acetaldehyde). Others: for ethanol oxidation:CYP2El

,Catalase,

REACTION TYPE.

Oxidation of alcohol to aldehyde:

R CH2OH R CHO

( )


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SUBSTRATES.

Aliphatic or aromatic alcohols.

COFACTORS.

NAD+.

ENZYME STRUCTURE. Zinc-containing dimer of two 40 kDa subunits.

TISSUE DISTRIBUTION.

Liver, kidney, lung and gastric mucosa.

POLYMORPHISM. 85% of Asians- class I isozymes (atypical ADHresponsible for rapid conversion of ethanol to acetaldehyde),

20% of Caucasians - atypical ADH



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ALDEHYDE DEHYDROGENASE (ALDH)

ENZYME STRUCTURE.

Tetramer of 54 kDa subunits (ALDH1 and ALDH2) or dimer of85 kDa subunits (ALDH)

TISSUE DISTRIBUTION

. Liver, kidney, lung, and gastric mucosa

SUBCELLULAR LOCATION. Cytosol (ALDH1 and ALDH3), mitochondria (ALDH2)

Oxidation of xenobiotic aldehydes to acids.

In particular, acetaldehyde formed from ethanol by alcohol

dehydrogenase is oxidized to acetic acid by ALDH, which isfurther oxidized to carbon dioxide and water.

REACTION TYPE.

Oxidation of aldehyde to acid:

RCH2CHO RCH2COOH


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ALDEHYDE DEHYDROGENASE

SUBSTRATES.

Aliphatic or aromatic aldehydes.

COFACTORS : NAD+ or NADP+.

POLYMORPHISM.

50% of Asians have a defective ALDH2 gene

causing impaired ALDH2 activitY


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ALDEHYDE DEHYDROGENASE


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MOLYBDENUM HYDROXYLASES (MOLYBDOZYMES)


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MOLYBDENUM HYDROXYLASES (MOLYBDOZYMES)

Flavoprotein enzymes consists:two identical150 kDa subunits,

each of which contains FAD, molybdenum, in the form of a

pterin molybdenum cofactor ([MoVI(=S) (=O)]2+) and twoironsulfur (Fe2S2) centers (known as FeSIand FeSII).

An interaction between the molybdenum center with a

reducing substrate

Results in the reduction of the molybdenum cofactor, afterwhich reducing equivalents are transferred intramolecularly

to the flavin and ironsulfur centers, with reoxidation

occurring via the flavin moiety by molecular oxygen

ALDEHYDE OXIDASE XANTHINE OXIDO REDUCTASE

( Xanthine dehydrogenase and Xanthine Oxidase)

SULFITE REDUCTASE ( Oxidze sulfite, air poullutant)

XANTHINE OXIDOREDUCTASE (XOR)


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XANTHINE OXIDOREDUCTASE (XOR)

Xanthine dehydrogenase (XD) and Xanthine oxidase (XO);

Cytosol- Highest levels in heart, brain, liver, skeletal muscle,

pancreas, small intestine, colon, and placenta Two forms of the same enzyme that differ in the electron

acceptor.

XD: the final electron acceptor is NAD+ (dehydrogenase

activity),XO : the final electron acceptor is oxygen (oxidase activity).

XD is converted to XO by oxidation of cysteine residues

(Cys993 and Cys1326 of the human enzyme) and/or

proteolytic cleavage. Under normal physiologic conditions, XD is the predominant

form of the enzyme found in vivo.

However, during tissue processing, the dehydrogenase form

tends to be converted to oxidase form

XOR


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XOR

Ischemia/Hypoxia: XO levels increase- XOR gene

transcription, XD XO.

XO contributes to oxidative stress and lipid

peroxidation because the oxidase activity of XO

involves the reduction of molecular oxygen, which can

lead to the formation of reactive oxygen species

LPS, a bacterial endotoxin that triggers an acute

inflammatory response, increases XO activity both by

inducing XOR transcription and by converting XD to XO.

XOR


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XOR

First-pass elimination of purine derivatives (e.g., 6-

mercaptopurine and 2,6-dithiopurine), limits the therapeutic

effects of Certain prodrugs are activated by xanthine oxidase.

Eg: antiviral prodrugs 6-deoxyacyclovir and 2_-fluoroarabino-

dideoxypurine, which are relatively well absorbed after oral

dosing, are oxidized by xanthine oxidase to their respective activeforms, acyclovir and 2--fluoroarabino-dideoxyinosine, which are

otherwise poorly absorbed

Bioactivation of mitomycin C and related antineoplastic drugs,

although this bioactivation

Catalyzes, the sequential oxidation of hypoxanthine to xanthine

and uric acid


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Allopurinol: Hydroxylated coumarin

derivatives, such as umbelliferone (7-

hydroxycoumarin) and esculetin (7,8-dihydroxycoumarin

By competing with hypoxanthine and xanthine

for oxidation by XD/XO, inhibits the formation

of uric acid,.

Monomethylated xanthines ( except

theophylline and caffeine) oxidized to the

corresponding uric acid derivatives by XD/XO.


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ALDEHYDE OXIDASE


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ALDEHYDE OXIDASE Exists only in the oxidase form as it lacks an NAD+ binding site

High levels: in cytosol of liver, with considerably less activity in other

tissues Preference for oxidizing aromatic aldehydes ( benzaldehyde) over

aliphatic aldehydes. (acetaldehyde)

Transfers electrons to molecular oxygen, which can generate reactive

oxygen species and lead to oxidative stress and lipid peroxidation.

Oxidize a number of substituted pyrroles, pyridines, pyrimidines,

purines, pteridines, and iminium ions

plays an important role in the catabolism of biogenic amines and

catecholamines: physiologically important aldehydes- substrates:

homovanillyl aldehyde (formed from dopamine), 5-hydroxy-3-indoleacetaldehyde (formed from serotonin), and retinal, which is

converted to retinoic acid

In general, xenobiotics that are good substrates for aldehyd e oxidase

are poor substrates for cytochrome P450, and vice versa


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ALDEHYDE OXIDASE


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ALDEHYDE OXIDASE

Species difference : Dogs possess little or no aldehyde

oxidase activity.

6-oxidation of antiviral deoxyguanine prodrugs is catalyzedexclusively in rats by XD/XO, but by aldehyde oxidase in

humans

Drugs metabolized: Nicotine, citalopram, proprionaldehyde,

6-mercaptopurine, metyrapone, quinine, methotrexate,famciclovir ( to penciclovir)

Raloxifene and perphenazine : potent inhibitors

Under certain conditions, aldehyde oxidase and XOR can also

catalyze the reduction of xenobiotics, including azo-reduction(e.g., 4-dimethylaminoazobenzene), nitro-reduction (e.g., 1-

nitropyrene), N-oxide reduction (e.g., S-(-)-nicotine-1-N-oxide),

nitrosamine eduction (e.g., N-nitrosodiphenylamine),sulfite

reduction

MONOAMINE OXIDASE (MAO)


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MAO, DIAMINE OXIDASE and POLYAMINE OXIDASE

SUBSTRATES.

Primary, secondary, and tertiary naturally occurring .amines:

Monoamines: serotonin (5-hydroxytryptamine)

Diamine: putrescine and Monoacetylated derivatives of the

polyamines spermine and spermidine.

Oxidative deamination(Primary amine)ammonia & aldehyde

Oxidative deamination (Secondary) -- primary amine and an

aldehyde. (The products of the former reactioni.e., an

aldehyde and ammoniaare those produced during the

reductive biotransformation of certain oximes by aldehyde

oxidase)

The aldehydes formed are usually oxidized further by other

enzymes to the corresponding carboxylic acids, although in

some cases they are reduced to alcohols.



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Related to the metabolism of exogenous tyramine and the

cheeseeffect produced as a result of the ingestion of large

amounts of tyramine-containing foods Catalyzes the oxidative deamination of biogenic amines


Ubiquitous,throughout the brain, and is in outer membrane of

mitochondria the liver, kidney, intestine, and blood plateletslymphocytes.

SUBCELLULAR LOCATION. : Mitochondria, some MAO activity in

the microsomal fraction.

REACTION TYPE.

Oxidative deamination of amines:

RCH2NR1R2 RCHO + NHR1R2

MAO


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Substrates ; drugs- milacemide (dealkylated metabolite of propranolol ) , ,

primaquine, haloperidol, doxylamine, -phenylethylamine, tryptophan

analogs known as triptans: sumatriptan, zolmitriptan, and rizatriptan.

Endogenous: tyramine, catecholamines (dopamine, norepinephrine,

epinephrine), tryptophan derivatives (tryptamine, serotonin),

Isoforms : MAO-A and MAO-B.

OXIDIZE INHIBITED BY

MAO-A serotonin (5-hydroxytryptamine) clorgyline

norepinephrine phenelzine, metabolite of propranolol,

MAO-B -phenylethylamine l-deprenyl (selegiline).

benzylamine phenelzine

Species differences in the substrate specificity of MAO -dopamine is

oxidized by MAO-B in humans, but by MAO-A in rats, and by both enzymes

in several other mammalian species.

Most tissues contain both forms of the enzyme, each encoded by a

distinct gene, although some tissues express only one MAO.


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MAO

Th i i f MPTP (1 h l 4 h l


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The activation of MPTP (1-methyl-4-phenyl-

1,2,5,6-tetrahydropyridine)to its neurotoxic

metabolite is catalyzed predominantly by MAO

B:

haloperidol- to toxic pyridinium

Parkinsons disease in humans: elevated levels ofMAO-B( dopamine destruction)


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DIAMINE OXIDASE


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DIAMINE OXIDASE

Cytosolic, copper-containing, pyridoxal phosphate-

dependent enzyme present in liver, kidney, intestine, and

placenta. Substrates: Histamine and simple alkyl diamines with a chain

length of 4 (putrescine) or 5 (cadaverine) carbon atoms.

Diamines with carbon chains longer than 9 are not substrates

for DAO, although they can be oxidized by MAO. DAO/ similar enzyme is present in cardiovascular tissue -

cardiotoxic effects of allylamine, which is converted by

oxidative deamination to acrolein.

No DAO in brain : The major pathway of histaminemetabolism in the brain is by methylation

SEMICARBAZIDE-SENSITIVE AMINE OXIDASE (SSAO)


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SEMICARBAZIDE-SENSITIVE AMINE OXIDASE (SSAO)

Copper-containing enzyme that catalyzes

fundamentally the same reaction catalyzed by

monoamine oxidase:

Distinguished from MAO: By its sensitivity to

inhibitors (it is inhibited by semicarbazide but not byclorgyline, deprenyl, or pargyline, whereas the

opposite is true for MAO), and

it is found on various cell surfaces and in plasma,

(MAO- found in mitochondria).

SULFOTRANSFERASE (ST)


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TISSUE DISTRIBUTION. Liver, kidney, adrenals, lung, brain, jejunum, and blood

platelets, and, to a lesser extent, skin and muscle.

SUBCELLULAR LOCATION. Cytosol.

Homodimers in vivo with a molecular weight 32-34 kDa.

Mediates conjugation reaction of a compound at a low

concentration

In General a high-affinity and low-capacity reaction .

REACTION TYPE. Sulfation:

(a) O-sulfation: R-OH R-SO3H

(b) N-sulfation: R-NHCOR R-NCOR

SUBSTRATES. SO3H

Nucleophilic moieties of such molecules as phenol, alcohol, and

arylamine.

COFACTOR. 3-Phosphoadenosine-5-phosphosulfate (PAPS).



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ISOZYMES.

Six different phenol sulfotransferases (PST

Seven different steroid/ bile acid sulfotransferases have been

characterized in rats. In human

Four subfamilies: TS ST (thermostable ST or PST), TL ST

(thermolabile ST, or monoamine ST), EST (estrogen ST),DHEA ST(dehydroepiandrosterone ST).

POLYMORPHISM.

Bimodal frequency distribution of DHEA ST activity suggests that

approximately 75 % of the population are poor metabolizersSPECIES DIFFERENCES.

The pig and opossum are defective in their capability regarding

sulfate conjugation of phenolic compounds.

RELATIONSHIPS BETWEEN UDPGT AND ST


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RELATIONSHIPS BETWEEN UDPGT AND ST

Often, UDPGT and ST are considered to be complementary

to each other for conjugation of the same substrates, except

in connection with acyl glucuronidation, which cannot be

replaced by sulfation.

In general, glucuronidation is considered a low-affinity (Km)

and high-capacity (Vmax) reaction, whereas sulfation is known

as a high-affinity and

low-capacity conjugation.

Thus, at low substrate concentrations sulfation may be more

predominant, but as concentration increases, glucuronidation

becomes quantitatively more important.

N-ACETYL TRANSFERASE (NAT)


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( )

The first genetic polymorphism described for an enzyme involved

in human drug metabolism was for NAT

Molecular weight of 26.5 kDaTISSUE DISTRIBUTION.

Liver (in the Kupffer cells, not in the hepatocytes), spleen, lung,

intestine.

SUBCELLULAR LOCATION. : Cytosol.REACTION TYPE.: Acetylation on amine moiety.

R- NH2 R-NH-COCH3

R-SO2-NH2 R-SO2NH-COCH3

SUBSTRATES. Aromatic amines (RNH2),sulfonamides (RSO2NH2),or hydrazine (RNHNH2) derivatives.

COFACTOR. : Acetyl-coenzyme A(CoA).

N-ACETYL TRANSFERASE (NAT)


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N ACETYL TRANSFERASE (NAT)

ISOZYMES.

NAT1 and NAT2 enzymes

POLYMORPHISM.

4060% of Caucasians and 1030% of Asians

are slow acetylators.SPECIES DIFFERENCES.

Dogs and guinea pigs are deficient

GLUTATHIONE S-TRANSFERASE (GST)


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Integral part of the phase II detoxification system.

Protects cells from oxidative- and chemical-induced toxicity and

stress by catalyzing the glutathione conjugation reaction with an

electrophilic moiety of lipophilic and often toxic xenobiotics

REACTION TYPE.

Glutathione conjugation.

RX RSglutathione + X- (X: halide, sulfate, or phosphate)

RC = CCOR RCCCOR

Sglutathione

ENZYME STRUCTURE.

Dimer in vivo with a molecular weight of 2428 kDa.


Liver, gut, kidney, testis, adrenal, and lung.

SUBCELLULAR LOCATION

. Cytosol (major) and endoplasmic reticulum (minor).

GLUTATHIONE S-TRANSFERASE (GST)


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GLUTATHIONE S TRANSFERASE (GST)

ISOZYMES.

Five classes of cytosolic enzymes, , , , 0- , and and one class of

microsomal enzyme.POLYMORPHISM: GSTM1 (the class enzyme): 4050% of individuals; GSTT1

(the class enzyme): 1030% of Europeans have a deficiency.

SUBSTRATES.

Lipophilic and have an electrophilic moiety.

Glutathione conjugates of xenobiotics. In liver are usually excreted in bileand urine/ further metabolized to mercapturic acids(kidney, excr. Urine).

Substrates with reactive or good leaving groups : epoxide, halide, sulfate,

phosphate, or nitro moiety attached to an allylic or a benzylic carbon.

Facilitated by electron-withdrawing groups, such as CHO, COOR, COR,

orCN, adjacent to the electrophilic moiety of the compounds.

COFACTOR: Glutathione, a tripeptide cofactor (GSH, L-y-glutamyl-L-

cysteinylglycine (Gly-Cys-Glu)), is present in virtually all tissues, often in

relatively high (0.1 10mM) concentrations.

METHYL TRANSFERASE


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Methylation of endogenous substrates such as histamine,

catecholamines, and norepinephrine. And some drugs

COFACTOR.: S-adenosylmethionine(SAM).

TISSUE DISTRIBUTION:

Liver, brain, lung, kidney, adrenals, skin, and erythrocytes.

SUBCELLULAR LOCATION.: Cytosol.

ISOZYMES.: Four different enzymes can perform S-, N-, or 0-

methylation

POLYMORPHISM. 0.3% of the European population have a

deficiency in thiopurine S-methyltransferase activity.

In general, methylation of a compound produces a less polar

metabolite than the parent compound, and thus, unlike other

conjugation reactions, tends to decrease the rate of its

excretion.

REACTION TYPE 0 N S methylation:


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REACTION TYPE. 0, N, S-methylation:

0, N, S-methylation

AMINO ACID N-ACYLTRANSFERASE


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Exogenous carboxylic acid, ( acetates), activated to coenzyme A derivatives

(acyl CoA thioether) in vivo by acyl-CoA synthetase

Further conjugated with endogenous amines such as amino acids by acyl-CoA:amino acid

N-acyltransferase; Amino acid conjugates eliminated primarily in urine by tubular activesecretion mechanisms

COFACTORS.

Coenzyme A (CoA-SH) for acyl-CoA synthetase

Amino acids: glycine, glutamine, ornithine, arginine, and taurine, for acyl-CoA:amino acid Nacyltransferase.

TISSUE DISTRIBUTION. Liver and kidney.

SUBCELLULAR LOCATION. : Mitochondria and endoplasmic reticulum for

acyl-CoA synthetase, and cytosol and mitochondria for acyl-CoA:amino

acid N-acyltransferase.

SPECIES DIFFERENCES: The amino acid used for conjugation

Eg: conjugation of bile acids occurs with both glycine and taurine in most

species, whereas in cats and dogs, conjugation of bile acids occurs only

with taurine


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AMINO ACID N-ACYLTRANSFERASE


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THANK YOU

nonmicrosomal enzymes

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