Download Biotransformation of xenobiotics

Biotransformation of
xenobiotics
Eva Samcová
Absorption and distribution
Human exposure to foreign compounds are both
intentional and unintentional (exposure to drugs by
self-administration or by physician – intentional,
environmental pollutant contamination –unintentional)
Absorption depends on the chemical structure of
substance
Majority of xenobiotics unrelated to endogennic
substances are absorbed by passive diffusion
● Partition coefficient lipid/water – predominant feature
of passive diffusion and also the concentration in and
outside of the cell
P > 1, greater amount of substance is in lipid = easy
diffusion
P < 1, diffusion proceeds less readily
● Charged species do not easily cross membranes
(examples benzoic acid and aniline, pH)
Transport into the cell
Active transport, facilitated diffusion, membrane
processes such a phagocytosis and pinocytosis
Active transport has substantial impact on elimination
of xenobiotics
Especially relevant is the phagocytosis of particles in
the alveoli of the lung. The reticuloendothelial (Kupffer)
cells of liver also engage in this activity
Xenobiotics, which can interact with carrier proteins or
other transport-specific cellular constituents, often
handle the current transport system provided similarity
to the parent compound (fluorouracil vs. uracil, βaminopropane sulfonic acid vs. alanine)
Biotransformation
Overall effect of xenobiotic metabolism
(biotransformation) is to rid the organism of a foreign
compound converting it to a more polar substance.
Xenobiotic compounds could be broadly defined as
substances which cannot be broken down to generate
energy or be assimilated into biosynthetic pathway
The enzyme systems which cope with these compounds
are required to be quite nonspecific.
Lipophilic substances tend to the accumulation in
adipose tissue
Xenobiotics and their metabolites are bound in blood
to proteins or cell structure
The liver is the main place of biotransformation
● The smooth endoplasmic reticulum (SER) is the
center of the xenobiotic oxidative activity of the liver cell
Biotransformation
Biotransformation proceeds in many tissues especially in
the places of entering and excretion that is in the
intestine, lung, skin and kidney.
Subcellular localisation:
- enzymes of endoplasmic reticulum
- cytosol
● Enzyme induction : can be defined as the qualitative
and quantitative changes in the metabolism of
xenobiotics brought about by exposure to the same or
some other xenobiotic.
Enzymes taking part of xenobiotic transformation are
used also in biotransformation of endogenic compounds
Metabolism of xenobiotics run in two phases
Biological reactions occur at rates convenient for the
maintenance of life because of enzymic catalysis
Phase I – biotransformation = introduction (hydroxylation) or
unmasking (reduction or hydrolysis) of a polar group
The result – increasing of polarity
Phase II – conjugation = synthesis of small endogennic
molecules with polar function group from the Phase I
(introduction on the hydrophilic moiety)
The result – further increasing of polarity and thereby increasig
of excretion by urine or bile
Phase I reactions
Incorporation of new functional groups, into apolar compound
The most important oxidative reactions : hydroxylation, epoxide
formation, dealkyation and deamination
Reductive reactions : reduction of carbonyl-, azo- or nitro
compounds, dehalogenation
Methylation and Desulfuration
Reactions of biotransformation can lead to more toxic
substances than the parent compound was
Monooxygenases (cytochrome P450) – presence of NADPH and
molecule of oxygen
General reaction:
R-H + O2 + NADPH + H+ → R-OH + NADP+ + H2O
Cytochrome P-450 refers to a family of heme proteins
Cytochrome P450 contains Fe3+ and binds to its structure
xenobiotics and oxygen
Enzyme include in Cytochrom P450 NADPH-cytochrome
reductase – reduces Fe3+ of cytochrome P450 to Fe2+. This
system is involved in sER system of electron transport (similarly
how it is in mitochodrion).
Enzyme system, Cytochrome P450 , is bound on the
phospholipide part of sER membrane
Induction agent of enzymes „monooxygeneses“ can be
phenobarbital
Oxidation
Hydroxylation
: of aliphatic and compounds
R-CH2-CH2-CH3 → R-CH2-CHOH-CH3
● Oxidation can proceed also on the nitrogen (in presence of
NADPH and O2, results in phenylhydroxylamine and
phenylnitrosamine)
C6H5-NH2 → C6H5-NHOH → C6H5-N=O
● N-, O- or S-dealkylation :
R – NH – CH3 → R-NH2 + HCHO
● Deamination :
R – CH – NH2 → R – C=O + NH3
│
CH3
│
CH3
Oxidation of alcohols (ethanol)
1. step – oxidation to acetaldehyde with production of NADH + H+
by means of alcohol dehydrogenase. Alcohol dehydrogenase is
placed exclusively in cytosol.
The resulted acetaldehyde is transfered through mitochondrion
membrane into mitosole, where acetaldehyde is oxidized to
acetate ( 2. step)‚ enzyme aldehyde dehydrogenase. A acetate
after activation to Acetyl-CoA is metabolized in TCA.
NADH as a product of the first step reaction must be transfer by
means of malate-aspartate shuttle to mitochondrion, where is
used in the respiratory chain.
Besides above given enzyme in cytosol, enzymes catalase
(peroxisomal in peroxidase reaction) is also involved in ethanol
degradation
H2O2 + H2S (substrate ethanol) → 2 H2O + S (substrate)
A follow-up reactions
Reductive reaction proceed on the sER and in cytoplasm:
Reduction of carbonyl group :
R-CO-R1+ NADPH+H+ → R-CHOH-R1+ NADP+
● Hydrolytic reactions : breakdown of ether, amide, ester, but
also C-N bonds in hydrazide, carbamate, and nitrile
● Cyclization reactions or opening of the cycle of toxic
substances (xenobiotics)
II. Phase - conjugation
Endogenic substances are needed
Endergonic character of those reactions
Considerable increasing of polarity, conjugated
substance can not penetrate biological membranes and
therefore is excluded from organism (by kidney up 300
kDa) by bile in higher molecular mass)
The conjugation proceeds in liver (predominantly) but
also in another tissues, very often in the place of
entrance and excretion of toxic substance from the body
Endogenic compound must be activated
Glucuronate – the most common conjugation
agent
Component of activation of glucuronate is UTP
Activated form is UDP-glucuronate –yield of O- and Nglucuronides
The mechanism is the same like in case of endogenic
substances
H2SO4,activated compound : active sulfate PAPS
Acetate, activated acetic acid: Acetyl-CoA,
Glycine
Glutathione, conjugates polycyclic, aromatic compounds, but
even alkylhalogenic compounds. It forms mercapturic acids.
Example of biotransformation
Biotransformation of vinylchloride
Intake by inhalation, partly exhaled, partly
metabolized in hepatocytes
CH2=CHCl → chlorethylenoxide → ClCH2-COOH
+ GSH → HOOC─CH2─S─CH2─COOH
thiodiacetic acid
Biosynthesis of mercapturic acids
HO.CH2.CH2─S─CH2─CH─COOH
‫׀‬
NH─CO─CH3
Example of biotransformation
C6H5CH3 → C6H5COOH→
C6H5CONHCH2COOH
Narcotic effect
Absorption by lung (53%)
84% from absorbed toluene is metabolized to
benzoic acid. After conjugation with glycine hippuric acid is formed which is excreted by
urine very fast.
Excretion of xenobiotics
By urine, feces, exhalation (in minor way by
perspiration and salivas)
Excretion by kidney is effected by urine pH :
Basic xenobiotics are excreted in acid pH
because they are ionized in acid pH
Weak acids are excreted in basic pH, because
they are ionized in basic pH
Poisoning by phenobarbital – administration of
hydrogen carbonate, because phenobarbital is
weak acid with pKa = 7,2
Rate of xenobiotic excretion from the
body
Concentration level of
xenobiotics (maximum)
decrease exponentially
with time :
Ct = concent. in time t
Half-life of excretion (T) :
time when xenobiotic
has ½ of original
concentration in blood.
dc/dt = k . c
c – concentration of
xenobiotic in blood
ct = c0 . e-kt
T = ln 2/ k
Excretion of xenobiotic by feces and exhalation
By feces
Liver → bile → intestine
Rather high-molecular substances
Antibiotics (tetracykline) can damage intestinal
microflora
By exhalation
Only in case of sufficient xenobiotic
concentration in blood
Chemical disasters
Bophal (1984) poisoning by methylisocyanate
(metabolite in production of carbamate
insecticide) H3C-N=C=O
Minamata disease – poisoning by organic
mercury, seed corn treated by CH3-Hg+ was
used like food for people, domestic animals
and fishes (1956 –Minamata, 1972 – Basra
Seveso (1976) – poisoning by 2,3,7,8tetrachlorodibenzo-p-dioxin (TCDD) during
production of germicide
bis(trichlorhydroxyfenyl)methane