Environmental issues and hazards in the chemical research laboratory

Ulf Ellervik

Lecture 5: Uptake, metabolism and excretion of xenobiotics

Lecture 5 - outline1. Introduction2. Uptake of chemicals3. Distribution4. Cellular transportation5. Metabolism - Phase I6. Metabolism Phase II7. Excretion8. Specific examples

1. Introductionuptake

distribution

excretion

gastro-intestinal

tract

skin lungs intravenous other(mucous

membranes)

faeces urine exhaled air secretion

circulatory system

liver

gall

kidney lung

bladder

portal vein

glands interstitial liquid

fat bone other tissue

2. Uptake of chemicals2.1 By the skin

The skin-is a barrier which protects the organism -regulates the body temperature-does not have a specialized transport system

epidermis

dermis

blood vessels

2. Uptake of chemicals2.1 By the skin

epidermis

dermis

blood vessels

Uptake of chemicals by the skin depends on:-the general condition (brusies etc)-the thickness (arms thin, palms thick) -the water content of the epidermis (gloves)

Many compounds enter easily by the skin (dichloromethane, methanol)

DMSO increase the permeability for other compounds (used in veterinary medicine)

2. Uptake of chemicals2.2 By the gastro-intestinal tract

mechanical degradation chemical

degradationpH = 2

enzymatic degradation bacterial

degradation

Total surface 300 m2 (a tennis court)

2. Uptake of chemicals2.2 By the gastro-intestinal tract

The absorption takes place in villi

Toxic compounds sometimes fit into transport systems

examples:

Some compounds pass through the system

example:

-thallium and cobalt fit into the iron transporter

-lead fit into the calcium transporter

-elemental mercury

2. Uptake of chemicals2.2 By the gastro-intestinal tract

some compounds are modified by the low pH...

2. Uptake of chemicals2.2 By the gastro-intestinal tract

...or by the bacteria (reductive)

MUTAGENIC!

2. Uptake of chemicals2.3 By the lungs

-100 m2 (a badminton court)-the exchange takes place in the alveoli where the distance between the blood and the air is only 1 μm

2. Uptake of chemicals2.3 By the lungsAerosols are particles or droplets that are sufficientlysmall to hover in the air Dust (solid particles) Fume (particles formed by combustion) Smoke (particles formed by combustion of organic material) Fog, mist (liquid droplets) Smog (particles from car exhausts)

2. Uptake of chemicals2.3 By the lungs

The uptake is dependent on size-small particles (< 1 μm) goes down in the alveoli and cancause e.g. silicosis or asbestosis-bigger particles are stopped in the mucus membranesand transported by the mucociliary escalator:

2. Uptake of chemicals2.3 By the lungs-Hydrophilic gases are usually absorbed by the mucusmembranes

-Very reactive gases (HCl, NH3, SO2) are irritating andblocks respiration and can therefore not be inhaled inlarger quantities

-Chemicals with intermediate reactivity and lipophilicity(phosgene, ozone, isocyanates) can be inhaled and giveinjuries at all levels in the lungsThey harm the epithelial cells (by lipid peroxidation) so thatthese are leaking fluid - pulmonary oedema (internaldrowning) which takes up to 48 h to develop

2. Uptake of chemicals2.4 Bioaccumulation

Chemicals that are relatively stable and not rapidly converted may be absorbed by another organism that feeds on the first. (biomagnification)

example: polyhalogenated aromatic hydrocarbons (PCB)

Chemicals can be concentrated in organisms (bioconcentration) for example in fat

3. Distribution of chemicalsuptake

distribution

excretion

gastro-intestinal

tract

skin lungs intravenous other(mucous

membranes)

faeces urine exhaled air secretion

circulatory system

liver

gall

kidney lung

bladder

portal vein

glands interstitial liquid

fat bone other tissue

3. Distribution of chemicals3.1 AccumulationAbsorbed compounds are distributed by the blood but aresometimes accumulated in target organs.

Brain, nerves, fatty tissue: lipophilic compounds

Bones: inorganic lead, barium, strontium, fluorine, tin

Liver: mercury vapour, manganese, organic lead

Kidneys: cadmium, mercury

Thyroid gland: iodine

Hair: Arsenic

3. Distribution of chemicals3.2 Biological barriersThe endothelial cells of the blood capillaries in the brain are held together and are surrounded by glial cells. This makes the passage of compounds difficult and constitute the blood-brain barrier.

Problem: elemental mercury is lipophilic enough to pass the blood-brain barrier. If it is oxidized to Hg2+ it will not get out again...

The placenta regulates the flow of chemicals to the fetus

3. Distribution of chemicals3.3 Uptake versus distribution

The fate of a compound is directed by distribution coefficients

example 1: styrene (A) blood/air >30 1,1,1-trichloroethane (B) blood/air <5

saturation

3. Distribution of chemicals3.3 Uptake versus distribution

The fate of a compound is directed by distribution coefficients

example 2: toluene 12 1200 trichloroethylene 16 15

blood/air oil/blood

4. Cellular distribution

How can xenobiotics enter the cells?

4. Cellular distribution4.1 Diffusion

Transport through membranes by diffusion depends on:-temperature-molecular weight-polarity (pH)-concentration gradient

Transport from the cell membrane to the cell nucleus takes a few seconds

4. Cellular distribution4.2 Passive transport by proteins

Ion channels an be used by hydrophilic compounds

4. Cellular distribution4.3 Facilitated diffusion

Carrier proteins transport only one type of compoundand the capacity is limited

example: glucose transporter can also carry xylose and mannose

4. Cellular distribution4.4 Active transport

Active transport requires energy and can build up a concentration gradient

4. Cellular distribution4.5 Exocytosis and endocytosis Larger molecules are transported in vesicles by endo- or exocytosis

pinocytosis - a fluid is incorporatedphagocytosis - a solid is incorporated

5 Metabolism - Phase I

Properties suitable for excretion of toxic compounds:

Is the compound useful in the normal metabolism?

If not, it has to be excreted in order not to be accumulated.

Low molecular weight Exhaled fromthe lungs

metabolism break bonds or converts functional groups

5 Metabolism - Phase I

Properties suitable for excretion of toxic compounds:

Is the compound useful in the normal metabolism?

If not, it has to be excreted in order not to be accumulated.

Molecular weight > 500 Excreted with the bile

the compound is conjugated with some endogenous compound

5 Metabolism - Phase I

Properties suitable for excretion of toxic compounds:

Is the compound useful in the normal metabolism?

If not, it has to be excreted in order not to be accumulated.

High water solubility Excreted with the urine

functional groups are converted or the compound is conjugated

with polar compounds

5 Metabolism - Phase IBiotransformation is not equal to detoxification!

The metabolism will effect the biological activity of a compound

5 Metabolism - Phase ISummary of important Phase I and Phase II reactions

5 Metabolism - Phase I5.1 Oxidation Oxidations are performed by the cytochrome P-450 system

5 Metabolism - Phase I5.1 Oxidation Oxidations are performed by the cytochrome P-450 system

5 Metabolism - Phase I5.1 Oxidation Problem: reactive oxygen species are formed

5 Metabolism - Phase I5.1 Oxidation

Formation of hydroxyl radicals:

5 Metabolism - Phase I5.1 Oxidation

Formation of hydroxyl radicals:

Defence systems:

5 Metabolism - Phase I5.1 Oxidation

Epoxidation of alkenes

5 Metabolism - Phase I5.1 Oxidation

Oxidation of alkynes

5 Metabolism - Phase I5.1 Oxidation

Oxidation of aromatic compounds

5 Metabolism - Phase I5.1 Oxidation

Carbon hydroxylation

5 Metabolism - Phase I5.1 Oxidation

Oxidative dealkylation

5 Metabolism - Phase I5.1 Oxidation

Oxidation of halogenated compounds

5 Metabolism - Phase I5.1 Oxidation

Oxidation of heteroatoms

5 Metabolism - Phase I5.1 Oxidation

Oxidation of alcohols

5 Metabolism - Phase I5.2 Reduction

5 Metabolism - Phase I5.3 Hydrolysis

6 Metabolism - Phase II6.1 Conjugation with sulfate

6 Metabolism - Phase II6.2 Conjugation with glucoronic acid

6 Metabolism - Phase II6.2 Conjugation with glucoronic acid

Deconjugation

6 Metabolism - Phase II6.3 Conjugation with amino acids

6 Metabolism - Phase II6.4 Conjugation with glutathione

6 Metabolism - Phase II6.4 Conjugation with glutathione

The adducts are not always benign

7. Excretion of chemicals7.1 By the lungs

-volatile compounds are exhaled by the lungs

7. Excretion of chemicals7.1 By the lungs

-volatile compounds are exhaled by the lungs

7.2 By the liver-The liver metabolize compoundsSome compounds (e.g. mercury) are however excretedby bile which contains amphiphathic compounds suchas cholesterol and bile salts-The liver is the normal excretion way for compoundswith a molecular mass of more than 500 g/mol inhumans (lower for rats)-Compounds excreted by the liver can be uptaken bythe gastro-intestinal tract....

7. Excretion of chemicals7.3 By the kidneys

The kidneys is the most important excretion way

Some animals concentrate the urine (e.g. rats)

7. Excretion of chemicals

Milk is the major excretion way for DDT in breast-feeding women!

Cl Cl

ClClCl

8. Specific examples8.1 Hexane and heptane

8. Specific examples8.2 Benzene

8. Specific examples8.2 Benzene

8. Specific examples8.3 Hydroquinones

8. Specific examples8.4 Toluene

8. Specific examples8.5 Naphthalenes

8. Specific examples8.6 Styrene

good substrate for epoxide hydrolase

8. Specific examples8.7 PAH (polycyclic aromatic

hydrocarbons)

not a good substrate for epoxide hydrolase

8. Specific examples8.8 Dichloromethane

8. Specific examples8.8 Trichloroethylene

8. Specific examples8.8 Trichloroethylene

8. Specific examples8.9 Urethane

8. Specific examples8.10 Acetonitrile

8. Specific examples8.11 Aromatic amines

8. Specific examples8.11 Aromatic amines

A-list! B-list!

8. Specific examples8.12 Paracetamol

- limited amount of PAPS available for conjugation -more than a few grams a of paracetamol each day can give liver damage

8. Specific examples8.13 N,N-dimethyl formamide

8. Specific examples8.14 Dimethyl sulfoxide

8. Specific examples8.15 Acidosis

pH of the blood must be regulated to 7.35-7.40pH <7 or >8 is lethalthe blood is buffered and we can excrete CO2 in the lungs and acids with the urine

8. Specific examples8.15 Acidosis

pKa = 3.8

pKa = 4.8

pKa = 1.2

8. Specific examples8.15 Haemoglobin

8. Specific examples8.16 Fluoroacetic acid

8. Specific examples8.17 HCN