Lecture 4: Uptake, Metabolism and Elimination of Pollutants

What feature in toxicology is most important?

Most chemicals must enter the organism to exert a toxic effect

What is ADME?

• Absorption

• Distribution

• Metabolism

• Excretion

What are the exposure routes?

• Inhalation

• Dermal contact

• Ingestion

• Injection

Absorption vs adsorption)

• Absorption occurs where molecules are drawn into the bulk of the phase and penetrate/distribute into the material

• Adsorption occurs when molecules adhere to the surface of the phase

Absorption

• Transfer of chemical from exposure site to system circulation

• Usually (but not always) through the bloodstream (systemic circulation)

• Factors affecting absorption rate:

  • Route of exposure

  • Dosage of concentration

  • Physicochemical properties of chemicals

  • Physiological factors

To enter an organism

A chemical must cross a cell membrane no matter what the route of entry (ingestion, inhalation, gills, skin). A cell membrane consists of a bilayer of lipids.

What are the four ways in which chemicals can pass through lipid bilayers?

• Diffusion

• Facilitated diffusion

• Active transport

• Endocytosis

Diffusion

The movement of a chemical molecule from a place of higher concentration to a place of lower concentration (along concentration gradient). This process does not require much energy. This process also never stops; net movement is what stops.

Facilitated diffusion

A form of diffusion that is aided by transmembrane proteins (channel protein and carrier proteins), that cross the membrane. It does not travel through these bilayer lipids

Channel proteins demonstrate weak selectivity of molecules based on size and charge status. Carrier proteins are more selective (only allows one type of protein to pass through).

Chemicals that use facilitated diffusion function

Glucose (sugar) is absorbed through these carrier proteins. It does not go through the bilayer lipids because the molecules are too large and too polar (water loving).

Active transport

Active transport acts against the concentration gradient. It transports molecules from where concentration is lower to places of higher concentration. This process requires energy.

Endocytosis

A general process by which cells absorb external material by engulfing it within the cell membrane. The two types of endocytosis are:

• Phagocytosis

  • a cellular process for ingesting and eliminating solid particles larger than 0.5 μm in diameter, including microorganisms and foreign substances

• Pinocytosis

  • a cellular process for ingesting and eliminating liquids and dissolved substances

Chemicals that use active transport

Sodium ions

How do pollutants cross membranes?

• As long as there is a favourable concentration gradient, any chemical can cross over

• Proteins allow chemicals to cross over when they get tricked:

  • Only chemicals with a similar size, volume and favourable concentration gradient can cross by facilitated diffusion

  • Only chemicals with a very similar size, volume or 3D shape can cross by active transport

    • e.g. ionic radius of cadmium (Cd²⁺ = 95 pm) and calcium (Ca²⁺ = 100 pm) are very similar, so Cd²⁺ can bind to proteins that transport Ca²⁺ across membranes

Do all chemicals cross the membrane equally?

Physicochemical properties control entry:

• Ionic state

  • neutral molecules (not charged) cross membranes better than charged molecules (because they are nonpolar and oil loving)

• Molecular size

  • smaller molecules cross membranes more easily (lower effort)

• Lipophilicity

  • increased lipophilicity increases the ability to cross membranes

• Viscosity

  • increased viscosity decreases the ability to cross membranes (lower effort)T

• Concentration

  • increased conc increases the ability to cross membranes

The Gastrointestinal tract (GIT)

This route of entry applies to pollutants in food, solids or liquids that are swallowed .

Parts of the body involved in the ingestion of a pollutant

• Mouth and teeth

  • to chew and break food in smaller pieces thus increasing surface area to volume ratio

  • small amounts of food also absorbed in mouth

• Oesophagus

  • conduit from the mouth to the stomach

• Stomach

  • receptacle for chewed foods

  • acid and enzyme excretion which breakdown the structure of the food particles

  • food leaves the stomach and enters the small intestine

• Small and large intestine

  • the principal organs that absorb food substances and pollutant into the organism’s body

The intestines as a route of absorption

• Intestines are designed to maximise absorption

  • small intestine is ~2.7 to 5 m long in adults

  • large intestine is ~1.5 m long in adults

  • intestines have three sets of ridges (plicae circulares→ villi → microvilli) to increase their surface area (which increases absorption of food including toxicants)

Respiratory system (land animals)

This route of entry applies to pollutants in air, particles or vapour that is inhaled

Parts of the body involved in the inhalation of a pollutant

• Trachea

• Bronchi (primary and secondary)

• Bronchioles

• Alveolar duct

• Aleveoli

The aleveoli as a route of absorption

• Gas exchange occurs between alveoli and capillaries

• Gases can dissolve and diffuse between the lungs and the circulatory system

  • oxygen diffuses into red blood cells

  • carbon dioxide diffuses into alveolus

• Lungs maximise gas exchange by increasing surface area (adult lung area is ~50 - 70 m²)

Defences of the respiratory system

1. Nose has nasal hairs (act as crude filter) and mucosa cells that line the nose and secrete mucus (trap particles)

2. Medium sized particles will hit the larynx and will not go further than the mouth

3. Lung structure and air velocity

   1. our airway will double 23 times which helps with particle removal (area increases)

   2. Bernoulli’s principle suggests that high velocity keeps small and large particles suspended, whereas lower velocity causes larger particles to strike mucus covered surfaces

4. The mucociliary escalator is made of Goblet cells that line the respiratory system (mouth, nose to bronchioles) and secrete mucus (phlegm, sputum or spit)

   1. mucus layer is 5 to 10 µm thick

   2. other cells lining the respiratory system are called ciliated cells which have cilia (tiny hairlike projections)

5. Coughing and sneezing

Particles sizes and penetration into the respiratory system

• 10 µm (coarse)

  • mass = 1

  • particle no. = 1

  • surface area per particle = 1

  • total surface area per mass = 1

    • filtered in proximal airways

    • may irritate skin and mucosa

• 2.5 µm (fine)

  • mass = 1

  • particle no. = 64

  • surface area per particle = 0.0625

  • total surface area per mass = 4

    • reached peripheral airways

    • cannot enter systemic circulation

• 0.1 µm (ultrafine)

  • mass = 1

  • particle no. = 1,000,000

  • surface area per particle = 0.0001

  • total surface area per mass = 100

    • higher adsorbed toxic material on surface

    • may enter systemic circulation

COVID and respiratory system

When coronavirus infects cells in the respiratory system, they cut off cilia and stop their cleaning function

Respiratory system (aquatic organisms)

Gills in aquatic organisms perform the same function as lungs in terrestrial organisms (exchange gases and bring O₂ into the organism). They are the main route of entry for pollutants in aquatic organisms.

Skin

The skin is a selective barrier some substances can penetrate and others can not

Distribution within the organism

Partitioning within the blood to kidneys and liver, bone and tissues.

T_f + biomolecule → T_b

T_f the free fraction of biologically active and can cause toxic effects

T_b the bound fraction is not biologically active and does not cause toxic effects

Blood phases

Blood has three main phases:

• Water

• Lipids

• Protein

Examples of partitioning in blood

• DDT

  • log KOW = 6.2

  • aqueous solubility (mg/L) = 0.003

  • % free = 0.1

  • % bound = 99.9

• Parathion

  • log KOW = 3.8

  • aqueous solubility (mg/L) = 6.5

  • % free = 1.3

  • % bound = 98.7

• Nicotine

  • log KOW = 1.3

  • aqueous solubility (mg/L) = very soluble

  • % free = 75.0

  • % bound = 25.0

Persistent Organic Pollutants (POPs such as DDT) are preferably stored in fat cells. What do you think would happen to blood concentrations of POPs if a person lost weight?

We would expect POP concentrations in blood to increase.

Do you think that organisms with a higher fat content would be able to
tolerate more or less of a fat-soluble pollutant?

We would expect organisms with a higher fat content would be able to tolerate higher concentrations of a fat-soluble pollutant.

Metabolism and biotransformation

Many foreign chemicals (xenobiotics) enter the body. Organisms have developed defences against these chemicals. Decreasing order of important for biotransformation:

• Liver

• Kidney

• Lungs

• Intestine

Metabolism (biotransformation vs bioactivation)

• Biotransformation reactions are catalysed by enzymes

  • Enzymes are proteins that facilitate particular biochemical reactions to occur

Phase 1 reactions of biotransformation

• Phase 1 reactions involve oxidation, reduction and hydrolysis of the chemical

• Phase 1 enzymes are cytochrome P450 system and mixed function oxygenase

• These reactions add or expose:

  • Hydroxyl groups (OH^-)

  • Amino groups (NH₂)

  • Thiol groups (SH)

  • Carboxyl groups (COOH)

    • these increase the chemicals’ polarity, aqueous solubility and make it more susceptible to Phase 2 reaction

Phase 2 reactions of biotransformation

• These are called biosynthetic reactions or conjugate reactions, and they require energy to occur

• Conjugation process involves adding endogenous molecules to the foreign molecules that have already been hydrolysed in Phase 1 metabolism

  • e.g. glucuronic acid and sulfate will attach these molecules to the Phase 1 metabolite to become more water soluble and available for excretion (pee)

Example of Phase 2 reactions

Di(2-ethylhexyl) phthalate (DEHP) is a commonly used plasticiser. Polymers when first produced as plastics are useless so plasticisers are added to make the product malleable. The issue is that these plastics are binded to the material and easily leached. So it is likely everyone has DEHP in their body — however it can be metabolised and excreted easily.

Interesting feature of biotransformation

• Most enzymes are highly specific — they catalyse one specific chemical reaction

  • but enzymes in metabolism (biotransformation and bioactivation) are not

• Animals and humans are not born with a full complement of metabolic enzymes

  • chemical concentrations in infants and kids are much higher than adults

Elimination of pollutants

• The main routes of excretion are:

  • Urine

  • Faeces

  • Respiratory system

pee and poo are generally the most important but it depends on the physicochemical properties of pollutant

Urinary system

• The kidney is a filter of water-soluble material

• T_f can bypass to bladder whereas T_b cannot bypass due to larger molecule weight

Faecal system

• Mainly excrete molecules with a larger weight

When we lose fat where do they go?

85% leaves via our lungs as CO2 and the rest as
water through urine and sweat etc.

Respiratory system elimination (lungs)

The lungs only eliminate gases or volatile pollutants. e.g. carbon dioxide, alcohol

Respiratory system elimination (gills)

• Fastest for chemical with low lipophilicity (log KOW < 3) and/or high polarity

• Slow for non-polar chemicals

• Driven by diffusion — must be a favourable gradient (higher internal conc and lower external conc)

Elimination by other organisms

• Fish lay fat rich eggs

• Crustaceans and insects moult their exoskeleton

• Birds incorporate metals into their feathers

• Many aquatic organisms secrete metals into granules

Elimination by plants

• They have a range of different elimination routes compared to humans and mammals:

  • Transformation (equivalent to animals)

  • Leaching

  • Evaporation from leaves (equivalent to animals’ lungs or gills)

  • Dying and falling leaves (equivalent to animals’ hair, nails etc)

  • Exudation from roots

  • Herbivore grazing

Biomagnification

When toxins stored in fat cells increase in their concentration as they move along the food chain. (e.g. diotoxins from burning plastics → settle in pastures → consumed by cattle → eaten by humans → accumulate in from mother to infant)

Breast milk

Numerous studies have shown that mothers milk contains a range of organic chemicals e.g. POPS

Overall picture of distribution and excretion