Mechanisms of Toxicity

Concentration, duration of exposure

The _______ of the toxicant and _________ contributes to toxicity.

Foreign

The body recognizes the toxicant as _____ and attempts to eliminate it and repair any damage.

Ultimate toxicant

The chemical species that reacts with the endogenous target molecule or critically alters the biological environment, initiating structural and/or functional alterations that result in toxicity.

Toxicant

All of these can be a ______.

• The original chemical

• A metabolite or reactive species generation during the biotransformation of the toxicant

• An endogenous molecule

Endogenous molecule

Bilirubin is considered a(n)…

Treatment with sulfonamides bind albumin, allowing free bilirubin which can cause brain damage.

Endogenous moelcule

Lipid radicals are considered a(n)…

Carbon tetrachloride reacts with unsaturated fatty acids, which initiates a chain of lipid per oxidation that causes hepatotoxicity.

Delivery from site of exposure to target

What is the first step in the mechanism of toxicity?

Absorption

The transfer of a chemical from the site of exposure into the systemic circulation.

• Lipid solubility is the most important factor for toxic agents.

• Affected by concentration, area, characteristics of epithelial tissue, & microcirculation.

Presystemic Elimination

Also called 1st pass metabolism.

Generally reduces the toxic effects of chemicals but can cause injury to the digestive mucosa, liver, and lungs.

Factors that facilitate diffusion

These are examples of…

• Porosity of the capillary & endothelium

• Specialized transport across the plasma membrane

• Accumulation in cell organelles

• Reversible intracellular binding

Factors that oppose diffusion

These are examples of…

• Binding to plasma proteins

• Specialized barriers

• Distribution to tissues/storage sites

• Association with intracellular binding proteins

• Export from cells (P-glycoprotein)

Accumulation in cell organelles

• Happens in lysosomes & mitochondria

• Amphiphatic xenobiotics with a protonatable amine group

Reversible intracellular binding

This is an example of…

• Organic & inorganic cations & PAH bind to melanin

Association with intracellular binding proteins

This is an example of…

• Metallothionein binds ROS & heavy metals

Highly hydrophilic & ionized

The kidney and liver efficiently remove _____________ chemicals such as organic acids & bases.

Nonvolatile, highly lipophilic chemicals

There are no efficient elimination mechanisms for…

Volatile, nonreactive toxicants and liquids

These chemicals diffuse from pulmonary capillaries into the alveoli and are exhaled.

Increases the concentration & the time of exposure by slowing urine flow

Toxicants delivered to renal tubules may diffuse back across the tubular cells, dependent on lipid solubility & ionization.

What are the effects of this?

Reabsorbed by diffusion

Toxicants delivered to the GI tract may be _________ across the intestinal mucosa.

Toxication, metabolic activation

• Biotransformation to harmful products

• Renders xenobiotics indiscriminately reactive towards molecules with susceptible functional groups.

• Generates:

  • Electrophiles

  • Free radicals

  • Nucleophiles

  • Redox-active reactants

Elimination of the ultimate toxicant or preventing its formation

Toxication competes with…

Detoxication of toxicants with no functional groups

• Benzene & toluene

• Two phases— OH or COO, then glucoronic acid/sulfuric acid is added to the functional group by a transferase.

Detoxication of nucleophiles

Conjugation at the nucleophilic functional group (OH, SH)

Detoxication of electrophiles

Conjugations with the SH of glutathione; epoxide hydrolase, carboxyl esterase, among others.

Detoxication of free radicals

Specific enzyme for different types of free radicals— for example, superoxide dismutase catalyze the reaction of O2- to oxygen or hydrogen peroxide.

Protein toxin

Extra & intracellular proteases— for example, toxins with disulfide binds are inactivated by thioredoxin.

Overwhelm

When detoxication fails, toxicants can ______ the system.

Inactivate

Toxicants can _______ detoxicating enzymes.

Reversed

Conjugations reactions can be…

Harmful byproducts

Sometimes, detoxication generates potentially…

Reaction of the ultimate toxicant with the target molecule

What is the second step in the mechanism of toxicity?

Target molecule

Toxicity is typically mediated by a reaction of the ultimate toxicant with a target molecule.

Appropriate reactivity, accessibility, critical function

Target molecules must have…

Cellular dysfunction & resultant toxicities

What is the third step in the mechanism of toxicity?

Dysfunction of target molecule

These are examples of…

• Activation: agonist, activator

• Inhibition: antagonist

• Interference with DNA template

Destruction of target molecule

These are examples…

• Crosslinking

• Fragmentation

• Drug-Protein adduct

Chemicals that cause DNA adducts

• Leads to DNA mutations which can activate cell death pathways.

• If mutations activate oncogenes or inactivate tumor suppressors, it can lead to uncontrolled cell proliferation & cancer.

Chemicals that cause protein adducts

• Leads to protein dysfunction which can activate cell death pathways.

• These can lead to autoimmunity.

• If _________ can activate oncogenes or inactivate tumor suppressors, it can lead to uncontrolled cell proliferation & cancer.

Chemicals that cause oxidative stress

Can oxidize DNA or proteins leading to DNA mutations or protein dysfunction (benzene).

Chemicals that activate or inactivate ion channels

• Can cause widespread cellular dysfunction, cell death, & many physiological symptoms.

• Na, Ca, K levels are extremely important in neurotransmission, muscle contraction, & nearly every cellular function.

Chemicals that inhibit cellular respiration

Inhibitors of proteins or enzymes involved in oxygen consumption, fuel utilization, and ATP production will cause energy depletions & death.

Chemicals that specifically interact with protein targets

Chemicals that inhibit the production of cellular building blocks (nucleotides, lipids, amino acids).

Chemicals that specifically interact with protein targets

Chemicals that inhibit enzymatic processes of bioactive metabolites that alter ion channels & metabolism.

Target molecule-independent toxicity

These are…

1. Chemicals that alter H+ concentrations

2. Solvents or detergents that alter the lipids of cell membranes

3. Occupying a site or space

3 primary metabolic disorders jeopardizing cell survival

1. ATP depletion

2. Sustained rise in intracellular calcium

3. Overproduction of ROS & RNS

ATP

Plays a central role in cellular maintenance both as a chemical for biosynthesis and as the major source of energy.

ATP

• Drives ion transporters to maintain cellular ion gradients

• Used in biosynthetic reactions (phosphorylation & adenylation)

• Essential for protein degradation

• Used for signal transduction regulation (kinase phosphorylation)

• Incorporated into DNA

• Required for muscle contraction & neurotransmission

• Required for polymerization of cytoskeleton

• Necessary in cell division

• Needed for the maintenance of cell morphology

Direct consequences of ATP depletion

• Compromised ion pumps lead to a loss of ionic & volume regulatory controls, and excessive Ca/Na influx

• End result is necrosis

Agents that impair ATP synthesis

• Inhibitors of electron transport

• Inhibitors of oxygen delivery

• Inhibitors of ADP phosphorylation

• Chemicals causing mitochondrial DNA damage

Calcium

Involved in:

• Signal transduction regulation & exocytosis

• Muscle contraction

• Cytoskeletal polymerization

• Neurotransmission & synaptic plasticity

• Enzyme induction

• Transporters

Calcium levels

• Highly regulated

• 10,000 fold difference between extracellular & intracellular levels

• Maintained by impermeability of the plasma membrane

• Transport mechanisms remove it from the cytoplasm

• Sources are extracellular, or ER / mitochondria

Excitotoxicity

Consequence of increased intracellular calcium

Depletion of energy reserves, dysfunction of microfilament, activation of hydrolytic enzymes, generation of ROS/RNS

What are the four consequences of increased intracellular calcium?

Oxidative stress

Imbalance of cellular oxidants & antioxidants

• Direct generation of reactive oxygen & nitrogen species

• Indirect, caused by increase in Ca & CYPs

Consequences of reactive oxygen & nitrogen species

1. Can directly oxidize & affect protein function & can mutate DNA leading to cellular dysfunction

2. DNA damage

3. Oxidative inactivation of Ca-ATPase & elevate Ca

4. Drain ATP reserves

5. Compromise ATP synthesis

6. Lipid peroxidation, cell swelling, cell rupture

Repair or disrepair

What is the fourth step in the mechanism of toxicity?

Progression of toxicity

If damage resulting from toxicant exposure is not repaired, it can cause damage at higher levels of the biological hierarchy & influence the…

Examples:

1. Molecular repair: proteins, lipids, DNA

2. Cellular repair: neuron repair, tissue repair

3. Toxicity from disrepair: tissue necrosis, fibrosis, carcinogenesis

4. Failure of repair

Damaged molecules may be repaired in different ways

1. Some chemical groups can be removed

2. Removal of damage & insertion of correct repaired part

3. Molecule is degraded & resynthesized

Molecular repair of proteins

• Oxidation of thiols can be reversed by enzymatic reduction

• Oxidized hemoglobin can be repaired by electron transfer

• Molecular chaperone proteins help refold proteins

• Proteolytic degradation

Molecular repair of lipids

• Peroxided ____ are repaired by a complex process that involves a series of reductants & requires NADPH

• May be degraded & resynthesized entirely

Molecular repair of DNA

• Some modifications can be directly reversed by specific enzymes

• Various repair pathways are in place to removed damage

Peripheral neuron repair

• Can be repaired, do NOT multiply

• Requires macrophages & Schwann cells

  • removes debris, produce growth factors

  • schwann cells guide & chemically lure axons to reinnervate target cells

• Reserve cells can take over the functions of lost neurons

Apoptosis

• Active deletion of damaged cells, initiated by cell injury

• Specific cellular program REQUIRED

• Only has value as a repair process if the tissue is made up of constantly renewing or conditionally dividing cells

• Not valuable in organs that contain non-replicating & non-replaceable cells

• Eliminates cells with mutagenic DNA

Necrosis

• Unprogrammed cell death

• Passive form of cell death induced by accidental damage of tissue

• Does not involve activation of any specific cellular program

• Cells do this in response to damage

  • Chemicals, viruses, infection, cancer, inflammation, ischemia

Necrosis

• Early loss of plasma membrane integrity & welling of the cell body followed by cell bursting

• Mitochondria & other processes contain substances that are damaging to surrounding cells

• Released upon bursting & cause inflammation

Apoptosis

• Cell shrinks as its nuclear & cytoplasmic materials condense

• Breaks into membrane-bound fragments that are phagocytosed

• Intracellular constituents are not released where they might have deleterious effects on neighboring cells

Regeneration of tissue

• Proliferation

• Involves both the lost cells & the extracellular matrix

• Reintegration of the newly formed elements into tissues & organs

Cadherins

Allow adjacent cells to adhere to each other C

Connexins

Connect neighboring cells internally by associations of these proteins into gap junctions

Integrins

Link cells to the extracellular matrix

Inflammation

• Alteration of microcirculation & accumulation of ____ cells are initiated by macrophages secreting cytokines in response to tissue damage.

• Cytokines stimulate neighboring cells to release mediators that induce dilation of local microvasculature—> capillary permeabilization

• These cells also facilitate the egress of circulating leukocytes into the injured tissue by releasing chemoattractants & expressing cell-adhesion molecules.

An organism has mechanisms that…

1. Counteract the delivery of toxicant, such as detoxication

2. Reverse the toxic injury, such as repair mechanisms

3. Offset some dysfunctions, such adaptive responses

Is not

Toxicity (is/is not) an inevitable consequence of toxicant exposure.

Toxicity

Develops if the toxicants exhausts or impairs the protective mechanisms and/or overrides the adaptability of biological systems.