The document discusses various topics in toxicology including: 1) Toxicology addresses questions about health effects of exposures like pesticides and determines dose-response relationships in animal experiments. 2) Toxicology also deals with how chemicals and waste affect individual health. 3) Toxic substances are classified including heavy metals, solvents, radiation, dioxins, pesticides, and animal toxins. Animal toxins come from venomous or poisonous animals and can include toxins from snakes, fish, insects and other species.

1. TOXICOLOGY
UNIT ii

2. • Toxicology addresses a variety of questions. For example, in agriculture,
toxicology determines the possible health effects from exposure to pesticides or
herbicides, or the effect of animal feed additives, such as growth factors, on
people. Toxicology is also used in laboratory experiments on animals to establish
dose-response relationships.
• Toxicology also deals with the way chemicals and waste products affect the
health of an individual.
• Toxic substances are classified into the following:
A. Heavy Metals-Metals differ from other toxic substances in that they are neither
created nor destroyed by humans. Their use by humans plays an important role in
determining their potential for health effects. Their effect on health could occur
through at least two mechanisms: first, by increasing the presence of heavy metals
in air, water, soil, and food, and second, by changing the structure of the chemical.
For example, chromium III can be converted to or from chromium VI, the more
toxic form of the metal.
INTRODUCTION

3. B. Solvents and Vapors
Nearly everyone is exposed to solvents. Occupational exposures can range from the use
of “white-out” by administrative personnel, to the use of chemicals by technicians in a
nail salon. When a solvent evaporates, the vapors may also pose a threat to the exposed
population.
C. Radiation and Radioactive Materials
Radiation is the release and propagation of energy in space or through a material
medium in the form of waves, the transfer of heat or light by waves of energy, or the
stream of particles from a nuclear reactor
D. Dioxin/Furans
Dioxin, (or TCDD) was originally discovered as a contaminant in the herbicide Agent
Orange. Dioxin is also a by-product of chlorine processing in paper producing industries.

4. E. Pesticides
The EPA defines pesticide as any substance or mixture of substances intended to
prevent, destroy, repel, or mitigate any pest. Pesticides may also be described as
any physical, chemical, or biological agent that will kill an undesirable plant or
animal pest
G. Animal Toxins
These toxins can result from venomous or poisonous animal releases. Venomous
animals are usually defined as those that are capable of producing a poison in a
highly developed gland or group of cells, and can deliver that toxin through biting
or stinging. Poisonous animals are generally regarded as those whose tissues,
either in part or in their whole, are toxic.

5. Animal Toxins
• Some species from practically all phyla of animals produce toxins for either
offensive or defensive purposes.
• Some are passively venomous, often following inadvertent ingestion, whereas
others are actively venomous, injecting poisons through specially adapted stings
or mouthparts.
• It may be more appropriate to refer to the latter group only as venomous and to
refer to the former simply as poisonous.
• The chemistry of animal toxins extends from enzymes and neurotoxic and
cardiotoxic peptides and proteins to many small molecules such as biogenic
amines, alkaloids, glycosides, terpenes and others.
• In many cases the venoms are complex mixtures that include both proteins and
small molecules and depend on the interaction of the components for the full
expression of their toxic effect. For example, bee venom contains a biogenic
amine, histamine, three peptides, and two enzymes.
• The venoms and defensive secretions of insects may also contain many relatively
simple toxicants or irritants such as formic acid, benzoquinone, and other
quinines, or terpenes such as citronellal.
• Bites and stings from the Hymenoptera (ants, bees, wasps, and hornets) result in
5 to60 fatal anaphylactic reactions each year in the United States. According to
experts, about 0.3% to 3.0% of the US population experiences anaphylactic
reactions from insect stings and bites.

6. • Snake venoms have been studied extensively; their effects are due, in general, to toxins that are
peptides with 60 to 70 amino acids.
• These toxins are cardiotoxic or neurotoxic, and their effects are usually accentuated by the
phospholipases, peptidases, proteases, and other enzymes present in venoms. These enzymes
may affect the blood clotting mechanisms and damage blood vessels. Snake bites are responsible
for less than 10 deaths per year in the United States but many thousand worldwide.
• Many fish species, over 700 species worldwide, are either directly toxic or upon
ingestion are poisonous to humans.
• A classic example is the toxin produced by the puffer fishes (Sphaeroides spp.) called tetrodotoxin
(TTX). Tetrodotoxin is concentrated in the gonads, liver, intestine, and skin, and poisonings occurs
most frequently in Japan and other Asian countries where the flesh, considered a delicacy, is eaten
as “fugu.”
• Death occurs within 5 to 30 minutes and the fatality rate is about 60%.
• TTX is an inhibitor of the voltage-sensitive Na channel (like saxitoxin); it may also be found in some
salamanders and may be bacterial in origin.
Animal Toxins

7. Animal Toxins
Toxins and other natural products generally provide great benefit to society. For example,
some of the most widely used drugs and therapeutics like streptomycin, the
aminoglycoside antibiotic from soil bacteria, and acetylsalicylic acid (aspirin), the
nonsteroidal anti-inflammatory from willow tree bark, are used by millions of people
everyday to improve health and well-being. On the other hand, adverse encounters with
toxins like fish and shellfish toxins, plant, and insect toxins do result in harm to humans.
Some Components of Bee Venom
Compound Effect
Biogenic amine Pain, vasodilation, increased capillary
permeability
Histamine, Peptides
Apamine CNS effects
Melittin Hemolytic, serotonin release, cardiotoxic
Mast cell degranulating peptide Enzymes Histamine release from mast cells
Phospholipase A Hyaluronidase Increased spreading and penetration of
tissues

8. • A systemic toxin is one that affects the entire body or many organs rather than a
specific site.
• Potassium cyanide affects virtually every cell and organ in the body by interfering
with the cell’s ability to utilize oxygen.
• Toxins may also affect only specific tissues or organs while not producing damage
to the body as a whole.
• These specific sites are known as target organs or target tissues
• Benzene is a specific organ toxin in that it is primarily toxic to the blood forming
tissues.
• Lead is also a specific organ toxic, however it has three target organs (CNS, kidney
and hematopoietic system).
Targeted organ

9. • Toxicity is the intrinsic capacity of a chemical agent to affect an organism adversely.
• Xenobiotics is a term for “foreign substances”, that is, foreign to the organism. Its
opposite is endogenous compounds. Xenobiotics include drugs, industrial chemicals,
naturally occurring poisons and environmental pollutants.
• Risk is the probability of a specific adverse effect to occur. It is often expressed as the
percentage of cases in a given population and during a specific time period
• The dose-effect relationship is the relationship between dose and effect on the
individual level. An increase in dose may increase the intensity of an effect, or a
more severe effect may result. A dose-effect curve may be obtained at the level of
the whole organism, the cell or the target molecule.
• The dose-response relationship is the relationship between dose and the percentage
of individuals showing a specific effect. With increasing dose a greater number of
individuals in the exposed population will usually be affected.
• It is essential IN toxicology to establish dose-effect and dose-response relationships
Exposure, Dose and Response

10. All of these substances may also be further classified according to their:
• Effect on target organs (liver, kidney, hematopoietic system),
• Use (pesticide, solvent, food additive),
• Source of the agent (animal and plant toxins),
• Effects (cancer mutation, liver injury),
• Physical state (gas, dust, liquid),
• Labeling requirements (explosive, flammable, oxidizer),
• Chemistry (aromatic amine, halogenated hydrocarbon), or
• Poisoning potential (extremely toxic, very toxic, slightly toxic)

11. • Routes and Sites of Exposure
• Ingestion (Gastrointestinal Tract)
• Inhalation (Lungs)
• Dermal/Topical (Skin)
• Injection - intravenous, intramuscular, intraperitoneal
• Typical Effectiveness of Route of Exposure
Exposure pathways

12. Transport processes
• Diffusion: In order to enter the organism and reach a site where damage is
produced, a foreign substance has to pass several barriers, including cells and their
membranes.
• Most toxic substances pass through membranes passively by diffusion. This may
occur for small water-soluble molecules by passage through aqueous channels or,
for fat-soluble ones, by dissolution into and diffusion through the lipid part of the
membrane.
• Ethanol, a small molecule that is both water and fat soluble, diffuses rapidly
through cell membranes.
• Diffusion of weak acids and bases. Weak acids and bases may readily pass
membranes in their non-ionized, fat-soluble form while ionized forms are too polar
to pass.
• The degree of ionization of these substances depends on pH. If a pH gradient exists
across a membrane they will therefore accumulate on one side.
• The urinary excretion of weak acids and bases is highly dependent on urinary pH.
Uptake and Disposition

13. Uptake and Disposition
Active transport:
• Some substances are actively transported across cell membranes. This
transport is mediated by carrier proteins in a process analogous to that of
enzymes
• Phagocytosis is a process where specialized cells such as macrophages engulf
particles for subsequent digestion. This transport process is important, for
example, for the removal of particles in the alveoli.

14. • Absorption is the uptake of a substance from the environment into the organism.
The term usually includes not only the entrance into the barrier tissue but also the
further transport into circulating blood.
• Pulmonary absorption. The lungs are the primary route of deposition and absorption
of small airborne particles, gases, vapours and aerosols.
• For highly water-soluble gases and vapours a significant part of the uptake occurs in
the nose and the respiratory tree, but for less soluble substances it primarily takes
place in the lung alveoli.
• The alveoli have a very large surface area (about 100 m2 in humans).
• In addition, the diffusion barrier is extremely small, with only two thin cell layers and
a distance in the order of micrometers from alveolar air to systemic blood circulation.
• This makes the lungs very efficient not only in the exchange of oxygen and carbon
dioxide but also of other gases and vapours.
Absorption

15. • Once a living organism has been exposed to a toxicant, the compound must get into
the body and to its target site in an active form in order to cause an adverse effect.
• The body has defenses:
• Membrane barriers
• passive and facilitated diffusion, active transport
• Biotransformation enzymes, antioxidants
• Elimination mechanisms
• Inhalation: readily absorb gases into the blood stream via the alveoli.(Large alveolar
surface, high blood flow, and proximity of blood to alveolar air)
• Ingestion: absorption through GI tract stomach (acids), small intestine (long contact
time, large surface area)
• 1st Pass Effect (liver can modify)
• Dermal: absorption through epidermis (stratum corneum), then dermis; site and
condition of skin
Absorption

16. • The distribution of a substance within the organism is a dynamic process which
depends on uptake and elimination rates, as well as the blood flow to the different
tissues and their affinities for the substance.
• Water-soluble, small, uncharged molecules, univalent cations, and most anions
diffuse easily and will eventually reach a relatively even distribution in the body.
• Volume of distribution is the amount of a substance in the body at a given time,
divided by the concentration in blood, plasma or serum at that time.
• The value has no meaning as a physical volume, as many substances are not
uniformly distributed in the organism.
• A volume of distribution of less than one l/kg body weight indicates preferential
distribution in the blood (or serum or plasma), whereas a value above one
indicates a preference for peripheral tissues such as adipose tissue for fat soluble
substances.
Distribution

17. • Blood carries the agent to and from its site of action, storage depots, organs of
transformation, and organs of elimination
• Rate of distribution (rapid) dependent upon
• blood flow
• characteristics of toxicant
(affinity for the tissue and the partition coefficient)
• Distribution may change over time
Distribution

18. • Metabolism is the process by which the administered chemical (parent
compounds) are modified by the organism by enzymatic reactions.
• objective: make chemical agents more water soluble and easier to excrete
• decrease lipid solubility --> decrease amount at target
• increase ionization --> increase excretion rate --> decrease toxicity
• Bioactivation: Biotransformation can result in the formation of reactive
metabolites
Metabolism

19. • Acute toxicity describes the adverse effects of a substance that result either
from a single exposure or from multiple exposures in a short space of time
(usually less than 24 hours).
• Symptoms may vary, severity can range from mild to lethal.
• Chronic toxicity is a property of a substance that has toxic effects on a
living organism, when that organism is exposed to the substance
continuously or repeatedly.
• Carcinogenic, mutagenic, teratogenic effects can be expected.
Effects of acute and chronic poisoning

20. • Accumulation is the build-up of a substance in a tissue or organ to higher
levels than in blood or plasma.
• It may also refer to a gradual build-up over time in the organism.
• Many xenobiotics are highly fat soluble and tend to accumulate in
adipose tissue, while others have a special affinity for bone.
• For example, calcium in bone may be exchanged for cations of lead,
strontium, barium and radium, and hydroxyl groups in bone may be
exchanged for fluoride.
Accumulation

21. • Elimination is the disappearance of a substance in the body. Elimination may involve
excretion from the body or transformation to other substances not captured by a
specific method of measurement. The rate of disappearance may be expressed by
the elimination rate constant, biological half-time or clearance.
• Concentration-time curve. The curve of concentration in blood (or plasma) versus
time is a convenient way of describing uptake and disposition of a xenobiotic.
• Area under the curve (AUC) is the integral of concentration in blood (plasma) over
time. When metabolic saturation and other non-linear processes are absent, AUC is
proportional to the absorbed amount of substance.
• Biological half-time (or half-life) is the time needed after the end of exposure to
reduce the amount in the organism to one-half. As it is often difficult to assess the
total amount of a substance, measurements such as the concentration in blood
(plasma) are used. The half-time should be used with caution, as it may change, for
example, with dose and length of exposure. In addition, many substances have
complex decay curves with several half-times.
Elimination

22. • Toxicants are eliminated from the body by several routes
• Urinary excretion
• water soluble products are filtered out of the blood by the kidney and excreted
into the urine
• Exhalation
• Volatile compounds are exhaled by breathing
• Biliary Excretion via Fecal Excretion
• Compounds can be extracted by the liver and excreted into the bile. The bile
drains into the small intestine and is eliminated in the feces.
• Milk
• Sweat
• Saliva
EXCRETION
KIDNEY
FEACES
LUNGS

23. • Excretion is the exit of a substance and its biotransformation products from the
organism.
• Excretion in urine and bile. The kidneys are the most important excretory organs.
Some substances, especially acids with high molecular weights, are excreted with
bile. A fraction of biliary excreted substances may be reabsorbed in the intestines.
This process, enterohepatic circulation, is common for conjugated substances
following intestinal hydrolysis of the conjugate.
• Other routes of excretion. Some substances, such as organic solvents and breakdown
products such as acetone, are volatile enough so that a considerable fraction may be
excreted by exhalation after inhalation. Small water-soluble molecules as well as fat-
soluble ones are readily secreted to the foetus via the placenta, and into milk in
mammals. For the mother, lactation can be a quantitatively important excretory
pathway for persistent fat-soluble chemicals. The offspring may be secondarily
exposed via the mother during pregnancy as well as during lactation. Water-soluble
compounds may to some extent be excreted in sweat and saliva. These routes are
generally of minor importance. However, as a large volume of saliva is produced and
swallowed, saliva excretion may contribute to reabsorption of the compound. Some
metals such as mercury are excreted by binding permanently to the sulphydryl
groups of the keratin in the hair.
Excretion

24. • Biotransformation is a process which leads to a metabolic conversion of foreign
compounds (xenobiotics) in the body. The process is often referred to as
metabolism of xenobiotics.
• As a general rule metabolism converts lipid-soluble xenobiotics to large, water
soluble metabolites that can be effectively excreted.
• The liver is the main site of biotransformation. All xenobiotics taken up from the
intestine are transported to the liver by a single blood vessel. If taken up in small
quantities a foreign substance may be completely metabolized in the liver before
reaching the general circulation and other organs (first pass effect). Inhaled
xenobiotics are distributed via the general circulation to the liver. In that case
only a fraction of the dose is metabolized in the liver before reaching other
organs.
• Liver cells contain several enzymes that oxidize xenobiotics. This oxidation
generally activates the compound—it becomes more reactive than the parent
molecule. In most cases the oxidized metabolite is further metabolized by other
enzymes in a second phase. These enzymes conjugate the metabolite with an
endogenous substrate, so that the molecule becomes larger and more polar. This
facilitates excretion.
Biotransformation

25. • Enzymes that metabolize xenobiotics are also present in other organs such as the lungs
and kidneys. In these organs they may play specific and qualitatively important roles in
the metabolism of certain xenobiotics. Metabolites formed in one organ may be
further metabolized in a second organ. Bacteria in the intestine may also participate in
biotransformation.
• Metabolites of xenobiotics can be excreted by the kidneys or via the bile. They can also
be exhaled via the lungs, or bound to endogenous molecules in the body.
• The relationship between biotransformation and toxicity is complex. Biotransformation
can be seen as a necessary process for survival. It protects the organism against
toxicity by preventing accumulation of harmful substances in the body.
• However, reactive intermediary metabolites may be formed in biotransformation, and
these are potentially harmful. This is called metabolic activation. Thus,
biotransformation may also induce toxicity. Oxidized, intermediary metabolites that
are not conjugated can bind to and damage cellular structures. If, for example, a
xenobiotic metabolite binds to DNA, a mutation can be induced (see “Genetic
toxicology”). If the biotransformation system is overloaded, a massive destruction of
essential proteins or lipid membranes may occur. This can result in cell death.
Biotransformation