Biotransformation of Toxicants

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Biotransformation of Toxicants

• Chapter 5

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What Happens to Toxicants After Absorption and
Before Excretion?

• Metabolism
• More specifically, Biotransformation
• The most important sites of these reactions
• Liver (Primary Organ)
• Lungs
• Stomach
• Intestine
• Skin
• Kidneys

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Perfect Metabolism
Toxicants ?
Excretion ?
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Biotransformation

• All organisms are exposed constantly and
  unavoidably to foreign chemicals or Xenobiotics
• What property allows most substances to be
  absorbed?
• Lipid-loving (lipophilic)
• This same property makes them difficult if not
  impossible to excrete

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Biotransformation

• So, if most substances enter the body as
  lipophilic (lipid-loving) what form of Xenobiotic
  do we want to end up with?
• Generally, it would be optimal to transform
  all xenobiotics to water-loving substances
  (Hydrophilic)
• What forms of Xenobiotics are we talking about?

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Lipid
Hydrophilic
Polar
Lipophilic
Water
Highly Lipophilic
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2 General Types of Biotransformation

• Phase I
• Phase II

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Phase I

• DEGREDATION REACTIONS
• (Kent calls these Catabolic Reactions)
• Oxidation
• Reduction
• Hydrolysis

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Phase II

• CONJUGATION REACTIONS
• (Kent calls these binding reactions)
• Glucuronide formation
• Sufate conjugation
• Methylation
• Acetylation
• Amino Acid conjugation
• Glutathione conjugation

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Biotransformation Reactions

• Phase I and II reactions may occur simultaneously
  or sequentially
• We typically represent them sequentially for
  clarity and this is the way we will represent
  them in class.

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XENOBIOTICS
Highly Lipophilic
Lipophilic
Polar
Hydrophilic
Accumulation in Fat
Phase I Bioactivation or Inactivation
Polar
Phase II Bioinactivation
Hydrophilic
Extracellular Mobilization Biliary
Excretion Plasma Circulation Renal
Excretion
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Biotransformation

• Phase I

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Biotransformation Phase I
LIPOPHILIC
POLAR

• As a result of an oxidation, reduction, or
  hydrolysis reaction

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Oxidation-Reduction Reactions

• Oxidation occurs when a molecule combines with
  oxygen, loses hydrogen, or loses one or more
  electrons
• Reduction occurs when a molecule combines with
  hydrogen, loses oxygen, or gains one or more
  electrons

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Overview of Phase I Reactions

• Reductions not very common.
• Example the reduction of azo and nitro compounds
  yields a corresponding amine.

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Hydrolysis

• Hydrolysis Cleavage of a xenobiotic by the
  addition of water. Ex. Benzene to Phenol

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OXIDATION MOST IMPORTANT Phase I Reactions

• What is the nature of an oxidative process?
• The reaction will consume O2
• Most important reaction is P-450 oxidation
  process
• P-450 is an important component of the system -
  Mixed-Function Oxidase (MFO)
• Primarily, these reactions occur in hepatic
  endoplasmic reticulum

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P-450 MFO System

• The name of this system refers to the ability to
  incorporate one atom of molecular oxygen into the
  substrate and reduce the other atom of oxygen to
  water.

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P-450 MFO Oxidation Process
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Phase 1 From Lipophilic To PolarEXAMPLE
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Bioinactivation Bioactivation

• Biotransformation often leads to changes in a
  molecule, which increases its water solubility
  and improves its excretion
• These changes often shorten the duration of the
  toxic effect.

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Bioinactivation

• Remember, in general, there is a direct
  relationship between the concentration of a
  substance and the intensity of the toxic effect.
  (the greater the concentration, the more intense
  the toxic effect)
• Therefore, bioinactivation or deactivation
  generally means a decrease in the intensity of
  the toxic effect.

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Bioactivation

• Biotransformation reactions, which yield products
  having a higher toxicity than the parent
  compound, are referred to as BIOACTIVATION
  reactions.
• Example Benzoapyrene undergoes a series of
  metabolic reactions producing a variety of
  chemical intermediates (metabolites). One
  intermediate 7,8-dihydrodiol,9,10-oxide is
  believed to be the cancer-causing agent.

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Bioactivation - Another Example

• Parathion, an OP, is oxidized (Phase I) to
  Paraoxon (which has an extremely high affinity
  for acetylcholinesterase (AchE)).
• We call this increase in toxicity - BIOACTIVATION
  
• Fortunately, Paraoxon can subsequently be
  hydrolized to fairly harmless components (which
  is known as BIOINACTIVATION)

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Metabolic Pathway for the Bioactivation of
Acetaminophen
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Bioactivation Bioinactivation
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Bioactivation

• Another example is the hydrolysis reaction we
  discussed earlier where benzene is hydrolized to
  phenol.
• During this conversion, a highly reactive
  intermediate, called an EPOXIDE, is formed.
• This epoxide is shortlived, but can interact with
  nucleophilic groups such as proteins and DNA

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Hydroxylation of Benzene
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Metabolism of Benzene in the Liver
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Summary of Phase I Reactions

• Organisms are able to change the biological
  activity of xenobiotics by enzymatic conversion
  of reactions
• Such reactions usually make these substances more
  polar and thus more easily excreted from the body

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Summary of Phase I Reactions

• Generally, the first step in the
  biotransformation process is the addition of a
  polar handle by a Phase I reaction.
• The reactions often take place under the control
  of enzymes from the MFO system.
• This system is a collection of enzymes, which can
  catalyze the oxidation of a great many substances.

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Summary of Phase I Reactions

• The MFO system primarily oxidizes lipophilic
  compounds and is responsible for the introduction
  of a polar group into the substrate molecule.
• Molecular oxygen is activated by the P-450 MFO
  and one atom is incorporated into the substrate
  and the other is reduced to water.

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Summary of Phase I Reactions

• Biotransformation of xenobiotics leads to changes
  in their biological activity.
• Toxicity is often times reduced (BIOINACTIVATION)
  but a large number of BIOACTIVATION reactions are
  known to take place, especially among oxidative
  reactions.

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Summary of Phase I Reactions

• The introduction of a polar group to a xenobiotic
  may give the compound a sufficiently hydrophilic
  character for rapid excretion
• Most of the time, this is NOT the case and the
  xenobiotic must undergo Phase II reactions in
  order to be eliminated

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Biotransformation Phase II
POLAR
HYDROPHILIC
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Phase II Conjugation Reactions

• A Conjugate is simply the chemical reaction
  that results in the union of two compounds
• Most conjugates result in a more soluble (water)
  compound
• This is one of the bodys mechanisms for
  improving the excretion of a toxicant

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Phase II Reactions

• In these reactions, the conjugate is generally an
  endogenous hydrophilic compound.
• The result is a substance with sufficient
  hydrophilic quality (character) to allow rapid
  excretion.
• The conjugation reactions can occur with a
  variety of substances

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Phase II Reactions

• These metabolic intermediates are conjugated with
  the polar group which is often added or unmasked
  by the Phase I reactions.
• Again, benzene serves as a good example

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Phase II Reactions

• The Phase II reactions are present in virtually
  every organism (like the P-450 enzymes)
• The Phase II reactions are designed to
  BIOINACTIVATE xenobiotics however, there are a
  few notable exceptions where the more water
  soluble compound is more BIOACTIVE than its
  parent compound

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Phase II - Reactions

• Glucuronide Formation
• Sulfate Conjugation
• Methylation
• Acetylation
• Amino Acid Conjugation
• Glutathione Conjugation

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Phase II - Glutathione Conjugation

• The resulting conjugation yields mercapturic
  acids
• These reactions take place with a variety of
  substances, which often have a reactive group,
  such as a halogen atom.
• An example of G.C. is dichloronitrobenzene and
  bromocyclohexane with glutathione.
• These substances form mercapturic acids and are
  excreted

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Glutathione Conjugation
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Phase II Reactions

• The product of Phase II reactions are usually
  highly water-soluble and are therefore readily
  excreted by organisms.
• Most conjugates (phase II products) have a very
  low biological activity.
• Most phase II reactions are BIOINACTIVATION
  reactions or detoxification reactions.

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Summary of Phase II Reactions

• In phase II reactions, polar xenobiotics and
  xenobiotics which have undergone a phase I
  reaction are conjugated with endogenous
  substances in the hopes of making them more
  water-soluble and more easily excreted.

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Factors Affecting Metabolism

• The rate of metabolism is dependent upon
• Those factors that affect the metabolic processes
  directly, and
• Those factors that affect the transport of toxic
  substance to tissues where metabolism occurs

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Factors Affecting the Metabolic Process

• Age
• Species
• Sex
• Nutritional Status
• Disease
• Enzyme Induction or Inhibition
• Genetics

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Example

• Parathion is much more toxic to newborn and young
  infants because the P-450 enzymes are not present
  or very limited.
• Therefore, biotransformation and subsequent
  detoxification of parathion in these critters
  does not occur or not effectively.

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Example

• A balanced diet will provide the necessary
  protein, essential metals, minerals (i.e.
  calcium, copper, zinc) to assist normal cellular
  enzymatic activities associated with
  biotransformation.
• Such diets result in a decrease in the bodys
  ability to carry out metabolic reactions used in
  detoxification.

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Example

• Liver Cirrhosis often associated with excessive
  alcohol drinking, causes liver cells
  (hepatocytes) to die and they are replaced with
  connective tissue (does not have the same
  function as the hepatocyte). Therefore, the
  liver has less ability to detoxify.

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Factors Affecting Transport and Absporption

• As we know, toxic substances must be transported
  to the appropriate tissue in order for metabolism
  to occur.
• In general, organic substances because of their
  high lipid solubility are easily absorbed and
  undergo metabolism
• Lipid-insoluble substances with large molecular
  weights will NOT be delivered as rapidly to
  active metabolic sites (i.e. liver)

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Other Factors

• Perfusion rate higher perfusion rates allow
  quicker metabolism
• Binding (particularly protein binding as with the
  blood) covalent versus non-covalent
• Size larger (i.e. protein bound molecules)
  slows removal