Toxicology 300 Midterm 1

What are xenobiotics?

Any toxic agent/foreign chemical in the body

What is the boomerang paradigm?

The concept that chemicals put out into the environment come back around and effect us

• Replaced dilution paradigm

What was the incident at Minimata Bay, Japan?

Waste water concentrated with methylmercury released

• Caused minimata disease/cat dancing disease

Who identified the central concept of toxicology (dose response relationship): “Dose defines the poison

Paracelsus

What is LD50?

Dosage (mg/kg body weight) that cases death in 50% of exposed animals

What is toxicokoinectics?

What the body does to the xenobiotic

• Processes include absorption, distribution, metabolism, and excretion (ADME)

• Determines the dose of a xenobiotic

What is toxicodynamics?

What the xenobiotic does to the body

• Effect of the xenobiotic on cellular and physiological processes

• determines to xenobiotic

What are the membranes of animal cells?

Phospholipid bilayers

How does the phospholipid bilayer impact xenobiotics?

The lipophilicity of a xenobiotic is the most important factor, allowing it to diffuse across cell membranes

• Lipophilic can pass

• Lipophobic cannot

What are the modes of movement of a chemical across cell membranes?

1) Passive transport/simple diffusion

• Chemicals follow its conc gradient

• Transcellular

2) Filtration/bulk flow

• Passive movement through cell junctions due to pressure gradient

• Paracellular

3) Facilitated diffusion

• Passive transport following conc gradient but requires a transporter to assist movement

4) Active transport

• Movement going against conc gradient, requires ATP

What is the most common absorption pathway for xenobiotics?

Passive transport

What determines if xenobiotics can move across lipid membrane using passive transport?

• Octonol:water partition coefficient (Kow)

• High log Kow (>4) indicates high lipophilicity —> potential for accumulation and toxicity

  • Ratio of how much of an analyte in water and octonol are in each phase (conc in oct/conc in water)

How do weak organic acids and bases transport through passive diffusion?

Chemicals exist in both ionized and nonionized forms in solution - preparation depends on pka of xenobiotic and pH of solution

• Only the non ionized form can passively diffuse across cell membrane

  • Protonated acid = nonionized (HA), non-protonted acid = ionized (A-)

  • Protonated base = ionized (HB+), non-protonated base = nonionized (B)

• In acidic pH, more acid is in nonionized form (more transport)

• In basic pH, more base is in nonionized from (more transport)

How do you determine the ratio of nonionized vs ionized forms of xenobiotics?

Henderson-Hasselbalch equation

• Log[Protonated/nonprotonated] = pka - pH

  • Log[HA/A-] or [BH+/B]

What is the important factor of xenobiotics for filtration (bulk flow)?

• Size of xenobiotic

What are the major families of transporters for facilitated diffusion?

• Organic anion transporters (OATs)

• organic cation transporters (OCTs)

What are the families of active transporters?

ATP-binding cassette (ABC proteins) - all transporters

Include:

• Multi-drug resistance proteins (MDRs and MRPs)

• Breast cancer resistance protein (BCRP)

What are some of the major routes of xenobiotic exposure?

• GI tract absorption (ingestion)

  • Most important route

  • Most absorption in small intestine because of high SA

• Inhalation (lung) absorption

  • Gasses, vapours, particulates

• Dermal (skin) absorption

  • Damaged skin

  • Highly lipophilic chemical

  • Important route for amphibians

• Clinical (injections)

What are “compartments”

The different locations of the body a xenobiotic is distributed to

• Initial absorption to “central compartment” (systemic circulation) is followed by distribution to “peripheral compartments” (other organs and tissues, specifically highly perfused tissues)

What occurs after absorption?

Immediate rapid distribution of xenobiotic, especially to well perfused tissues

What are the four main factors that influence distribution?

1. Blood flow (perfusion)

• Depends on cardiac output - volume of blood pumped x heart rate

2. Physiochemical properties

• Lipid solubility, pka (ionized vs non ionized), size, ect

3. Binding of xenobiotic to plasma proteins (albumen) and cellular binding proteins

4. Barriers

• Ex:

  • Blood brain barrier (tightly bound endothelial cells and active transporters pump out xenobiotics)

  • Placental barrier (Must assume an xenobiotic entering maternal circulation is capable of crossing placenta)

What is the most abundant plasma protein for xenobiotic binding?

Albumen

• Has affinity to bind xenobiotics

Why is plasma protein binding important for xenobiotic distribution?

Creates a free:bound equilibrium of xenobiotic in blood due to reversible binding (weak chemical bonds)

• Only free xenobiotics can diffuse out of blood into tissues

  • As free xenobiotics are excreted, more is released from plasma proteins to maintain equilibria

What happens to the free:bound equilibrium during a high dose of xenobiotics?

Not enough protein binding sites (“seats”) causes more free ratio of xenobiotic

What are some examples of cellular (tissue) protein binding?

• Liver and kidney

  • Have high binding affinity for some xenobiotics

• Adipose tissue

  • Important storage depot for highly lipophilic xenobiotics (high log Low)

• Bone

  • Binds certain xenobiotics like heavy metals (ex: lead)

What is Volume of Distribution (VD)?

The apparent fluid volume in which a xenobiotic appear to be dissolved (how widely a xenobiotic is distributed throughout the body)

• A proportionality constant

• VD = total dose (mg) / plasma xenobiotic conc (mg/L)

  • High VD means extensive distribution and high affinity for tissues

  • Low VD means xenobiotic is restricted mainly to blood plasma, due to Hugh plasma protein binding

• Can be influenced by pka - ioniozed vs nonionized

What is biotransformation?

The enzyme catalyzed conversion of one xenobiotic to another

• Transforms xenobiotic to a more water-soluble metabolites - essential to terminating biological activity and eliminate them from body

What is so important about biotransformation?

Most important determinant of the duration of action of xenobiotics in the body

What is detoxification?

Biotransformation results in a less toxic metabolite

• Most common

What is bioactivation?

Biotransformation that results in a more toxic metabolite

What are the two phases of biotransformation?

• Phase 1: Biotransformation enzymes modify the xenobiotic molecule mainly by oxidation - addition of an -OH (hydroxyl)

• Phase 2: Synthetic reactions that conjugate (combine) the xenobiotic with a highly polar endogenous compound (very water soluble) in the cell (e.g. carbohydrate, sulphate, or acetate)

What is the most important site for biotransformation?

the liver

What is the major Phase I oxidative enzyme?

Cytochrome P450-dependent monooxygenases (CYPs)

• Located in smooth endoplasmic reticulum

• Add a polar functional group (-OH) to lipophilic xenobiotic

  • Catalyze insertion of an oxygen atom

• Xenobiotix (R-H) + O2 + NADPH —> Metabolite (R-OH) + H2O + NADP+

What are some major CYP enzyme families?

• CYP1A2

• CYP2E1

• CYP3A4

What is a broad substrate specificity?

When one enzyme can biotransform many xenobiotics

What are overlapping substrate specificities?

When one xenobiotic can be biotransformed by several enzyme

What type of biotransformation can CYP cause?

Detoxify and bioactivate

What is First Pass Xenobitoic Biotransformation?

• When the hepatic portal venous system (portal vein) delivers all substances absorbed from the GI tract to the liver - has extensive biotransformation capacity - before it reaches the systemic circulation

  • Can result in complete inactivation of certain drugs

    • Certain drugs need to be administered alternatively (i.e intravenously)

What is oral bioavailability?

The fraction of an orally administered drug that reaches the systemic circulation in an unchanged form

• Bioavailability = AUC(oral) '/ AUC(IV)

  • AUC = area under curve

  • AUC(IV) = 100% bioavailable