Toxicokinetics and Toxicodynamics

Question 1

describe biotransformation (3)

Answer 1

1. enzyme-driven reactions acting on toxicants in phase I and phase II reactions
2. mostly in the liver but also in the lungs and kidneys (less so but some also in skin, intestine, testes, and placenta)
3. outcomes are detoxification/inactivation, bioactivation/toxification/lethal synthesis, or facilitating excretion by making more hydrophilic or larger

Question 2

describe the BASICS of phase I reactions (3)

Answer 2

1. simple degradation reactions that are ATP INdependent
2. add or expose and functional (ionizable) group that acts as a tag or a handle for the body via oxidation, reduction, or hydrolysis
3. metabolite excreted or proceeds to phase II

Question 3

describe cytochrome P450 enzymes (5)

Answer 3

1. cytochrome P450 (CytP450) are mixed function oxidases and the most important oxidases in phase I reactions!
2. they have multiple isoforms (isozymes) that act on a broad range of substrates
3. are located on the ER membrane (NOT IN THE CYTOSOL)
4. also called microsomal (MFOs)
5. have variable expression as they can either be induced or suppressed

Question 4

describe the oxidation reaction steps of phase I in relation to P450 enzymes (5)

Answer 4

1. oxidized CytP450(Fe3+) binds toxicant
2. enzyme-toxicant complex is reduced via NADPH and reductase
3. reduced complex binds O2
4. second reduction takes place, also via NADPH and reductase
5. the final complex splits into 3 products: functionalized toxicant metabolite, oxidized CytP450 (was regenerated), and H2O

Question 5

describe the BASICS of phase II reactions

Answer 5

1. conjugation reactions that are ATP dependent (unlike phase I) where polar groups from donor compounds are added to functional groups from phase I
2. the enzymes that accomplish these reactions are specific transferases that require specific cofactors that act as donors and are primarily cytosolic
3. this phase promotes excretion!! by making toxicants larger, charged, and water-soluble

Question 6

describe glucuronidation as it relates to phase II reactions

Answer 6

1. sugar conjugation: donor is uridine diphosphate glucuronic acid (UDPGA) and the enzyme is glucuronsyl transferase (DEFICIENT IN CATS); enzyme is bound to ER membrane
2. this reaction pathway is low affinity and high capacity, meaning it can handle a lot and works for many different substrates
3. this pathway is inducible (like CytP450)

Question 7

describe sulfation/sulfonation as it relates to phase II reactions

Answer 7

1. similar substrates as glucuronidation (VERY IMPORTANT REACTION IN CATS)
2. enzymes are sulfotransferases
3. donors/cofactors are PAPS, ATP, and inorganic sulfates to conjugate sulfonate to toxicant
4. this reaction is high affinity but low capacity and not readily inducible, meaning it is easy to overwhelm
5. acetaminophen can take either the glucuronidation or the sulfonation route through phase II, but cats cannot do glucuronidation, which is why they are so susceptible to acetaminophen toxicity, because the sulfonation pathway is lower capacity and higher affinity than glucuronidation

Question 8

describe glutathione conjugation as it relates to phase II reactions

Answer 8

1. glutathione is the most important intracellular antioxidant!!
2. uses electrophilic substrates like reactive intermediates and epoxides
3. donors are glutathione (a tripeptide) and a thiol group
4. enzymes are GSTs, found in the cytosol or on the ER, but are not “required” for this reaction

Question 9

describe the reaction steps of glutathione phase II conjugation (3)

Answer 9

1. formation of glutathione-toxicant (metabolite) conjugate via thiol group and GST
2. recycling of glutathione peptides via peptidases
3. formation of final conjugate via acetyltransferase and mercapturic acid metabolite

Question 10

what are the 3 steps of the mechanism of toxicity?

Answer 10

1. delivery from site of exposure to target
2. reaction of the ultimate toxicant with the target molecule(s)
3. cellular dysfunction and resultant toxicities

Question 11

describe the 2 general mechanisms of toxicant-target interactions

Answer 11

1. direct interaction with target:
   1a. non-covalent, reversible
   1b. covalent, irreversible
   1c. hydrogen abstraction
   1d. electron transfer
   1e. enzymatic
2. indirect: disrupt intracellular environment (like disrupting pH gradient to mess up everything else)

Question 12

what are 3 common toxicant targets?

Answer 12

1. proteins: their enzymes, receptors, or transporters
2. lipids: cellular or in subcellular membranes
3. nucleic acids

Question 13

if the role of the target molecule is cell regulation (signaling) what are the 2 types of effects a toxicant can have?

Answer 13

1. dysregulation of gene expression: inappropriate cell division, apoptosis, or protein synthesis
2. dysregulation of ongoing cell function: example is inappropriate neuromuscular activity

Question 14

if the role of a target molecule is cell maintenance, what are the 2 types of effects a toxicant can have?

Answer 14

1. impaired internal maintenance, leading to cell injury/death
2. impaired external maintenance: example is impaired hemostasis leads to bleeding

Question 15

what are the 4 toxic mechanism categories?

Answer 15

1. specific protein interactions
2. non-selective opportunistic reactions
3. toxican is electrophilic or biotransformation/bioactivation
4. oxygen reduction

Question 16

describe the specific protein interaction mechanisms of toxicity (3)

Answer 16

1. enzyme systems: toxicant is an enzyme or a substrate or causes enzyme induction or suppression
2. receptor systems: toxicants can be agonists or antagonists
3. transport systems: toxicants can hitch a ride on hemoglobin or hijack cellular pumps

Question 17

describe the non-selective, opportunistic mechanisms of toxicity

Answer 17

just act on whatever proteins, lipids, or nucleic acids are nearby

Question 18

describe the electrophilic or biotransformation mechanism of action of toxicity

Answer 18

can form an electrophilic metabolite, a free radical (small molecule or atom with unpaired electrons), or a reactive intermediate

Question 19

describe the oxidative damage mechanisms of toxicity (2)

Answer 19

1. complete oxygen reduction: low risk of ROS formation
2. partial oxygen reduction: forms ROS!!

Question 20

list and describe the 3 toxic ROSs

Answer 20

1. superoxide anion (O2-): attack proteins
2. hydrogen peroxide (H2O2): crosses membranes and attacks proteins, free amino acids, and nucleic acids
3. hydroxyl radical (OH): attacks protein thiols, lipids, nucleic acids, and carbohydrates

Question 21

what are the 2 types of protective mechanism that the body has against toxicants?

Answer 21

1. enzymatic
2. non-enzymatic