Biology 445/545 Biology of Toxins

Biology 445/545 – Biology of Toxins

Study Questions

Grab your dictionary and look up the definition of "toxic". Is oxygen toxic? Is water? Are there any substances that aren't toxic? What determines whether something is toxic?
Look up the definition of xenobiotic? How is the word relevant to toxicology?
Give a good definition of a toxin. Why do organisms produce toxins?
What is a venom?

Cell Biology Review (you should be able to answer these from your cell biology background)

What is an enzyme? Why are enzymes important to organisms?
Describe the basic model of an enzyme-catalyzed reaction. What is the active site? What is the significance of the “lock-&-key” hypothesis?
What do biologists mean by the term “gene expression”? What is involved in the expression of genes? What is the consequence of gene expression?
Give a brief description of the events involved transcription and translation. What enzyme plays a key role it the process? What is the function of ribosomes?
Defend or refute the statement “What a cell is and what it does is dependent on the proteins it’s synthesizing.”.
Draw a cartoon rendition of a plasma membrane. Why is the current model of plasma membrane structure referred to as the Fluid Mosaic model?
Which type of molecule – polar or non-polar – moves through a membrane more readily? Why is this?
What functions do plasma membrane proteins perform?
Compare and contrast a cell surface receptor with an enzyme. Why are these receptors important?
Using an appropriate diagram (that you produce), discuss signal transduction in cells.
What is the “cytoskeleton”, and why is it important to cells? What can happen if the cytoskeleton is disrupted?
Diagram and describe endocytosis. What role do cell surface receptors play in it?

Principles of Toxicology

What is water intoxication? How does it develop, and why is it dangerous? What is cerebral edema? Discuss Ecstasy in the context of water intoxication and brain herniation.
What was the take-home message from our discussion of water and oxygen toxicity?

***End 1st lecture***

What do each of the letters in the acronym ADAME stand for?
With respect to toxins, what is meant by the term “absorption”? By the term “distribution”?
Describe the routes of absorption of a toxin. From the perspective of an organism producing a toxin, discuss the relative strengths/drawbacks of each route. (Hint: you’ll want to include distribution pathways in your discussion.)
What organ system generally plays the key role in distribution of toxins?
Present a block diagram of a human’s body fluid compartments, along with some rough statement about their respective relative volumes (you can use terms like “biggest”, “smallest”, “lots”, “little”, etc.).
Discuss the properties of the barriers that separate the various body fluid compartments, and speculate about the implications of the barrier’s properties for organisms ‘trying’ to develop toxins that target specific tissues.
What is the primary function of your kidney, and which body fluid compartment does it work on?
For a toxicologist, what is meant by the term “action”? By the term “response”? How do the two terms relate to one another? What are some examples of responses used by toxicologists in their study of toxins? Give an example of a plant toxin that produces a subtle, but potentially serious, response.
What are the three factors that determine the response of an organism to a particular toxin?
What is meant by the term “dose”? Contrast administered dose with effective dose.
Why do toxicologists use mass-specific doses of a toxin (i.e., mg toxin/kg body mass) when studying the effects of a toxin?
What are the five factors that determine the effective dose of a particular toxin?
Discuss exposure pathways and their impact on the effective dose of a toxin. Which pathways tend to produce the highest effective doses, and why? Which pathways tend to produce the lowest effective doses, and why?
What is meant by the term pharmacokinetics?
Construct a diagram that depicts the basic model used by toxicologists to understand pharmacokinetics of xenobiotics. Use this model to discuss the pharmacokinetics of a xenobiotic that is administered as

a single IV bolus
What is meant by the term “half-life”? (Google this.)
a single GI tract bolus
What other administration route might be modeled in this way?
repeated GI tract doses

Be sure to include appropriate graphs in your discussion!

What is meant by the terms Area Under the Curve (AUC), and what is its significance to a budding toxicologist such as yourself?
Why are lipophilic toxins often more toxic than more polar, hydrophilic toxins?
What is a potential problem for an organism that depends on a toxin that attack intracellular targets?

***End 2nd lecture***

In toxicology, what is meant by the term “response”?
Graph the following data and estimate the LD50 for the two toxins:

% of Individuals Showing Response
Dose
( mg/kg ) /

Toxin 1

Toxin 2

0 / 0 / 0
10 / 0 / 0
20 / 2 / 0
30 / 18 / 3
40 / 56 / 96
50 / 83 / 100
60 / 94 / 100
70 / 97 / 100
80 / 99 / 100

In the preceding question, what would you call the NOAEL for Toxin 1? For Toxin 2? What is he linear range for each of the two toxins?
With regard to the toxins used in the preceding question, which toxin would be considered to be more toxic? Justify your answer.
What’s the difference between LD50 and ED50?
Suppose you’re a toxicologist working for a pharmaceutical firm, and have been charged with determining whether a newly-developed chemical would be a good candidate for use in the treatment of brain cancer. What data would you want in order to make this assessment, and what experiment(s) would you perform to gather those data? Now, using an appropriate graph to facilitate your discussion, describe how you would come to a conclusion about the chemical’s suitability as a chemotherapeutic agent.
Use a graph to help you explain what is meant by the toxicological term Margin of Safety.
In terms of therapeutic effect and adverse effects, what must one take into account when deciding whether a particular drug may be approved for medical use?
Describe an experiment whose results indicate the presence of intraspecific ( = within a species = among the individuals of a species) variation in susceptibility to a toxin. Draw a graph that illustrates that variation. Can you suggest any explanations for this variation in susceptibility?
Why do the elderly and very young tend to be more susceptible to many toxins?
What do biologists mean by the terms ontogeny or ontogenetic? Senescent?
Can you suggest any reason(s) why physiological condition can affect susceptibility to toxins?
What do biologist mean when they talk about genetic variation in susceptibility to a toxin?
Use the graph you drew as part of your response to question 40 as part of an explanation for how resistance to an antibiotic can evolve. Would your explanation apply to other systems of toxins and the organisms targeted by them?
Citing experimental evidence when you can, provide as detailed an answer as you can to each of the following questions:

Why do bacteria and fungi produce antibiotics?
Why don’t all bacteria and fungi produce antibiotics?
Why aren’t all bacteria resistant to antibiotics?

I described an experiment whose results suggested that it was costly for bacteria to produce and maintain antibiotic resistance mechanisms. Besides the fact that antibiotic resistance is costly, what other inference can you draw from the results of that experiment? (Hint: think cost:benefit ratio.)
What did the results of the experiment described in the preceding question indicate about the importance of the environment in the ecology and evolution of organisms?
What are the two most prevalent mechanisms of antibiotic action and the three most frequently observed mechanisms by which bacteria are able to resist the effects of antibiotics. Do you think these resistance mechanisms are genetically based? Justify your answer.
What is meant by the terms bacteriostatic and bactericidal?
Is bacterial resistance to antibiotic action observed outside the hospital or agricultural setting, i.e., in nature? If so, what does that suggest?
Why do meat producers give antibiotic-laced feed to cattle, sheep, and pigs? Can you suggest any explanations for the observed effect? (note: I didn’t suggest any in lecture)
Do you think your answers to question 51 could be generalized to other toxin-producing organisms? If so, how?
Give me two bits of evidence that indicates that toxins are costly for the organisms than produce them. Give me four examples of possible ecological/evolutionary costs incurred by an organism that produces a toxin.
What is meant by the term inducible? What can we infer when we learn that a system, such as a toxin, is inducible?
The take-home message from our discussion of cost-benefit ratios is…?

***End 3rd lecture***

What is meant by the term teratogenic? How did Thalidomide produce its teratogenic effect?
Would you expect there would be a ‘barrier’ between a female mammal’s blood and that of her developing fetus? Justify your answer. Would you expect this barrier would be highly permeable, or highly impermeable? Justify your answer.
What take-home message(s) do you draw from the comparative data I gave you about the effects of ethanol, thalidomide, and gasoline in humans and other mammals?
What does the term metabolism mean with respect to toxins?
Define and discuss biotransformation.
What is the primary objective of the body’s metabolic ‘attack’ on toxins, and how is this objective achieved (3 processes)?
In terms of excretion, how does oxidizing a xenobiotic help?
What beneficial effect does decreasing lipid solubility have on a toxin? How about increasing its size?
Does biotransformation necessarily render a xenobiotic non-toxic?
What is the outcome of Phase I biotransformation? Where in the body does Phase I biotransformation occur?
What is cytochrome P450, what does it do, and why is it important to toxicology?
Which phase of biotransformation is P450 involved in?
What is the phylogenetic distribution of P450?
Describe the evolution of P450s.
What is gene fusion?
What is gene duplication, and how would it contribute to rapid evolutionary change in a protein?
What environmental ‘events’ seem to have been important in evolution and diversification of P450s?
What evidence do we have that P450s are important in plants? What do plants use them for?
Discuss regulation of P450 levels in cells. What is upregulation? Downregulation? How do those processes relate to gene expression?
Give three implications of the fact that P450s are inducible.
Which P450 family is the most diverse in humans? What are some compounds that it biotransforms?
Discuss a downside to CYP2E1 function.
Which P450 family is most abundant in humans? In what two organs is it especially abundant?
In terms of drug metabolism, what is the significance of the CYP3A P450 subfamily? Give some examples of drugs that are biotransformed by it.
What do we mean when we say that P450s are non-specific? From a toxicology standpoint, why is this lack of specificity important?
What are some processes that are regulated by histamine, and what is the effect of histamine on each? How is histamine able to accomplish this regulation?
What is meant by the term agonist? Antagonist?
What is an antihistamine? What do we use them for?
What is a commonly used antihistamine, and what is a downside to its use?
How do first-generation antihistamines produce drowsiness?

***End 4th lecture***

How do second-generation antihistamines avoid causing drowsiness?
Describe in detail the serious problem associated with use of some second-generation antihistamines such as Allegra. In this context, what is the significance of compounds such as tricyclic antidepressants, statins and grapefruit juice?
What do we mean by the term drug interaction? Using suitable diagrams, describe & discuss the three main ways they come about.
What are the consequences for AUC for the drugs involved in each of the three types of interaction you just described?
How do binding affinities relate to drug interactions?
What is the significance for drug interactions of the relative lack of substrate specificity of P450s? Suggest some hypotheses that might account for that lack of specificity.
What is meant by the term genetic polymorphism, and what is its significance for toxicology?
What happens during Phase II of biotransformation, and what is the outcome? Where does it occur? What are some of the functional groups that play a role in Phase II biotransformation?
Suggest reasons for why biotransformation involves two distinct phases.
What is meant by the term bioactivation? Using acetaminophen as an example, discuss bioactivation and some of its consequences. What is meant by arylation?
We said that acetaminophen overdoses now account for the majority of toxic liver failure cases in the United States. Are these overdoses intentional (suicide, homicide) or inadvertent? If you believe the latter, how is it possible for inadvertent acetaminophen overdoses to occur?
What is the name of the compound used in emergency treatment of acetaminophen overdoses, and how does it work?
Would acetaminophen overdose be more, or less, of a problem for heavy consumers of alcohol? Provide a detailed justification for your answer. What evidence is there that producers of OTC medicines containing acetaminophen believe that alcohol consumption is associated with an increased risk of acetaminophen-induced hepatotoxicity?

***End 5th lecture***

Discuss excretion of toxins. By what avenue are most toxins excreted? What are the benefit(s) and drawback(s) to this avenue?
Describe the excretion of toxins in the bile. Give me one benefit and one drawback to excreting toxins via the bile. Discuss how the green color of bile is produced. Why can rh negative babies born to rh positive mothers suffer from jaundice, and what is the term applied to the condition that produces the jaundice?
Tell me as much as you can about milk sickness. How does it occur? What are the “Trembles”, and how do they relate to milk sickness? Who’s most at risk for developing it? Who’s least at risk? What role does P450 play in milk sickness?
Why is White Snakeroot more toxic in the western and northern parts of its range? What does that suggest to you? (hint: think cost:benefit from the plant’s perspective)
What are cytotoxins?
Give the source(s) and mechanism of action for each of the following:
Phospholipases, hyaluronidases
Hemolysins and leukocidins
Sphingomyelinase D
Sialidase, neuraminidase
Cyanide, cyanogenic compounds
Rotenone
Hypoglycin A
Taxol
How does the ability to produce hyaluronidase benefit organisms such as venomous snakes or bacteria?
What is meant by the term “suicide inhibitor”?
Why do infections by certain bacteria, such as Group A -hemolyticStreptococcus pyrogenes, increase our risk of developing autoimmune disease? Hint: you should include a brief discussion of how our immune system learns to discriminate ‘self’ from ‘non-self’.
What is the underlying cause of the high-mortality outbreaks of hypoglycin A poisoning in its natural habitat, west Africa? I.e., why to the native peoples eat what they know is a deadly toxic fruit?
What is the medical use of taxol, and why is it effective in that application? What are some of its side-effects, and why do they develop?

***End 6th lecture***

What are the commonalities in the way Shigella and certain strains of E. coli attack cells lining the small intestine? What is different about Shigella once it gains entry to the cell’s cytoplasm?
Compare and contrast the way Salmonella typhi and Shigella dysenteriae attack their target cells. What are their primary target cells? Why do S. typhi and S. dysenteriae ‘want’ to remain inside their target cells?
What is the organism that produces phalloidin and -amanitin, and what is the mechanism ‘used’ by each toxin? What is unusual about the composition and structure of the two toxins? How is P450 involved?
Describe the A-B mechanism of toxin attack on a target cell. Why is it necessary?
Why was diphtheria known as the “White Strangler” and “Strangling Angel of Children”? (hint: as part of your answer to this question, you should tell me how diphtheria toxin works.)
How does Pseudomonas aeruginosa toxin work?
What is a biofilm, and how do bacteria benefit from producing them?
Do ricin and abrin use an AB mechanism?
How do ricin and abrin cause toxic effects in cells?
Chickens are approximately 75x less susceptible to ricin than are mice. What does that statement mean? Suggest some biochemical or physiological reasons for this. Can you think of any ecological or evolutionary reasons for the difference in susceptibility?
What is phospholipase A2, and what function does it normally perform? How have venomous snakes adapted the basic molecular structure to make it work for them?
What is the target of each of the following types of snake PLA2: cytolytic, myotoxic, cardiotoxic, hemotoxic?
Give a very general summary of the Russell’s Viper story, and tell me what the data suggest to biologists.

***End 7th lecture***

What is the Red Queen Hypothesis, and how is it relevant to the biology of toxins?
With respect to the California squirrel-rattlesnake story, suggest possible explanations for how some squirrel populations are able to be resistant to the snake’s venom.
Below is a map showing the NM distribution of a small rodent (the black area) and a venomous snake that preys on it (the red stippled area). The dark red area shows the zone of overlap of the two species’ distributions, which is the area where the snake species actually preys on the rodent.

Design a research project that would test whether coevolution of the snake and prey has occurred – tell me what kind of data you’d collect, where you’d sample, what results you’d expect to get from your experiments, etc.

What is MAPK, and why is it important to cells?
What are oncogenes?
Using a suitable diagram, present a detailed discussion of the G-protein signaling pathway.
Why do we say that the G-protein signaling pathway functions like an ON-OFF switch? What is a benefit of having a signaling pathway that works in this fashion?
What is adenyl cyclase, and what does it do?
How does pertussis toxin work? What is one of the symptoms of a person who’s infected by the bacterium that produces pertussis toxin? Why would the bacterium want its victim to exhibit this symptom?
Discuss the three components of anthrax toxin, tell me what each does, and tell me why the anthrax bacterium would want the three components to do what they do
Present a simplified diagram of your gastrointestinal ( = digestive) system, and briefly describe the function of each part.
Take copious notes during an episode of vomiting (yours, or someone else’s), and assess whether the flow chart I presented in lecture accurately described the sequence of events you observed.
Complete the following table: