Current and Emerging Paradigms in Environmental Toxicology

1. Current and Emerging Paradigms in Environmental Toxicology Lecture 2

2. Understanding the three basic functions in environmental toxicology • Interaction of toxicant (xenobiotic) with the environment • Determines amount (dose) of toxicant available to living organisms • Time component • Interaction of toxicant with site of action • Usually receptor on/in a cell • Receptor often a cell protein • Interaction of toxicant at the molecular level, leading to all higher level ecological effects • Line from molecular to ecological effects is poorly understood Note: see Figure 2.1 Note: f(letter) = function of process indicated by letter

3. Classification of toxicological effects • Chemical/physical-chemical characteristics • Bioaccumulation/biotransformation/biodegradation • Site of action • Biochemical monitoring • Physiological and behavioral effects • Population parameters • Community parameters • Ecosystem effects

4. Chemical/Physical-Chemical Characteristics • Interactions of xenobiotic compound with biological compounds determines toxicity • The degree of effect that is due to the physico-chemical characteristics of the compound is called the Structure-Activity Relationship (QSAR). • Has potential for allowing prediction of toxic effects based only on structure of xenobiotic. • Could save lots of time, money, effort while allowing a greater degree of protection

5. Bioaccumulation/Biotransformation/Biodegradation • Many things can happen to chemical between release to environment and arrival at the biological site of action • Bioaccumulation – increase in concentration of chemical in tissue relative to concentration in environment • More likely in lipid solubule/lipophilics • Biotransformation – chemical change in toxicant caused by biological tissue • May decrease [usually] or increase toxicity • Biodegradation– breakdown of a xenobiotic into a simpler chemical form • Could be the result of biotransformation • All above processes dependent on site specific conditions so direction and degree hard to predict

6. Receptor (site) and Mode of Action • Active site extremely important on determining mode of action • May cause very specific or very general effect • Active site may be on specific nucleic acids, enzymes, cell membranes or non-specific • Covered much more in Xenobiotic Metabolism part of course

7. Biochemical/Molecular Effects • Broad range of possible effects • Could be general (ex. general effect on DNA) or specific (ex. effect of specific portion of DNA) • Includes effects on chromosomes, enzyme systems, immunological system, etc.

8. Physiological and Behavioral • Biochemical/molecular effects manifested at higher organismal level • Classical means by which population health is assessed • Major drawback  extrapolation from individual effect to population and ultimately ecosystem effect • Can include pathology, oncogenesis, reproduction, mortality, osmo- and ionoregulation, behaviors (fish respiration, cough response), temperature preference, predator avoidance or prey detection

9. Population effects • Suitable for both field and laboratory evaluations • Well-developed protocols • Population size, density, age-structure, cycling, growth rate, genetics

10. Community effects • Evaluation of community structure extensively used in field studies • Many indices developed to quantify species composition • Most widely used  species diversity (biodiversity) • Most dramatic impact that can be observed • Decrease in species diversity usually = impact but sometimes reverse is true • Diversity can be misleading  can have same diversity after exposure but be the result of a completely different set of species Note: most of what we call communities are really assemblages because we do not understand most of the interactions among populations in a “community”

11. Ecosystem effects • Most ecosystem-level changes indicate a serious problem • Variables measured can include metabolism (energy capture, flow, loss), net productivity (gross productivity – respiration), biomass accumulation, rate of detrital breakdown, landscape alteration, species distribution, chemistry • Evaluation of effects must be system-specific

14. Ecosystem effects • Most environmental regulations aimed at protecting ecosystem structure and function but these are rarely measured when determining compliance

15. Complexity theory • Alternative to previous “classical) approach to environmental toxicology • Based on differences between organisms and ecosystems • Organisms – genetic structure is retained in all cells (redundant) and designed to maintain homeostasis --> most impacts not passed on to future generations • Ecosystems – no central repository of information like genome • History of past events written into ecosystem structure and function, nature of interactions  high complexity and non-linear relationships

16. Properties of Complex, Non-linear Structures • Exhibit both deterministic and stochastic properties • Causes and effects of events experienced by systems are not proportional • Different parts are linked and affect one another synergistically • Can undergo irreversible processes (because no system “memory”) • Dynamic (not in equilibrium) • Note: above properties may be useful in extrapolating toxicity test results on to highly variable ecosystems

17. Spatial and Temporal Scales • All previous functions vary over spatial and temporal scales • May appear disconnected but never are • Usually smaller spatial scale operates at a shorter temporal scale because of inherent differences when considering atom-level effects to ecosystem level effects (see Figure 2.4) • Type of environmental problem will be a function of spatial and temporal scale (see Figure 2.5)

18. Wednesday – Intro to toxicity testing