All authors are or have co-taught a general toxicology course at North Carolina State University and have insight into both the actual teaching process and the content of their areas of specialization. Thanks to all the authors and to the students and faculty of the Department of Environmental and Molecular Toxicology at North Carolina State University, and to Carolyn McNeill for much of the text editing.
INTRODUCTION
Introduction to Toxicology
- DEFINITION AND SCOPE, RELATIONSHIP TO OTHER SCIENCES, AND HISTORY
- Definition and Scope
- Relationship to Other Sciences
- A Brief History of Toxicology
- DOSE-RESPONSE RELATIONSHIPS
- SOURCES OF TOXIC COMPOUNDS
- Exposure Classes
- Use Classes
- MOVEMENT OF TOXICANTS IN THE ENVIRONMENT
In the United States, enforcement falls under such government agencies as the Environmental Protection Agency (EPA), the Food and Drug Administration (FDA), and the Occupational Safety and Health Administration (OSHA). Chemicals released into the environment rarely remain in the form or at the site of release.
Introduction to Biochemical
- INTRODUCTION
- CELL CULTURE TECHNIQUES
- Suspension Cell Culture
- Monolayer Cell Culture
- Indicators of Toxicity in Cultured Cells
- MOLECULAR TECHNIQUES
- Molecular Cloning
- cDNA and Genomic Libraries
- Northern and Southern Blot Analyses
- Polymerase Chain Reaction (PCR)
- Evaluation of Gene Expression, Regulation, and Function
- IMMUNOCHEMICAL TECHNIQUES
Some examples of the use of cultured cell lines in the study of toxicity effects are shown in Table 2.1. These primers are complementary to the sequence at each end of the DNA sequence to be amplified.
Toxicant Analysis and Quality Assurance Principles
- INTRODUCTION
- GENERAL POLICIES RELATED TO ANALYTICAL LABORATORIES Every analytical laboratory, governmental, private, or university, has a standard set of
- Standard Operating Procedures (SOPs)
- QA/QC Manuals
- Procedural Manuals
- Analytical Methods Files
- Laboratory Information Management System (LIMS)
- ANALYTICAL MEASUREMENT SYSTEM
- Analytical Instrument Calibration
- Quantitation Approaches and Techniques
- QUALITY ASSURANCE (QA) PROCEDURES
- QUALITY CONTROL (QC) PROCEDURES
- SUMMARY
Thus, it is necessary to have a system capable of measuring the compound of interest and to ensure the reliability of the data, the analytical process (instrument and analytical method) must be closely monitored. Audits should be conducted on a scheduled basis to verify that all aspects of the QA program are functioning adequately.
CLASSES OF TOXICANTS
Exposure Classes, Toxicants in Air, Water, Soil, Domestic and Occupational Settings
AIR POLLUTANTS .1 History
- Types of Air Pollutants
- Sources of Air Pollutants
- Examples of Air Pollutants
- Environmental Effects
One of the effects of sulfur oxide production is the formation of acid deposits, including acid rain. Most of the information on the effects of air pollution on humans comes from episodes of acute pollution, such as those in Donora and London.
WATER AND SOIL POLLUTANTS
- Sources of Water and Soil Pollutants
- Examples of Pollutants
Much of the acidity in rain can be neutralized by dissolving minerals in the soil, such as aluminum, calcium, magnesium, sodium and potassium, which are leached from the soil into surface water. This contamination resulted from irrigation of the soil with water containing cadmium released from industrial sources. Herbicides, due to the large amount used, are also a concern as potential toxic pollutants.
OCCUPATIONAL TOXICANTS
- Regulation of Exposure Levels
- Routes of Exposure
- Examples of Industrial Toxicants
Occupational exposure to chromium (Cr6+) causes dermatitis, ulcers on the palms and hands, perforation of the nasal septum (probably due to chromic acid), inflammation of the throat and liver, and bronchitis. Lead interferes with the biosynthesis of porphyrins and heme, and several screening tests for lead poisoning use this interaction by monitoring the inhibition of the enzyme δ-aminolevulinic acid dehydratase (ALAD) or the appearance of aminolevulinic acid (ALA) and coproporphorin (UCP) in the urine. Benzene was widely used in the rubber industry in the second half of the nineteenth century as a solvent for rubber latex.
Classes of Toxicants: Use Classes
INTRODUCTION
METALS .1 History
- Common Toxic Mechanisms and Sites of Action
- Lead
- Mercury
- Cadmium
- Chromium
- Arsenic
- Treatment of Metal Poisoning
Occupational exposure to metals in the form of metal dust makes the respiratory system a likely target. Mercury occurs in the environment in three main chemical forms: elemental mercury (Hg0), inorganic mercury (Hg+) and mercury (Hg2+) salts, and organic methylmercury (CH3Hg) and dimethylmercury (CH3HgCH3) compounds. The mercury was converted to the readily absorbed methylmercury by bacteria in the aquatic sediments.
AGRICULTURAL CHEMICALS (PESTICIDES) .1 Introduction
- Definitions and Terms
- Organochlorine Insecticides
- Organophosphorus Insecticides
- Carbamate Insecticides
- Botanical Insecticides
- Pyrethroid Insecticides
- New Insecticide Classes
- Herbicides
- Fungicides
- Rodenticides
- Fumigants
- Conclusions
One of the most widely used carbamate insecticides is carbaryl (1- napthyl methylcarbamate), a broad-spectrum insecticide (Figure 5.1). A member of the bipyridilium family of herbicides is the compound paraquat (1,1-dimethyl-4,4-bipyridinium ion as chloride salt) (Figure 5.1). Most rodenticides are classified as restricted use and are only applied by licensed pest control officers.
FOOD ADDITIVES AND CONTAMINANTS
Human poisonings associated with rodenticides usually result from accidental or suicidal ingestion of the compounds. This section has covered only a few of the pesticides available on the United States and world markets today. An understanding of the basic chemical processes affected by pesticides has led to the discovery and production of new families of chemicals.
TOXINS .1 History
- Microbial Toxins
- Mycotoxins
- Algal Toxins
- Plant Toxins
- Animal Toxins
The main pollution problems include mussels, clams and crabs in the Pacific Northwest of the United States and Canada. The main pollution problems include mussels, clams, crabs and fish in the Pacific Northwest and Northeast Atlantic. It is caused by chemicals from the okadaic acid family (okadaic acid+4-related compounds) produced by several species of dinophys dinoflagellates.
SOLVENTS
THERAPEUTIC DRUGS
A number of toxic effects on the blood have been documented, including agranulocytosis caused by chlorpromazine, hemolytic anemia caused by methyldopa, and megaloblastic anemia caused by methotrexate. Toxic effects on the eye have been noted and range from retinotoxicity caused by thioridazine to glaucoma caused by systemic corticosteroids.
DRUGS OF ABUSE
COMBUSTION PRODUCTS
COSMETICS
Skin reactions (dermatitis) are common side effects of drugs, an example being topical corticosteroids. Bromates used in some cold wave neutralizers can be acutely toxic if ingested, as can the ethanol used as a solvent in hair dyes and perfumes. Thioglycolates and thioglycerol used in cold wave lotion and hair removers and sodium hydroxide used in straighteners are also toxic if ingested.
TOXICANT PROCESSING IN VIVO
Absorption and Distribution of Toxicants
INTRODUCTION
Toxic disposition can also be affected by plasma protein binding in the bloodstream. These factors also affect movement from one compartment to another in the body during distribution as well as metabolism and excretion. Put differently, this is a study of how the body "handles" the toxin as it is reflected in the plasma concentration at different times.
CELL MEMBRANES
Some of the fatty acids are unsaturated and contribute significantly to the fluidity of the membrane. The amphipathic nature of the membrane creates a barrier to ionized, highly polar drugs, although it does not completely exclude them. These biochemical and biophysical differences are believed to be responsible for permeability differences in skin from different anatomical regions of the body.
MECHANISMS OF TRANSPORT
- Passive Diffusion
- Carrier-Mediated Membrane Transport
Eventually the concentration of unionized or unbound (free) toxicant is the same on both sides of the membrane. The diffusion coefficient or diffusivity of the toxicant, D, depends primarily on the solubility of the toxicant in the membrane and its molecular weight and molecular conformation. Therefore, membrane permeability is closely related to the lipid solubility of the toxicant in the membrane, as well.
PHYSICOCHEMICAL PROPERTIES RELEVANT TO DIFFUSION
- Ionization
- Partition Coefficients
The permeability, P (P = Pc×D), of a non-polar substance through a cell membrane depends on two physico-chemical factors: (1) solubility in the membrane (Pc), which can be expressed as a partition coefficient of the drug between the aqueous phase and membrane phase, and (2) diffusion or diffusion coefficient (D), which is a measure of the mobility of the drug molecules in the lipid. For an organic base (RNH2+H+⇔RNH3+) the reverse is true, and lowering the pH (increasing the concentration of H+) will favor the formation of the ionized form, while raising the pH (lowering the concentration of H+) will promote the formation of the non-ionized form. The partition coefficient is a measure of a chemical's ability to separate between two immiscible phases.
ROUTES OF ABSORPTION
- Extent of Absorption
- Gastrointestinal Absorption
- Dermal Absorption
- Respiratory Penetration
Therefore, most of the absorption will occur in the intestine (pH=6) and to some extent in the stomach (pH=1-3). The smaller the particle size of the poison, the greater the absorption and a chemical must be in aqueous solution to be absorbed in the GIT. The toxicant can now be reabsorbed from the GIT, prolonging the presence of the drug or toxicant in the systemic circulation.
TOXICANT DISTRIBUTION
- Physicochemical Properties and Protein Binding
Again, this is related to what fraction of the toxic substance is free or unbound (fu). Extensive plasma protein binding may affect renal clearance if glomerular filtration is the major elimination process in the kidney, but not if it is through active secretion in the kidney. There are other toxic substances that have a slightly larger Vd (eg 0.23 l/kg), and these toxic substances can be distributed in the extracellular compartment.
TOXICOKINETICS
Therefore, as the equations above indicate, the body clearance of a toxin is expressed in units of volume per time unit (e.g. L/h), and can be derived if we know the volume of distribution of the toxin. Note that it is the slope of the straight line for a semilog plot of toxin concentration versus time (Figure 6.12). In the previous equation, it is the elimination rate constant that is related to the half-life of the toxic substance described earlier in this chapter.
Metabolism of Toxicants
INTRODUCTION
PHASE I REACTIONS
- The Endoplasmic Reticulum, Microsomal Preparation, and Monooxygenations
- The Cytochrome P450-Dependent Monooxygenase System
- The Flavin-Containing Monooxygenase (FMO)
- Nonmicrosomal Oxidations
- Cooxidation by Cyclooxygenases
- Reduction Reactions
- Hydrolysis
- Epoxide Hydration
- DDT Dehydrochlorinase
Both are located in the endoplasmic reticulum of the cell and have been studied in many tissues and organisms. Functional FMO2 is found in 26% of the African-American population and perhaps also in the Hispanic population. These enzymes are located in the mitochondria or in the soluble cytoplasm of the cell.
PHASE II REACTIONS
- Glucuronide Conjugation
- Glucoside Conjugation
- Sulfate Conjugation
- Methyltransferases
Glucuronide conjugation generally results in products that are less biologically and chemically reactive. Sulfation and sulfate conjugate hydrolysis, catalyzed by various members of the sulfotransferase (SULT) and sulfatase enzyme superfamilies, play an important role in. CatecholO-methyltransferase occurs in the soluble fraction of several tissues and has been purified from rat liver.
- Glutathione S-Transferases (GSTs) and Mercapturic Acid Formation Although mercapturic acids, the N-acetylcysteine conjugates of xenobiotics, have been
- Cysteine Conjugate β-Lyase
- Acylation
- Phosphate Conjugation
Glutathione conjugation dramatically increases the water solubility of the metabolites compared to the parent compounds. However, it is found in the microsomes of the kidney and liver and is specific for acetyl-CoA as the actyl donor. Despite its great instability, this enzyme has been purified from the cytosolic fraction of the rat liver.
Reactive Metabolites
- INTRODUCTION
- ACTIVATION ENZYMES
- NATURE AND STABILITY OF REACTIVE METABOLITES
- Ultra-short-lived Metabolites
- Short-lived Metabolites
- Longer-lived Metabolites
- FATE OF REACTIVE METABOLITES
- Binding to Cellular Macromolecules
- Lipid Peroxidation
- Trapping and Removal: Role of Glutathione
- FACTORS AFFECTING TOXICITY OF REACTIVE METABOLITES
- Levels of Activating Enzymes
- Levels of Conjugating Enzymes
- Levels of Cofactors or Conjugating Chemicals
- EXAMPLES OF ACTIVATING REACTIONS
- Parathion
- Vinyl Chloride
- Methanol
- Aflatoxin B 1
- Carbon Tetrachloride
- Acetylaminofluorene
- Benzo(a)pyrene
- Acetaminophen
- Cycasin
- FUTURE DEVELOPMENTS
Often the CYP forms induced are those involved in the metabolism of the inducing agent. In the case of the hepatocarcinogen, 2-acetylaminofluorene (2-AAF), two activation steps are required to form the reactive metabolites (Figure 8.4). As long as glutathione (GSH) is available, most of the reactive intermediate can be detoxified.
Chemical and Physiological Influences on Xenobiotic Metabolism
- INTRODUCTION
- NUTRITIONAL EFFECTS
- Protein
- Carbohydrates
- Lipids
- Micronutrients
- Starvation and Dehydration
- Nutritional Requirements in Xenobiotic Metabolism
- PHYSIOLOGICAL EFFECTS .1 Development
- Gender Differences
- Hormones
- Pregnancy
- Disease
- Diurnal Rhythms
- COMPARATIVE AND GENETIC EFFECTS
- Variations Among Taxonomic Groups
For example, liver monoamine oxidase activity is decreased, whereas the activity of the same enzymes in the kidney is increased. Studies of the activity of the mentioned enzymes show no gender differences in the mouse; both sexes show an increase. Striking differences between species can be seen in the biological half-lives of different drugs.
