Cornerstones of Toxicology

From Beginning of Civilization

Toxicology is arguably the oldest scientific discipline, as the earliest humans had to recognize which plants, nuts, or fruits were safe to eat. More than likely through illnesses, and even possibly death, they soon recognized harmful and beneficial consequences associated with taking foreign materials into their bodies. From these early and basic observations, the concept involving the division of chemicals into two categories of beneficial and harmful chemicals evolved and has persisted to the present day. Technically, there is no strict line of demarcation to describe or distinguish beneficial or harmful chemicals, and the degrees of harmfulness and safeness appear to lie in the dose of the chemical, thereby making this concept the hinge pin of modern toxicology that is the dose makes the poison. All chemicals can cause toxic effects in large enough amounts, and humans are exposed to chemicals both deliberately and inadvertently. Most exposure of humans to chemicals is via naturally occurring compounds consumed in the diet from food plants.

Many of the Earliest Practitioners of Toxicology Were Women

It has been alleged that Lucrezia Borgia, daughter of Rodrigo Lenzuoli Borgia or Pope Alexander VI, who specialized in “faith-based” poisoning, was an early Italian who helped develop poisoning into a simple but fine art. It is said that the Borgias selected and laid down rare poisons in their cellars with as much thought as they had given to selecting and preserving their vintage wines. It is historically rumored that the Borgias served poisonous wine to members of the papal court, wealthy guests, suitors, and even family members, to their demise, to increase their wealth and power (Hayes and Kruger 2014). Lucrezia and her family members could all be considered early food toxicologists. Another early practitioner of toxicology was Catherine de Medici of Florence and Queen Consort of France. She tested and carefully studied the effects of various toxic concoctions on the poor and sick, noting the onset of action and symptoms that occurred (Knecht 1998), and could by today’s standards be considered an early experimental toxicologist. Goeie Mie (Good Mary) of Leiden, the Netherlands, was alleged to have poisoned at least 102 friends (27 died) and relatives between 1867 and 1884, distributing arsenic trioxide in hot milk to her victims after opening life insurance policies in their names (Gaute and Odell 1991). She would have been considered an early forensic toxicologist. Catherine Deshayes or “La Voisin” would be thought of as an early economic toxicologist because of her propensity to trade in selling poisons to wives who wished to rid themselves of their husbands (Ramsland 2005).

Historical Examples of Poisoning

Examples of historical cases of poisoning include Joseph Stalin, leader of the former Soviet Union, who reportedly died of brain hemorrhage. It has been suggested that he could have been poisoned, possibly by the Politburo who allegedly gave him warfarin (Hayes and Kruger 2014). It is reported that the U.S. Central Intelligence Agency (CIA) made attempts on the life of the Cuban dictator Fidel Castro by attempting to give him botulinum-laced pills in his food or drinks (http://www.edwardjayepstein.com/archived/castro.htm). More recently, in the history of high-profile poisonings, evidence suggests Viktor Yushchenko, the former Ukrainian President, was poisoned with dioxin (a polychlorinated dibenzodioxin) in an attempt to remove him from office. Another suspected casualty of poisoning is Napoleon Bonaparte who died in exile on the island of St. Helena in 1821. British doctors performed an autopsy and found evidence of stomach cancer, and this was given as the cause of his death. However, in the 1960s, unusually high levels of arsenic were reported in Napoleon’s hair. Arsenic was present in the drinking water, hair cream, and even the wallpaper in his cottage. It is historically rumored that Napoleon’s associate the Comte de Montholon, who was in charge of his food and wine, poisoned him slowly, first with arsenic and then with other medications (Weider, Forshufvud, and Forshufvud 1995).

From Killers to Healers: Toxicology Evolves, but Poisonings Still Occur

Over the years, there have been several cases of human exposures to chemicals that have led to devastating outcomes and in some instances death. In 2008, one such case of worldwide poisoning was the exposure of hundreds of thousands of Chinese children to melamine-contaminated milk, with ensuing kidney stones, kidney failure, and death in some of the children exposed (Xin and Stone 2008; Yang and Batlle 2008; Chiu 2008). Melamine was found in tainted pet food a year earlier with hundreds of thousands of dogs and cats being exposed (Dobson et al. 2008).

The government of Nigeria implemented tighter controls on chemical imports after 84 children between the ages of 2 months and 7 years died after consuming a tainted teething syrup, “My Pikin” (Okuonghae et al. 1992). Authorities believe diethylene glycol (component of antifreeze) was wrongly labeled as propylene glycol, a chemical generally recognized as safe for use in food and medicine, which was smuggled into Nigeria before being purchased by a Lagos-based company, which has since been shut down. To prevent future incidents, The National Agency for Food and Drug Administration and Control (NAFDAC) required all propylene glycol imports from India and China to be certified by the agency’s independent analysts in India and China before shipment and to be recertified upon entry into Nigeria. The first case was discovered on November 3 with symptoms including diarrhea, vomiting, fever, convulsions, and an inability to pass urine.

Diethylene glycol was used as a cheap replacement for the sweetener glycerin in cough syrup and more than 100 people, mostly children, died in Panama (http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5848a2.htm). In addition to the United States and Panama, diethylene glycol–tainted toothpaste was found in Australia, the Dominican Republic, Costa Rica, Honduras, and Nicaragua (http://www.nytimes.com/2007/06/02/us/02toothpaste.html?_r=0).

In 2009, 121 of the 287 children under 14 years of age in Longyan, China, had high blood lead levels (http://www.nytimes.com/2009/09/28/world/asia/28china.html). A smelting plant closed after more than 600 children were found to have lead poisoning; and in another case, 800 children living near a zinc and lead smelting plant were found to have high blood lead levels. In the United States, the Food and Drug Administration (FDA) has a limit of 0.5 µg/dl in products intended for infants and children and has banned the use of lead-soldered food cans.

Each year in the United States, 310,000 one- to five-year-old children are found to have unsafe levels of lead in their blood due to exposure to lead through dust and other sources (https://www.cdc.gov/nceh/lead/data/). Just recently, Virginia Tech University researchers brought proof of high lead levels in Flint, Michigan, water to public attention in September 2015, performing water tests in more than 250 Flint homes. Lead levels were high enough, up to 38 μg/dl, to warrant urgent government action (https://www.rt.com/usa/327363-flint-children-blood-lead-water/); however, in a similar problem with Washington, DC’s tap water a little over a decade ago, hundreds of homes were found to have stratospheric lead levels of 300 parts per billion (ppb) or more (Guidotti et al. 2007).

New Sweden, Maine, made national headlines in 2003 when a man poisoned the coffee urn with arsenic at the local Lutheran church, sickening 15 parishioners and killing 1 (http://www.nytimes.com/2003/05/09/us/poison-mystery-widens-after-a-suicide-note.html). Five days later, a church member, Daniel Bondeson, was found after apparently shooting himself and supposedly leaving a suicide note confessing to having something to do with the poisoning, but the note has never been released to the press or even to his sister.

China has a 150-ppb arsenic limit in foods such as rice. Rice grown in the United States has an average 260 ppb of arsenic; however, a 100-ppb action level has been proposed for inorganic arsenic in infant rice cereal (http://www.philrice.gov.ph/phl-rice-safe-from-arsenic/).

The U.S. Food and Drug Act, also known as the “Wiley Act,” was passed in 1906 in response to the exponential growth of American counterfeiters of patented medicine(s). The most notorious scheme involved importing empty bottles and filling them with phony concoctions and affixing fake labels from well-respected European companies. Harvey Wiley, a chief chemist of the Division of Chemistry (later Bureau of Chemistry), U.S. Department of Agriculture (USDA), Washington, DC, was a strong advocate for this act, hence, the nickname the Wiley Act. Wiley was also known as the “Crusading Chemist” and “Father” of the Pure Food and Drugs Act. Wiley’s department was responsible for examining food and drugs for “misbranding” or “adulteration,” and he used these newly found regulatory powers to pursue aggressive action toward the manufacturers of foods with chemical additives. His department later came under scrutiny and the Bureau of Chemistry’s powers were quickly checked by judicial decisions that focused and defined the bureau’s powers and set standards to be achieved for proof of fraudulent intent. In 1927, the regulatory Bureau of Chemistry was reorganized under a new USDA group, the Food, Drug, and Insecticide Organization. In 1930, the name was changed to the FDA (http://www.fda.gov/AboutFDA/WhatWeDo/History/Milestones/default.htm).

Paracelsus and the Concept of Dose

Paracelsus, born Philippus Aureolus Theophrastus Bombastus von Hohenheim, is credited as the founder of toxicology. His premise of poisons was that “All things are poison, and nothing is without poison; only the dose permits something not to be poisonous.” Simply stated, “The dose makes the poison.”

Traditionally, the study of poisons was centered on the inherent capacity of a chemical or compound to produce injury. Today, toxicology is more safety-driven, and modern toxicology uses chemicals as tools to understand molecular/cellular biology. One basic function of toxicology is to assess the likelihood of occurrence of adverse effects (qualitative): Is a chemical or compound safe? This is hazard identification. Also, toxicology studies the nature and mode of action of adverse effects (quantitative). At what concentration is it safe? This is known as dose–response assessment or hazard characterization.

Toxicology affects us every day. The purpose of toxicology is to provide workers a safe working environment and to ensure consumers’ products are safe to use as specified and under foreseeable misuse. Another area of toxicology is the environment or ecotoxicology, which is concerned with the quality of the air, water (surface and ground), soil, and bedrock, and aquatic wildlife (fresh and salt) and terrestrial organisms (flora and fauna). Toxicology provides information to risk managers about the nature and severity of effects on human health and the environment, as it relates to specific exposures. Ecotoxicology often drives remediation site cleanups. There are approximately 6,000,000 known chemicals: approximately 100,000 currently in use worldwide and 500 new chemicals being added annually. There is limited information available on many of these chemicals, which has spawned in the European Union regulations such as the formation of the Registration, Evaluation, and Authorization of Chemicals (REACH), which enforces enterprises that manufacture large volumes of chemicals to register those chemicals in a central database. Risk assessment involves hazard identification, dose–response, exposure, and risk characterization. Risk is defined as hazard times exposure.

Dose Matters

The key points of toxicology are (1) dose matters (and so does timing), (2) people differ, and (3) things change (Mitchell et al. 2004). When we explore the concept of dose, there are poisons such as botulinum toxin A (0.00001 mg/kg) that can be poisonous, yet this toxin can also heal. Clostridium botulinum is an anaerobic, gram-positive, spore former commonly found in soil that produces oval, subterminal endospores. One of the 7 strains (ABCDEG) is responsible for approximately 145 cases of poisoning annually and is 40 million times more powerful than cyanide. There are several types of botulism poisonings, which can be food borne (approximately 15%; home canning; relatively rare today) and via wounds (approximately 20%; often associated with black tar heroin injection, especially in California). Because of the widespread occurrence of spores of C. botulinum in the soil and the typical hand-to-mouth response of infants, C. botulinum spores are often consumed by young children. These spores can germinate in the intestine in infants (approximately 75–100 cases annually; 2nd month of life), causing often fatal outcomes. This poisoning can be treated with an antitoxin (human botulinum immunoglobulin) and supportive care.

Conversely, botulinum toxin is a poison that can heal. In its purified form (type A), it was the first bacterial toxin to be used as a medicine. In 1989, the FDA licensed Oculinum for treating 2 eye conditions characterized by excessive muscle contractions: blepharospasm (tic or twitch of eyelid) and strabismus (eyes not properly aligned). As a medicine, type A toxin can be used to control certain conditions marked by involuntary muscle contractions and can block muscle contractions. The toxin binds to nerve endings at the point where the nerves join muscles and blocks release of acetylcholine, thereby blocking muscle contractions. Another beneficial effect of botulinum toxin is its use in cosmetic applications, such as Botox and Botox Cosmetic (botulinum toxin A). Another is its use in the treatment of patients with cervical (neck) dystonia and to reduce the severity of abnormal head position and neck pain associated with cervical (neck) dystonia. Other applications of botulinum toxin include severe primary axillary hyperhidrosis (excessive sweating), achalasia (failure of the lower esophageal sphincter to relax), neuropathies, migraine and other headache disorders, although the evidence is conflicting in this indication, and overactive bladder and benign prostatic hyperplasia.

Dose matters as does timing. An example of this concept is thalidomide (100+ mg/kg) and its critical timing of exposure. Originally developed as a treatment for insomnia and morning sickness in the 1950s, thalidomide is an oral drug that has been shown to be highly active against myeloma. Many consider thalidomide to be the first new agent with major antimyeloma activity in more than 30 years. Thalidomide has been FDA approved for the treatment of not only myeloma but also erythema nodosum leprosum (treat and prevent skin conditions caused by Mycobacterium leprae). It also has had limited success in treating a variety of other diseases (Kaposi’s sarcoma, primary brain malignancies, chronic graft vs. host disease, Behcet’s disease, aphthous ulcers, systemic lupus erythematosus, adult Langerhans cell histiocytosis, rheumatoid arthritis, and Jessner’s lymphocytic infiltration of the skin). Thalidomide can inhibit the growth of HIV in test tubes (by selective tumor necrosis factor alpha inhibition) and may alleviate symptoms of HIV (Gunzler 1992; Emer 2009).

Interactions between chemicals and biological systems follow a dose–response relationship. Toxicity is quantified through the dose–response relationship. Individual change in severity of effect with dose is also called a dose–effect relationship. Population change is the proportion of the population responding with dose (people differ). There are different relationships for different effects, and the shape of the dose response curve gives information about population variability and toxicity of the compound. A key concept in toxicology is the quantitative relationship between the concentration of a xenobiotic (foreign chemical) in the body and the magnitude of its biological effect. The magnitude of the effect is usually a function of the amount of xenobiotic a person is exposed to. In any given population, there will be a range of sensitivities to a xenobiotic. It is extremely useful to know what is the average sensitivity of a population to a xenobiotic and what the average dose required to elicit a toxic response will be. This brings us back to the central tenet of toxicology that the dose makes the poison and that dose matters.

People Differ

Classic examples of the fact that people differ include allergies to food (e.g., peanuts and shellfish) and to drugs (e.g., penicillin). Food allergy in the United States and Europe affects approximately 2% to 4% of adults and 4% to 8% of young children. Severe allergenic reactions are relatively rare, with approximately 120,000 emergency room visits with less than 200 fatalities/year. Eight foods (peanuts, milk, wheat, eggs, shellfish, soybeans, crustaceans, and some tree nuts) account for approximately 90% of food allergies (United States); whereas 14 foods are listed as allergenic in Europe, which are regional and include molluscan shellfish, lupine, celery root, mustard and sesame seeds, and sulfites in addition to those outlined for the United States. Even more striking are the differences in the effects of the same chemical on a single individual that may be observed during various stages of life (in utero, neonate, young adult, elderly). For instance, infants have an immature immune system and limited phase II systems (more sulfur conjugation) and reduced kidney function. The elderly may be similar in their sensitivities to the adverse effects of chemicals. There is a difference in sensitivity of newborn rats and older rats to DDT. The LD50 of DDT in young male rats is greater than 4,000 mg/kg; whereas in adult rats at 1 year of age, it is approximately 225 mg/kg.

Acetaminophen (paracetamol) is metabolized 90% by sulfate or glucuronide conjugation (Phase II). The sulfate pathway predominates in children less than 12 years of age, while adults primarily use the glucuronide pathway. Chloramphenicol (chlornitromycin), an antibiotic for gram-positive/-negative (most anaerobic) organisms, is primarily metabolized by glucuronidation and is a poor substrate for sulfotransferase. Therefore, this antibiotic is extremely toxic to neonates (bone marrow) and is responsible for the “gray baby syndrome” consisting of progressive cyanosis, metabolic acidosis, vasomotor collapse, respiratory difficulty, and death (McIntyre and Choonara 2004).

A key concept in toxicology is the quantitative relationship between the concentration of a xenobiotic in the body and the magnitude of the biological effect it produces. The magnitude of the effect of a xenobiotic is usually a function of the amount of xenobiotic a person is exposed to or that reaches a receptor (i.e., “The Dose Makes the Poison”). In any given population, there will be a range of sensitivities to a xenobiotic. It is extremely useful to know what is the average sensitivity of a population to a xenobiotic, and what the average dose required to elicit a toxic response will be. Population dose–response function focuses on a specific end point made up of many individual dose–response functions. At each dose level, individual members of the population either do or do not respond. There is variation in sensitivity/susceptibility of individuals in the population. One can measure the proportion of population responding at each dose level or the level of the response at each dose level.

Why do we look so different or behave so differently? Why are some more able or more susceptible than others? What makes us unique? Individual genetic makeup (polymorphisms)! Changes in the number and order of genes add variety to the human genome. Inversions, insertions, deletions, and copy number variation all contribute to the individuality of our genetic makeup. The Major Histocompatibility Complex (MHC) is a large genomic region or gene family found in most vertebrates. It is the most gene-dense region of the mammalian genome and plays an important role in the immune system, autoimmunity, and reproductive success. The proteins encoded by the MHC are expressed on the surface of cells in all jawed vertebrates and display both self antigens (peptide fragments from the cell itself) and nonself antigens (e.g., fragments of invading microorganisms) to a type of white blood cell called a T cell that has the capacity to kill or co-ordinate the killing of pathogens or infected or malfunctioning cells. A polymorphism is a genetic variant that appears in at least 1% of a population. Examples of polymorphisms in humans are (1) ABO blood groups, (2) Rh factor, (3) MHC 1, 2, 3 (immune system, autoimmunity, and reproductive success), and CytP450s/Phase II metabolic enzymes. By setting the cutoff at 1%, it excludes spontaneous mutations that may have occurred in and spread through the descendants of a single family.

Allelic variants in aldehyde dehydrogenase-2 (ALDH2) cause decreased ability to clear acetaldehyde and other aldehyde substrates, with homozygous variants (ALDH2*2/2) having no activity and heterozygotes (ALDH2*1/2) having intermediate activity relative to the predominant wild type (ALDH2*1/1). These polymorphisms are associated with increased buildup of acetaldehyde following ethanol ingestion and increased immediate symptoms (flushing syndrome) and long-term cancer risks. The nonfunctional allele is rare in most populations but is common in Asians such that 40% are heterozygotes and 5% are homozygote variants.

Classical phenylketonuria (PKU) is an autosomal recessive disorder, caused by mutations in both alleles of the gene for phenylalanine hydroxylase (PAH), found on chromosome 12. In the body, PAH converts the amino acid phenylalanine to tyrosine, another amino acid. Mutations in both copies of the gene for PAH mean that the enzyme is inactive or is less efficient, and the concentration of phenylalanine in the body can build up to toxic levels. In some cases, mutations in PAH will result in a phenotypically mild form of PKU called hyperphenylalaninemia. Both diseases are the result of a variety of mutations in the PAH locus; in those cases where a patient is heterozygous for 2 mutations of PAH (i.e., each copy of the gene has a different mutation), the milder mutation will predominate. PKU is one of the commonest inherited disorders occurring in approximately 1 in 10,000 babies born in the United States whereby they inherit 2 mutant genes for the enzyme PAH. PAH normally breaks down the amino acid phenylalanine that is in excess of the body’s need for protein synthesis. Both parents must be defective to produce the disease. Test that measure how quickly injection of phenylalanine is removed from blood can distinguish a person who has 1 PKU gene from a person who has none. A person with 1 defective gene is perfectly healthy because the unmutated allele produces enough of the enzyme; however, these heterozygous individuals are “carriers” of the disease. Loss of PAH can result in mental retardation, organ damage, unusual posture, and can, in cases of maternal PKU, severely compromise pregnancy.

Subtle genomic changes known as silent polymorphisms lead to coding-equivalent changes in messenger RNA (mRNA). During translation, these mRNAs yield proteins with identical amino acid sequences but different shapes. In 2007, the NCI showed that a multidrug resistance gene with a single nucleotide, coding-equivalent sequence modification translates into a P-glycoprotein with a different conformation (Kimchi-Sarfaty et al. 2007). They speculate that the difference may result from timing or other subtle changes to the translation process caused by the sequence modification. The work points to a previously unrecognized and potentially profound role of single-nucleotide polymorphisms in health and disease.

The acetylation polymorphism illustrates another genetic polymorphism of a drug-metabolizing enzyme studied in the early era of pharmacogenetics (Grant et al. 1990). N-acetyltransferase (gene, NAT), a phase II conjugating liver enzyme, catalyzes the N-acetylation (usually deactivation) and O-acetylation (usually activation) of arylamine carcinogens and heterocyclic amines. The slow acetylator phenotype often experiences toxicity from drugs such as isoniazid, sulfonamides, procainamide, and hydralazine, whereas the fast acetylator phenotype may not respond to isoniazid and hydralazine in the management of tuberculosis and hypertension, respectively. During the development of isoniazid, isoniazid plasma concentrations were observed in a distinct bimodal population after a standard dose. Patients with the highest plasma isoniazid levels were generally slow acetylators and they suffered from peripheral nerve damage, while fast acetylators were not affected (Goel et al. 1992). Slow acetylators are also at risk for sulfonamide-induced toxicity and can suffer from idiopathic lupus erythematosus while taking procainamide (Sim 1989). The slow acetylator phenotype is an autosomal recessive trait. Studies have shown large variations of the slow acetylator phenotype among ethnic groups: 40% to 70% of Caucasians and African Americans, 10% to 20% of Japanese and Canadian Eskimo, more than 80% of Egyptians, and certain Jewish populations are slow acetylators (Bell et al. 1993; Cascorbi et al. 1995; Delomenie et al. 1996; Hamdy et al. 2003; Lemos and Regateiro 1998; Lin et al. 1993, 1994; Okkels et al. 1997; Peloquin et al. 1997; Smith et al. 1997). In East Asia, the further north the geographic origin of the population, the lower the frequency of the slow acetylator gene. The reason for this trend is unknown, but it has been speculated that differences in dietary habits or the chemical or physical environment may be contributing factors.

Allelic variation at the NAT2 gene locus accounts for the polymorphism seen with acetylation of substrate drugs (Lin et al. 1993; Sabbagh and Darlu 2005; Sabbagh et al. 2008; Sabbagh et al. 2011; Vatsis, Martell, and Weber 1991). There are 27 NAT2 alleles that have been reported. NAT2 is an unusual gene because it consists of open-reading frames (i.e., protein-coding regions) with no introns. Most variant NAT2 alleles involve 2 or 3 point mutations. For example, the variant NAT2*5B differs from the wild type at 3 nucleotide positions: 341, 481, and 803; NAT2*6A has 2 changes at positions 282 and 590; NAT*13 has 1 point mutation at 282; and NAT2*7A has 2 changes at positions 282 and 857. NAT2*5B and *6A account for 72% to 75% of all the variant NAT2 alleles, which includes at least 94% of all variant alleles in Caucasians, Japanese, and Hispanics and 83% of the NAT2 alleles in African Americans. NAT2*5B is the most common allele in Caucasians (40–46%) but occurs at a very low frequency in Japanese (0.5%). NAT*6A, *7B, and *13 share a mutation at C282 T. NAT*5A,*5B, *6A, *7A, *7B, and *13 are associated with the slow acetylator phenotype as a result of a decrease in the amount of NAT2 protein. The protein expressed from NAT2*5A, *5B, and *5C genes has lower activities than *6A and *7B, whereas *13 has normal activity.

Currently, the importance of these variants in NAT2 is most studied for their association with a modestly increased risk for cancers, possibly because of prolonged exposure of the body to chemicals, drugs, or metabolites compared with fast acetylators (Peloquin et al. 1997; Ayaz et al. 2008; Terry and Goodman 2006; Schnakenberg et al. 1998; Hein et al. 1992). A preliminary result suggested that impaired isoniazid metabolism is associated with point mutations in NAT2 in a small Japanese population (Kita et al. 2001). This exciting result awaits large population studies to establish clearly the relationship between the NAT2 genotype and isoniazid acetylation. It may still take some time to establish the clinical utility of NAT2 genotype analysis to independently predict isoniazid acetylation. However, genotype NAT2 mutations could be an addition to the traditional therapeutic drug monitoring for isoniazid in the near future.

The emerging field of “Pharmacogenomics” or “Toxicogenomics” offers the potential to identify and protect subsets of people predisposed to toxicity from chemicals or drugs. This approach allows one to identify people with different chemical/drug sensitivity (people differ).

Things Change

Perfumes were an important part of the court life in Ancient Egypt. Cleopatra’s perfume factory was (still is) at the southern end of the Dead Sea. Cleopatra used to drink turpentine (terebinth from Pistacia terebinthus) to make her urine smell of violets. Presumably terpenes within the oil being metabolically converted to ionones, with the volatile organic compounds methanethiol, dimethyl sulfide, dimethyl disulfide, bis(methylthio)methane, dimethyl sulfoxide, and dimethyl sulfone, are responsible for the smell. As a chemical passes through the body, it will encounter a number of enzymes that accelerate chemical reactions (intermediary metabolism) that are necessary for growth, maintenance of integrity, and continuance of life. Distinctive microenvironments exist at the active sites of each enzyme to assist chemical interconversions. It is, therefore, not unexpected that a chemical undergoes chemical alteration(s) as it traverses a living system. What is eliminated from the biological system is not always the same as what entered the system originally. Sometimes the host (i.e., the organism that is exposed to the chemical) plays a critical role in the outcome of toxicity. The process by which this occurs is called biotransformation/metabolism and involves chemical reactions within the organism in which 1 chemical is changed to another. Chemicals can enter the body and be absorbed by the body by a variety of means such as ingestion, inhalation, and dermal absorption. They are distributed through many bodily compartments and finally excreted. What happens in between entry and exit, though, plays a key role in toxicity.

Biotransformation usually decreases the ability, and in turn, the likelihood, of a molecule interacting with a biological system. The probability of potential damage is offset or lowered. Usually, but not always, the process makes the molecule more polar (frequently acidic) and thus more water soluble. As such, the system can more easily remove the molecule via the kidneys (urine) or liver (bile). An example is the enzyme epoxide hydrolase converting ethylene oxide, a chemical with genotoxic and other chronic toxic properties, into the somewhat less toxic ethylene glycol. On occasions, the chemical may be activated and converted into a more dangerous species whereby the body inadvertently poisons itself. For example, chloroform can be converted to phosgene. Phosgene (COCl₂), a poisonous gas used as a chemical weapon in World War I, is a highly toxic gas or liquid that is classified as a pulmonary irritant. Exposure to phosgene gas produces delayed onset noncardiogenic pulmonary edema. Exposures to 50 ppm may be rapidly fatal (http://emergency.cdc.gov/agent/phosgene/basics/facts.asp).