Exposure with subsequent damage necessitates the actual absorption of a toxic compound into the body.  In order to be absorbed, the compound must naturally cross cell membranes.  The absorption of toxic chemicals is dependent upon the same factors as that of any biochemical constituent or drug, namely

1) its structure
2) the contributions of that structure to factors of transport such as
a) size
b) lipid solubility
c) charge
3) the specific transport process
a) channels
b) passive diffusion, as determined by Fick's Law, Rate = (K)(Area)(Concentration Difference)
c) active transport
d) facilitated diffusion
e) phagocytosis or pinocytosis

Sites of Absorption

Skin -- The skin usually serves as a protective barrier due to the layer of dead cells and the tightly layered keratin of the epidermis that must be traversed before reaching blood vessels.  Typically, only very lipophilic compounds will be absorbed from the skin.  However, when exposure to  lipophilic compounds does occur, absorption may be high, due to the large surface area of the skin and toxicity may be profound (even resulting in death).  Disturbances in the protective nature of the skin (as in abraded areas) will increase the absorption of even non-lipophilic chemicals.

Lungs -- Usually toxicants such as gases, vapours (from solvents), and aerosols are absorbed by this route, although particulate matter (asbestos, lead) may also be inhaled.  When exposure occurs by this route, absorption is much greater due to the lack of a protective layer, the large surface area (50-100 m²), the large blood supply, and the relative thinness of the cell layer that must be crossed (only 2 cells thick in the alveoli).  Two primary factors of absorption in the lung are

1) Blood flow, which is a major factor in the absorption of lipid compounds and
2) Respiratory rate, which often determines the extent of absorption of hydrophilic compounds.
Typically, small lipophilic compounds are very rapidly absorbed.  Compounds in solution are often absorbed by pinocytosis and particulates by phagocytosis.  NOTE that larger particulates (on the order of 3 microns or larger) are often not absorbed but deposited on the respiratory tree, where they may be encysted and potentially cause damage in this manner (as in asbestos).  Typically the smaller the particle, the deeper in the lung it may penetrate, following the general scheme below:
Size less that 1 micron -- reaches the alveoli
Between 2 and 5 microns -- reach the trachea and bronchus
Greater than 5 microns -- deposited in the nasopharynx

Gastrointestinal Tract -- the great determinants for absorption from the GIT are pH and lipophilicity.  The pH of the alimentary canal changes from 7 (mouth) to 2 (stomach) to 6 (small intestine) with corresponding changes in the absorption of chemicals.  Weak acids, un-ionised in the stomach, will have greater absorption in the stomach while weak bases are absorbed more in the small intestine.  HOWEVER, due to the high degree of blood flow in the small intestine and its large surface area, almost all substances may be absorbed from the SI.
Some factors of the GI tract may actually decrease the likelihood of absorption.  For instance acid labile compounds may undergo acid hydrolysis in the stomach and therefore be inactivated prior to absorption.  Degradation may also occur in the SI by bacterial or intestinal wall enzymes, effectively inactivating toxicants.
Other factors may either increase or decrease the toxicity of a compound by altering the rate of absorption, such as the presence of food (and food type -- fatty foods, et c.) and the vehicle used in formulation of the toxicant.

A second important concept that influences the toxicity of a specific compound is its half-life (t_½) or the time required to reduce the body (or plasma) concentration of a compound by one-half.  In toxicology, the longer the half-life, the greater the risk of adverse effects.  Additionally, repeated exposures are more likely to cause toxicity (or accumulate in the body) with longer half-lives.  NOTE that half-life is essentially determined by elimination (metabolism and excretion) of the compound.  Since the AUC is in great part determined by the absorption and distribution of a compound and its half-life, then the total dose of a toxicant and the AUC for that compound may be used to calculate  the total body clearance (TBC).

TBC_A = Total Dose / AUC = Dose_A / AUC_A = 100 mg / 20 mg/ml min = 5 ml/min for compound A.

TBC_B = Total Dose / AUC = Dose_B / AUC_B = 100 mg / 5 mg/ml min = 20 ml/min for compound B.

METABOLISM
Metabolism is a primary mechanism of removing toxic chemicals from the body.  Usually the process converts the toxicant to a more hydrophilic, inactive form.  However, toxic metabolites may be formed as with the production of oxalic acid from ethylene glycol or the toxic metabolites of paracetamol.  Generally metabolism

1) makes compounds more polar
2) changes the molecular weight/size
3) facilitates excretion                           so that
 
1) half-life is decreased
2) exposure is decreased
3) accumulation is less likely
4) biological activity is changed
5) duration of activity is changed

Phase I -- addition or exposure of a hydrophilic functional group
Phase II -- conjugation with a more polar group

Phase I Metabolism generally includes four types of chemical reactions: Oxidation, Reduction, Hydrolysis, and Hydration.

Oxidation reactions are most often mediated by the cytochrome P₄₅₀ enzyme system of the hepatic microsomes.  This system requires NADPH, electrons, and the co-factor magnesium.  Oxidation reactions follow the general scheme

Specific examples of oxidation type reactions include
Aromatic hydroxylation -- as in the addition of an alcohol to benzene to form the more polar phenol.
Aliphatic hydroxylation
De-alkylation of carbon chains from N, O, or S within the structure
R'-N-CH₃ -------> R'-NH + CH₃-X
Hydroxylation of Sulphur or Nitrogen on a compound.
Reduction reactions may be microsomal or cytosolic and are mediated by reductase enzymes.  They often involve the reduction of nitro and azo groups, aldehydes, ketones, epoxides, double bonds, and dehalogenation.

Hydrolysis most often occurs with esters and amides

RCOOC or RCONC   ---------->   RCOOH + HOC or HNC
Hydration most often occurs with epoxides.

Phase II (or Phase 2) Reactions -- Conjugation Reactions
These reactions involve the addition of a (usually more polar) group on the toxicant.  The addition of  a more polar group increases the water solubility (and therefore the elimination) of the compound.  Occasionally, conjugation reactions may increase the lipophilicity (and therefore the potential for toxicity) of a compound.

Sulphation -- A sulphate group is added to the compound.  The sulphate group is donated by phosphoadenosinephosphosulphate (PAPS) and the enzyme involved is sulphotransferase, which is a cytosolic enzyme.  Sulphation generally occurs with aromatic or aliphatic hydroxy, NOH, and amino functional groups.

Glucuronidation -- glucuronide (a water soluble carbohydrate) is conjugated to the compound.  (NOTE that other sugars may also be conjugated, such as ribose and xylose.)  This is mediated by the microsomal enzyme glucuronyl transferase with uridine diphosphate (UDP) glucuronic acid serving as the donor.  Glucuronide conjugation generally occurs with hydroxy (OH), carboxy (COOH), amino (NH), and thiol (SOH) functional groups.

Glutathione conjugation -- Glutathione (GSH) is a tripeptide especially present in the liver.  The important functional group for conjugation reactions is sulphydryl (SH).  The conjugation may occur microsomally (the catalytic enzyme is glutathione transferase) or in the cytosol, where glutathione may spontaneously conjugate active intermediary compounds without enzymatic activity.  Structures that may undergo GSH conjugation include aromatic, heterocyclic, alicyclic, and aliphatic epoxides, aromatic halogens, nitro compounds, and unsaturated aliphatic compounds.  Once the conjugation has occurred, the product may be either excreted into the bile or undergo further (Phase 3) metabolism to form N-acetylcysteine conjugates or mercapturic acid derivatives.

Acetylation -- The addition of an acetyl group may actually decrease the water solubility of a compound.  Acetyl transferases are active in the cytosol of the liver, gastric mucosa, and blood cells.  The donor group and cofactor for the reaction is acetyl Coenzyme A.  Note that there are two isozymes of acetyl transferase.  One acts relatively quickly while the second is much slower.  The specific enzyme that is present is genetically predetermined and accounts for some individuals being "fast" acetylators and others "slow" acetylators.  This is especially important in the dosing and toxicity profile for drugs such as hydralazine, isoniazid, and procainamide, which may show increased toxicity in slow acetylating individuals.  Substrates for this reaction include aromatic amino compounds, sulphonamides, hydrazines, and hydrazides.

Amino Acid Conjugation -- Specific amino acids (most often glycine, but also cysteine and others -- often dependent upon the species) will be conjugated to compounds.  Conjugation most often occurs at the carboxy terminus and involves enzyme CoA as a cofactor and mitochondrial acylase as the catalyst.

Methylation -- The addition of a methyl group also may decrease the solubility of a compound.  Target groups of methylation (via methyl transferase) include hydroxy, amino, and thiol functions.  The classic example of methylation increasing the toxicity of a compound is mercury.  Inorganic mercury may be ionised to the mercuric ion and therefore is relatively water soluble.  The toxicity of inorganic mercury is limited by its limited absorption and presents primarily as nephrotoxicity.  However, methylation of the mercuric ion to methylmercury (by environmental micro-organisms or in vivo) greatly increases its lipid solubility and consequently its absorption and toxicity.  Methylmercury is extremely toxic to the CNS.


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EXCRETION OF TOXICANTS

URINARY excretion of toxicants is especially important for relatively small, water soluble chemicals.  There are four (4) primary mechanisms involved in urinary excretion.

1) Filtration -- from the blood to the glomerulus.  The primary factors that influence filtration are size of the molecule and glomerular filtration, as determined by blood flow and pressure.  Depending upon the toxic agent and/or disease state, glomerular filtration may increase or decrease.  Therefore, glomerular filtration rate may be used as an indicator for clearance of a compound that is excreted renally by filtration.

2) Passive diffusion -- from the interstitium to the renal tubules.  This is an important route for lipid soluble compounds.  NOTE that if the compound is non-ionised passive diffusion is high.  However, if it remains non-ionised in the tubule, reabsorption may take place just as easily.  Thus retention in the tubule (and passive diffusion as a means of renal excretion) requires ionisation of the compound in the tubule, which may be altered by pH of the tubular fluid.  The primary determinant of diffusion is the concentration gradient of the compound.

3) Active transport -- of chemicals from the interstitium to the tubular lumen by energy-dependent carrier proteins.  These protein transport systems are specific for weak acids or bases and may become saturated, setting a limit on the amount of material excreted in a given time period.

4) Facilitated diffusion -- a process that essentially resembles active transport, except  that it does not require energy.

NOTE that if a compound is filtered AND undergoes either diffusion or transport, its renal clearance will be greater than that predicted by glomerular filtration alone.  Conversely if a filtered substance is reabsorbed, its renal clearance will be less than that predicted by the GFR.
 

Factors that may affect the urinary excretion of a toxicant include plasma protein binding (given a compound that is highly bound, its ability to undergo glomerular filtration will be decreased, note that its ability to be actively transported will NOT be affected), and pH (as described above -- if the toxicant is a weak acid, alkalinisation of the urine will increase its excretion may maintaining it in the ionised state.  The converse is true for weak bases).  Age may also affect urinary excretion.  Neonates have a less well developed urinary system.  Consequently, they may not clear a toxicant as rapidly as an adult, which could increase the toxicity of the substance.

BILIARY excretion is important for large, polar compounds such as glutathione conjugated metabolites.  Biliary excretion involves the active secretion of weak organic acids, bases, and neutral compounds into the bile, which is then transported, via the bile duct, to the intestine.  Since it involves specific carrier transport systems, it may become saturated.  Once in the gut, the compound may be

1) excreted via the feces.

2) undergo enterohepatic cycling by reabsorption into the hepatic portal vein.  This often involves metabolic changes resulting in a more lipophilic compound that is mediated by the natural flora of the gut.  If the compound happens to be hepatotoxic, this enterohepatic cycling will increase the risk of liver damage.

3) undergo one cycle of enterohepatic cycling and thence back to systemic circulation and potential toxicity.
 

PULMONARY excretion occurs primarily with volatile compounds and gases.  The mechanism of excretion is primarily by passive diffusion.  Thus the greater the concentration of the toxicant reaching the lungs, the greater the amount excreted by the lungs.  The process is essentially the reverse of absorption.  Therefore, the more a compound is absorbed by the lungs, the greater its excretion; the less its pulmonary absorption, the less its excretion.

OTHER routes of excretion include milk, which can have important consequences (especially with lipid soluble toxicants such as DDT and divalent metals) in both the infant (for mothers breast-feeding) and the population in general (for the excretion of toxicants by cows into milk intended for human consumption).  Additional, minor routes of excretion include sweat, tears, saliva, and semen.

FACTORS OF TOXICITY
In addition to the role the chemistry of the specific toxicant plays (as described above) in relation to its absorption, distribution, metabolism, and excretion, other factors may influence the specific toxicity of a compound.

SPECIES -- Different species of animals respond differently to toxicants.  While an agent may be extremely toxic to mice, it may have relatively few toxic effects in humans.  This is one of the problems with toxicity testing, where a compound may show toxicity as described above OR may be relatively non-toxic in lower animals while the reverse may be true in humans.  A classic example of species dependent effects is the toxicity of MPTP, which has virtually no effect in mice but produces profound and sudden Parkinsonian like effects in primates and man.  Another classic example is the absence of a gag reflex in rats, who cannot vomit, while other species may use this to eliminate a toxicant prior to damage.

STRAIN/RACE -- Different strains within a species may also respond differently to toxicants.  Usually these differences are not as dramatic as those between species, but they may influence the toxic response.  In humans, these differences are most noted along racial lines.  A classic example is the effect of ethanol in Asians, who generally express less aldehyde dehydrogenase than non-Asians.  Therefore the signs of mild ethanol toxicity (flushing, GI upset, headache, et c.) are more pronounced in them.

GENDER -- These differences may reflect direct action of the female/male hormone or an indirect effect such as metabolism.  Again using ethanol as an example, men typically have higher levels of gastric alcohol dehydrogenase, relative to women.  Therefore, the alcohol is more rapidly metabolised prior to distribution in males vs.  females, thus contributing at least in part to the differences in ethanol intoxication between the genders.   The toxicity of dinitrotoluene serves as another good example.  It undergoes glucuronide conjugation.  However, in males it is excreted primarily by biliary mechanisms while in females it undergoes urinary excretion.  In males, dinitrotoluene glucuronide will then undergo enterohepatic cycling.  Since dinitrotoluene is a potent hepatic carcinogen, the incidence of hepatic tumours following exposure is much higher in males than in females.  This effect is dependent upon androgen.

HUMAN VARIABILITY -- In addition to the racial or gender differences described above, individuals may show a wide range of responses to a toxicant.  These may be gene-specific, as with slow vs. fast acetylators as described previously, or they may simply be due to some unknown factor, such as differences in receptor number or response.  These are often termed idiosyncratic reactions, those that occur in rare individuals but not in the general population frequently enough to allow a prediction of their incidence.  Additional factors that may contribute to individual variability include

personal habits such as diet/drug use (smoking and drinking which may induce liver enzymes),

environmental factors such as air and water (contaminants may induce or inhibit metabolic enzymes or act as promoters to increase the toxicity of other compounds upon exposure), and

pathological state, such as

liver disease (decrease the ability to metabolism a toxicant),
kidney disease (decrease the ability to excrete a toxicant),
heart disease (reduce its total clearance).