Calculated from the terminal slope of a plot of the natural logarithm of the concentration in the
central compartment as a function of time, this half-life is designated the biological half-life. It
is the parameter most frequently used to characterize the in vivo kinetic behavior of an exogenous
compound.
Other features of chemical kinetic behavior or of mode of administration may be incorporated into
the model as appropriate. For example, there may be more than one peripheral tissue compartment, as
in Figure 2.1; or absorption, which is never truly instantaneous even for intravenous injection, may be
first-order instead. An oral exposure, in which the rate of absorption is usually considered to be directly
proportional to the amount remaining available in the GI tract, is an example of first-order uptake.
The important group of models that incorporate non-first-order kinetics should also be mentioned.
Absorption and distribution are conventionally considered to be passive, first-order processes unless
observation dictates otherwise. However, elimination often is not first-order. Frequently this is because
excretion or metabolism is saturable, or capacity-limited, due to a limitation on the maximum number
of active transport sites in organs of excretion or the maximum number of active sites on metabolizing
enzymes. When all active elimination sites are occupied, the elimination process is said to be saturated.
Kinetically it is a zero-order process, operating at a constant maximum rate independent of the amount
or concentration of the chemical in the body. At very low concentrations at which relatively few
elimination sites are occupied, capacity-limited kinetics reduces to pseudo-first-order kinetics. Capac-
ity-limited kinetics is often referred to as
Michaelis–Menten kinetics
, after the authors of an early paper
analyzing and interpreting this type of kinetic behavior. Classical kinetic models incorporating
Michaelis–Menten elimination have been developed.
Figure 2.7 Plot of the logarithm of the concentration versus time for the linear one-compartment open model. C
0
is the concentration at time t = 0, assuming instantaneous distribution. (Reproduced with permission from
O’Flaherty, 1981, Figure 2.15a.)
2.4 DISPOSITION: DISTRIBUTION AND ELIMINATION
47
Most industrial or environmental exposures are not acute. Acute exposures do occur, but chronic
exposures are much more frequent in both industrial and environmental settings. When exposure is

reasons why biological half-life is such an important attribute. Together with exposure rate, it
determines mean steady-state blood level irrespective of whether exposure is continuous or intermit-
tent. However, the individual exposed to large amounts of a substance at wide intervals will experience
greater peak concentrations in blood and tissues following each new exposure than will an individual
exposed to the same total amount as frequent small exposures. If the large peak concentrations are
associated with toxicity or with saturation of elimination processes, then it becomes important to
consider the pattern of administration as well as the equivalent mean exposure rate.
Physiologically Based Kinetic Models
Physiologically based kinetic (PBK) models are simplified
but anatomically and physiologically reasonable models of the body. Tissues are selected or grouped
according to their perfusion (blood flow) characteristics and whether they are sites of absorption or
elimination (by excretion or metabolism). The model design process is facilitated by reference to
compilations of anatomic and physiologic data, including tissue and organ perfusion rates, that are
now widely available.
Within this general structural framework, the kinetic behavior of the selected chemical is modeled.
A key question is how the chemical is taken up into tissues. When flow-limited kinetics are assumed,
the chemical is presumed to be in equilibrium between each tissue group and the venous blood leaving
Figure 2.10 The relationship between average concentration C
__
(
n
)
, calculated for repetitive administation, and the
time course of concentration change during continuous administration of a hypothetical compound. C
max
and C
min
are the maximum and minimum concentrations in each time interval between doses, assuming instantaneous
distribution of each successive dose. (Reproduced with permission from O’Flaherty, 1981. Figure 5-4.)
2.4 DISPOSITION: DISTRIBUTION AND ELIMINATION

Intestine
Excretion
Kidney
Excretion
Figure 2.11 Schematic diagram of a physiologically-based model of the kinetic behavior of a volatile chemical
compound.
50
ABSORPTION, DISTRIBUTION, AND ELIMINATION OF TOXIC AGENTS
between liver and intestine is also included in the model. These features of the model are choices made
by the model developer, and reflect the known physicochemical behavior of the agent whose kinetics
are being modeled. Models for other chemicals will be quite different. A model for a nonvolatile
chemical would not include an explicit lung compartment, while models for bone-seeking elements
like lead and uranium include bone as a distinct tissue.
In a sense, classical and PBK models work in opposite directions. In classical descriptive kinetics,
model compartments having no necessary relationship to actual tissue volumes and clearances having
no necessary relationship to tissue blood flow are inferred from a set of concentration data. In contrast,
the PBK model is constructed from basic anatomic, physiologic, physicochemical, and metabolic
building blocks. It is then used to simulate concentrations under a defined set of conditions, and its
predictions are compared with observations. If the predictions are not accurate, some premise of the
model is at fault. The need for model revision can afford insight into the processes that control the
kinetic behavior of the chemical.
A PBK model for dichloromethane (DCM) forms the basis of a current human health risk
assessment. DCM is metabolized by two pathways, a capacity-limited oxidative pathway and first-
order conjugation with glutathione (for descriptions of these biotransformation processes, see Chapter
3). Either pathway was thought potentially capable of generating reactive intermediates involved in
the tumorigenicity of DCM in mice. Andersen et al. (1987) demonstrated that tumorigenicity correlated
well with the activity of the glutathione pathway, but not with the activity of the oxidative pathway.
These investigators scaled a PBK model developed for DCM from mouse to human and from high
dose to low dose in order to predict, based on studies carried out at high doses in mice, the risk associated
with human environmental exposure to DCM. The mouse-to-human scaling of metabolism relied on

determines the amount of the chemical present in a nonionized form.
It is to be expected that some control could be exerted over the rate of excretion of weak acids and
bases by adjusting urine pH. This type of treatment can be used very effectively in some cases.
Alkalinization of the urine by administration of bicarbonate has been used to treat salicylic acid
poisoning in humans. Alkalinization causes the weak acid to become more fully ionized; the ionized
molecule is excreted in the urine rather than reabsorbed.
There are also active secretory and reabsorptive processes in the renal tubules of the kidney. These
processes are specialized to handle endogenous compounds; active reabsorption helps to conserve the
essential nutrients, glucose and amino acids. These pathways can also be used by exogenous
compounds, provided the compounds have the structural and electronic configurations required by the
carrier molecules.
The renal clearance represents a hypothetical plasma volume cleared of solute by the net action of
all renal mechanisms during the specified period of time. A compound such as creatinine that is filtered
but not secreted or reabsorbed is cleared in adult humans at a rate of about 125 mL/min. Compounds
that are reabsorbed as well as filtered have clearances less than the creatinine clearance. Compounds
that are actively secreted can have clearances as large as the renal plasma flow, about 600 mL/min.
The presence of disease in the kidney can affect the half-life of a compound eliminated via the
kidney, just as the presence of disease in the liver can affect the half-life of a compound that is largely
biotransformed.
Excretion in the Liver
The liver is both the major metabolizing organ and a major excretory organ.
Large fractions of many toxicants absorbed from the gastrointestinal tract are eliminated in the liver
by metabolism or excretion before they can reach the systemic circulation, the hepatic first-pass effect.
In addition, metabolites formed in the liver may be excreted into the bile before they themselves have
had a chance to circulate. Although it does not excrete as many different compounds as the kidney
does, the liver is in an advantageous position with regard to excretion, particularly of metabolites.
There are at least three active systems for transport of organic compounds from liver into bile: one
for acids, one for bases, and one for neutral compounds. Certain metals are also excreted into bile
against a concentration gradient. These transport processes are efficient and can extract protein-bound
as well as free chemicals. The characteristics that determine whether a compound will be excreted in

proportional to its concentration in the blood. Essentially, pulmonary excretion is the reverse of the
uptake process, in that compounds with low solubility in the blood are perfusion-limited in their rate
of excretion, whereas those with high solubility are ventilation-limited. Highly lipophilic chemicals
that have accumulated in lipid depots may be present in expired air for a very long time after exposure.
Other Routes of Excretion
Skin, hair, sweat, nails, and milk are other, usually minor routes of
excretion. Hair can be a significant route of excretion for furred animals, and indeed the amount of a
metal in hair, like the amount of a volatile compound in exhaled air, can be used as an index of exposure
in both laboratory animals and humans. Hair is not quantitatively an important route of excretion in
humans, however. Sweat and nails are only rarely of interest as routes of excretion, simply because
loss by these routes is quantitatively so slight.
Milk may be a major route of excretion for some compounds. Milk has a relatively high fat content,
3–5 percent or even higher, and therefore compounds that are lipophilic may be excreted in milk to a
significant extent. Some of the toxicants known to be present in milk are the highly lipid-soluble
chlorinated hydrocarbons: for example, the polychlorinated biphenyls (PCBs) and DDT. Certain heavy
metals may also be excreted in milk. Lead is thought to be secreted into milk by the calcium transport
process.
2.5 SUMMARY
This chapter has conveyed some of the general biochemical and physiological principles that govern
absorption, distribution, and elimination of toxic agents, in particular
•
The importance of lipid solubility, molecular size, and degree of ionization to the rate at
which a molecule moves through a membrane by passive transfer or diffusion.
•
The characteristics of other transfer processes such as facilitated diffusion, active transport,
phagocytosis, and pinocytosis.
•
Absorption from the gastrointestinal tract with particular emphasis on the importance of pH
as a determinant of absorption of ionizable organic acids and bases as well as on compound-
specific and host-related factors such as lipid solubility and molecular size, the presence of

500–506 (1981).
Brodie, B. B., H. Kurz, and L. S. Shanker, “The importance of dissociation constant and lipid-solubility in
influencing the passage of drugs into the cerebrospinal fluid,” J. Pharmacol. Exp. Therap.
130:
20–25 (1960).
Chamberlain, A. C., M. J. Heard, P. Little, D. Newton, A. C. Wells, and R. D. Wiffen. Investigations into Lead from
Motor Vehicles, AERE. Publication N2R9198, Harwell, England, 1978.
Crouthamel, W. G., J. T. Doluisio, R. E. Johnson, and L. Diamond, “ Effect of mesenteric blood flow on intestinal
drug absorption,” J. Pharm. Sci.
59:
878–879 (1970).
English, J. C., R. D. R. Parker, R. P. Sharma, and S. G. Oberg, “ Toxicokinetics of nickel in rats after intratracheal
administration of a soluble and insoluble form,” Am. Ind. Hyg. Assoc. J.
42:
486–492 (1981).
Gariépy, L., D. Fenyves, and J P. Villeneuve, “Propranolol disposition in the rat: Variation in hepatic extraction
with unbound drug fraction,” J. Pharm. Sci.
81:
255–258 (1992).
Gregus, Z., and C. D. Klaassen, “Disposition of metals in rats: A comparative study of fecal, urinary, and biliary
excretion and tissue distribution of eighteen metals,” Toxicol. Appl. Pharmacol.
85:
24–38 (1986).
Guidotti, G., “ The structure of membrane transport systems,” Trends Biochem. Sci.
1:
11–12 (1976).
Hamilton, D. L., and M. W. Smith, “ Inhibition of intestinal calcium uptake by cadmium and the effect of a low
calcium diet on cadmium retention,” Environ. Res.
15:
175–184 (1978).

sequences, gene mapping, accession numbers, early trivial names of enzymes, and nomenclature,” DNA Cell
Biol.
12:
1–51 (1993).
O’Flaherty, E. J., Toxicants and Drugs: Kinetics and Dynamics, Wiley, New York, 1981.
O’Flaherty, E. J., “ Physiologically based models for bone-seeking elements. IV. Kinetics of lead disposition in
humans,” Toxicol. Appl. Pharmacol.
118:
16–29 (1993).
Rollins, D. E., and C. D. Klaassen, “Biliary excretion of drugs in man,” Clin. Pharmacokinet.
4:
368–379 (1979).
Schanker, L. S., and J. J. Jeffrey, “Active transport of foreign pyrimidines across the intestinal epithelium,” Nature
190:
727–728 (1961).
54
ABSORPTION, DISTRIBUTION, AND ELIMINATION OF TOXIC AGENTS
Sha’afi, R. I., C. M. Gary-Bobo, and A. K. Solomon, “ Permeability of red cell membranes to small hydrophilic
and lipophilic solutes,” J. Gen. Physiol. 58: 238–258 (1971).
U.S. Environmental Protection Agency, Update to the Health Risk Assessment Document and Addendum for
Dichloromethane: Pharmacokinetics, Mechanism of Action and Epidemiology, EPA 600/8-87/030A (1987).
Wagner, J. G., “ Properties of the Michaelis-Menten equation and its integrated form which are useful in
pharmacokinetics,” J. Pharmacokinet. Biopharmaceut. 1: 103–121 (1973).
Williams, R. T., “Interspecies scaling,” in T. Teorell, R. L. Dedrick, and P. G. Condliffe, eds., Pharmacology and
Pharmacokinetics, Plenum, New York, 1974, Table IV, p. 108.
REFERENCES AND SUGGESTED READING
55
3
Biotransformation: A Balance between
Bioactivation and Detoxification

Biotransformation
is defined as the chemical alteration of substances by reactions in the living
organism. For convenience, the conversion of xenobiotics is divided into two phases: metabolic
transformations (phase I reactions) and conjugation with natural body constituents (phase II reactions)
(Figure 3.3). The reactions of both of these phases are predominantly enzyme-catalyzed. A xenobiotic
does not necessarily undergo metabolism by a sequential combination of phase I followed by phase II
reactions for successful elimination. It may undergo phase I metabolism alone, phase II alone, and
occasionally, phase I reactions subsequent to phase II conjugations are encountered.
An important objective of biotransformation is to promote the excretion of absorbed chemicals by
the formation of water-soluble drug metabolites or products (
p
in Figure 3.1). Increased water solubility
is derived primarily from the phase II reactions since most conjugates exist in the ionized state at
physiological pH levels. This promotes excretion (
e
in Figure 3.1) by decreasing xenobiotic reabsorp-
Principles of Toxicology: Environmental and Industrial Applications, Second Edition
, Edited by Phillip L. Williams,
Robert C. James, and Stephen M. Roberts.
ISBN 0-471-29321-0 © 2000 John Wiley & Sons, Inc.
57
tion from the renal tubule following glomerular filtration or active secretion (
f
and
s
, respectively, in
Figures 3.1 and 3.2) and from the gastrointestinal tract following biliary secretion. Biotransformation
also decreases the entry of xenobiotics into cells of all organs and makes them more suitable for
secretion by active transport mechanisms into the bile and urine. Active secretion requires both energy
and a carrier protein and is capable of forcing molecules up a chemical gradient. Of the carrier

As stated above, the conversion of xenobiotics is divided into the two phases of metabolic transformation
and conjugation (Figure 3.3). The main chemical reactions involved in phase I or metabolic transformation,
in approximate order of capacity or importance, are oxidation, hydrolysis, and reduction. Of the phase II or
conjugation reactions, glucuronidations are generally the most prevalent in mammals, with the other
conjugations having lesser overall capacity. All conjugation reactions, except with glutathione, involve the
participation of energy-rich or activated cosubstrates. Conjugation with the cellular nucleophile, glutathione,
is an especially important mechanism for the sequestering of electrophilic intermediates generated during
phase I metabolism, and it can occur, albeit less efficiently, in the absence of enzyme.
As mentioned above, with reference to the generation of electophilic metabolites, biotransformation can
have a variety of effects on the biological reactivity of the xenobiotic. The chemical can be inactivated or
detoxified, can be changed into a more toxic substance (bioactivated), or can be changed into other chemical
entities having effects that differ both quantitatively and qualitatively from the parent compound (Table 3.1).
Generally, phase II metabolites are inactive, but important exceptions exist. Phase I metabolites
may or may not be inactive, and many are more reactive than the original xenobiotic. The greater
reactivity can be viewed as an unfortunate necessary prerequisite to conjugation, which is the step
contributing most to the facilitation of excretion (Figure 3.4).
Figure 3.2 The role of metabolism in increasing urinary excretion.
BIOTRANSFORMATION: A BALANCE BETWEEN BIOACTIVATION AND DETOXIFICATION 59
Figure 3.3
Xenobiotic metabolism summary; reaction characteristics and
flowchart.
60
TABLE 3.1 Pharmacologic Effects with Xenobiotic Metabolism
Phase I Phase II
Active to Inactive
Amphetamine —P450
→
phenylacetone Acetaminophen —UGT/ST
→
Cocaine —esterase

phenobarbital Thiobarbital —P450
→
barbital
Inactive to Active
Chloral hydrate —reductase
→
trichloroethanol
Prontosil —reductase
→
sulfanilamide
Sulindac —reductase
→
sulfide
Inactive to Toxic
Acetaminophen —P450
→

N-acetyl-p-benzoquinine imine
N-Hydroxyacetylaminofluorene —ST
→
Acetylhydrazine —P450
→
acetylcarbonium ion N-Hydroxymethylaminoazobenzene —ST
→
Aflatoxin —P450
→
aflatoxin-8,9 epoxide Tetrachloroethylene —GST
→
Malathion —P450
→

Although it is not the first tissue of the body to be exposed to chemicals, the liver receives the entire
chemical load absorbed from the gastrointestinal tract, which is the predominant portal of entry for
most xenobiotics (Figure 3.1). The xenobiotic metabolizing enzymes are present in high concentrations
and the organ itself has large bulk, approximately 5 percent of the total body weight. Xenobiotics
absorbed from the lungs and skin can also quickly move to the liver for metabolism. Once in the liver,
the highly vascular nature of the tissue and the intimate contact between blood and hepatocytes, which
contain the xenobiotic metabolizing enzymes, allows for the rapid diffusion of chemicals in and
metabolites out (Figure 3.5).
Although not a portal of entry, the kidney is an organ where xenobiotics are likely to be
concentrated during the excretion process, and this may be the reason for the relatively high level
of xenobiotic metabolizing enzymes in this tissue. Although the data presented in Table 3.2 are
from laboratory animals, there is little evidence to contraindicate the existence of a similar
distribution pattern in humans.
Within the liver, hepatocytes or parenchymal cells are the major site of drug biotransformation,
and within these cells it is the endoplasmic reticulum, which occupies about 15 percent of the
hepatocyte volume and contains 20 percent of the hepatocyte protein, which houses the bulk of
the critical drug metabolizing enzyme activity. (The nonparenchymal cells, including endothelial
and Kupffer cells, constitute 35 percent of liver cell number but only contribute 5–10 percent of
liver mass. Their drug metabolizing enzyme activities are typically less than 20 percent of that in
hepatocytes).
When liver is carefully homogenized, fragments of the endoplasmic reticulum are converted to
microsomes (an artifact of cell disruption). The drug-metabolizing enzymes located in the endoplasmic
reticulum are often referred to as
microsomal enzymes
, and it is often stated that chemicals are
metabolized by liver microsomes. Enriched microsomal fractions are usually obtained by differential
sedimentation, either as a suspension with cytoplasm (10,000
g
supernatant) or as a sediment free of
cytosol (105,000

p
-nitrophenol)
b
0.4 — 6.6 2.9
Glutathione
S
-transferase (DCNB)
b
5.3 — 21.9 7.4
Rat
Cytochrome P450 0.09 0.05 0.84 0.12 0.01
Ethoxyresorufin demethylase (P4501A) 0.003 0.001 0.034 0.001
Erythromycin demethylase (P4503A) — 0.12 0.47 0.06
UDP-glucuronosyltransferase (
p
-nitrophenol)
b
0.8 — 4.4 3.3
Glutathione
S
-transferase (DCNB)
b
2.1 — 76.4 3.8
a
All activities are expressed on a per milligram of protein basis (DCNB = 1,2-dichloro 4-nitrobenzene).
b
Litterst CL, Mimnaugh EG, Reagan RL, Gram TE,
Drug Metab. Disp.

3:

g
for 15
min
Light mitochondria sedimented as pellet
5 18,000
g
supernatant centrifuged at 105,000
g
for 60
min
Microsomes sedimented as pellet leaving nonturbid
cytosol in 0.2 M sucrose supernatant
Figure 3.6 Diagram of the subcellular localization and organization of major xenobiotic metabolizing enzymes
and necessary cofactors.
Abbreviations (clockwise) are ST = sulfotransferase; PAPS = adenosine 3
′
-phosphate 5
′
-phosphosulfate; GST = glutathione
S
-transferase; GSH = glutathione; AlcDH = alcohol dehydrogenase; ES = esterase; FP
1
= NADH cytochrome
b
5
reductase; b
5
=
cytochrome
b

monooxygenases
. The terminal oxidase is generally a hemoprotein called
cytochrome P450
but can be a flavoprotein.
Figure 3.7 Possible metabolic conversions of a simple hypothetical xenobiotic.
3.2 BIOTRANSFORMATION REACTIONS
65
Cytochrome P450 is a collective term for a group of related hemoproteins, all with a molecular
weight (MW) around 50,000 daltons, which as will be seen later, differ in their substrate selectivity
and in their ability to be induced and inhibited by drugs and chemicals (Table 3.4).
Cytochrome P450–catalyzed oxidations are categorized by the nature of the atom that is oxidized
(see Figure 3.8). Subsequent to the oxidation, the oxygen atom from molecular oxygen may be retained
within the major fragment of the chemical or it may be eliminated by molecular rearrangement (e.g.,
O
and
N
dealkylations).
Whatever the atom oxidized, or the name given to the reaction, the cytochrome P450–mediated
oxidation involves the same cyclic three-step series (Figure 3.9).
Step 1.
The xenobiotic [
X
] first binds to the cytochrome at a substrate binding site on the protein and
alters the conformation sufficiently to enable the efficient transfer of electrons to the heme from
NADPH via a nearby (see Figure 3.6) flavoprotein, NADPH cytochrome P450 reductase. (The
activity of this FAD- and FMN-containing flavoprotein is often determined experimentally using
exogenously added mitochondrial cytochrome
c
rather than microsomal cytochrome P450 as the
electron acceptor and so is often identified as NADPH cytochrome

the important feature for metabolism being a heteroatom (nitrogen, sulfur) presenting a lone pair of
electrons (Table 3.5).
Some compounds are metabolized both by flavin-containing monooxygenases and cytochrome
P450 but to different products. An example is dimethylaniline, which is metabolized to the
N
-oxide
by the flavoprotein and is
N
-demethylated by cytochrome P450.
Nonmicrosomal
Oxidations in other subcellular organelles can be catalyzed by flavoproteins (e.g.,
monoamine oxidase in mitochondria) or pyridine nucleotide linked dehydrogenases (e.g., alcohol and
aldehyde dehydrogenases in cytoplasm).
Dehydrogenase-catalyzed oxidations do not involve molecular oxygen. The oxidation of the
chemicals or drugs occurs through electron transfer to a pyridine nucleotide, usually NAD
+
. Most of
the dehydrogenases are cytoplasmic in location. The most noteworthy of this class of enzymes in
humans is the dehydrogenase responsible for the metabolism of ethanol. In contrast to the major
66
BIOTRANSFORMATION: A BALANCE BETWEEN BIOACTIVATION AND DETOXIFICATION
TABLE 3.4 Important Cytochrome P450s
a
Subfamily
Forms Present in
Tissue
Substrates/
Reactions Inducers InhibitorsRat Rabbit Human Mouse
1A 1,2 1,2 1,2 1,2 liver, EH ethoxyresorufin,
phenacetin deE, caffeine 3

4
,
4-nitrophenol-OH
ethanol, disulfiram
ketones, pyridine,
isoniazid
2F 1 2 lung 3-methylindole
naphthalene
67
TABLE 3.4 Important Cytochrome P450s
a
Subfamily
Forms Present in
Tissue
Substrates/
Reactions Inducers InhibitorsRat Rabbit Human Mouse
3A 1,2 6 3,4,5,7 11,13 erythromycin
N
deM,
TAO MI complex,
cyclosporine, quinidine,
testosterone, and cortisol
6
β
-OH, mephenytoin
(rat), benzphetamine,
nifedipine, and other
dihydropyridines
glucocorticoids DEX,
PCN, macrolides, TAO

PB = phenobarbital
RIF = rifampicin
TAO = troleandomycin
TCDD = 2,3,7,8-tetrachlorodibenzo-p-dioxin
68
Figure 3.8 Cytochrome P450–catalyzed oxidations.
3.2 BIOTRANSFORMATION REACTIONS
69
microsomal oxidizing enzyme, these enzymes are not subject to extensive induction (see discussion
later).
Monoamine oxidases, which are usually mitochondrial in location, oxidize by electron transfer to
a flavin group. Monoamine oxidases are responsible for the normal metabolism of neurotransmitters,
and exposure to agents, which are also metabolized by this enzyme, (e.g., tyramine) can result in
toxicities or pharmacological effects arising from accumulation of the unmetabolized neurotransmitter.
A neurotoxin of much recent interest, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), which
leads to Parkinson’s syndrome, is bioactivated by monoamine oxidase B (a form selectively inhibited
by deprenyl and located in serotonergic neurons in the brain). Environmental compounds or drugs that
are also tetrahydropyridines have been speculated to be causative agents in Parkinson’s disease in the
elderly.
Phase I; Hydrolyses
Hydrolysis reactions are catalyzed by esterases and amidases. While both can be microsomal, esterases
are predominantly cytosolic in location. Hydrolysis of amides and esters produces two reactive centers,
Figure 3.9 The cytochrome P450 oxidation cycle.
TABLE 3.5 Compounds Metabolized by the Flavin-Containing Monooxygenases
Heteroatom Class Examples
Nitrogen Tertiary amine N-Dimethylaniline, imipramine, amitryptyline
Secondary amine N-Methylaniline, desipramine, nortryptyline
Sulfur Thiocarbamides Thiourea, propylthiouracil, methimazole
Thioamides Thioacetamide
Thiols Dithiothreitol,

glucuronic acid from a uridinediphosphoglucuronic acid (UDPGA) cofactor to a carboxyl or hydroxyl
(phenol), or less often an amine group on the xenobiotic (or phase I metabolite) (Figure 3.3). The
UDPGA is generated from the abundant carbohydrate supply in the liver as glucose-1-phosphate, and
following the reaction with UTP, the resultant UDP-glucose is oxidized. The formation of the
glucuronide does not involve the acid group of glucuronic acid, so the conjugate retains acid and ionized
character at physiological pH, providing dramatic enhancement of water solubility and excretability
to the xenobiotic. Glucuronides are actively secreted into bile and in the proximal tubule of the kidney.
Xenobiotics conjugated as glucuronides can be released as either a phase I metabolite or the original
molecule by the action of glucuronidases of both mammalian and microbial origin.
UDP-glucuronosyltransferases occur in multiple forms. The most common classification utilized
for the enzymes responsible for the metabolism of xenobiotics are those (GT1) that conjugate planar
phenols (e.g., 1-naphthol, 4-nitrophenol) and are induced by polycyclic hydrocarbon-like molecules
(see Table 3.6) and those (GT2) that conjugate nonplanar phenols (e.g. morphine, chloramphenicol)
and are induced by phenobarbital and similar compounds. There are other forms which appear to be
more selective for endogenous substrates, notably those for the 17 hydroxysteroids (testosterone), the
3 hydroxysteroids (androsterone) and bilirubin. More recent studies using the powerful techniques of
molecular biology have provided a more rational classification system, but to aid the reader in
understanding the bulk of existing literature, the old system has been used in this chapter. Like
cytochrome P450s, UDP-glucuronosyltransferases are often substrate selective rather than substrate
specific, being able to metabolize a wide range of compounds poorly (e.g., 4-nitrophenol is conjugated
by almost all isozymes) while metabolizing substrates with particular characteristics very efficiently.
Also like cytochrome P450s, more than one form may be induced by a xenobiotic inducing agent (both
bilirubin and testosterone as well as morphine conjugations are induced by phenobarbital).
Phase II; Sulfation
Sulfate conjugation is an important alternative to glucuronidation for phenolic compounds and
occasionally arylamines. Sulfate availability within the cell may be limited, so this conjugation pathway
decreases in importance with higher xenobiotic or phenolic metabolite concentrations. The 3′-phos-
phoadenosine-5′-phosphosulfate (PAPS) cofactor from which the sulfate group is transferred is
generated from ATP and inorganic sulfate. The sulfate can be derived from the sulfur containing amino
acids, cysteine and methionine. The enzymes catalyzing the sulfate conjugations are a family of