PRINCIPLES OF TOXICOLOGY

72 BIOTRANSFORMATION: A BALANCE BETWEEN BIOACTIVATION AND DETOXIFICATION
Phase I; Reductions
Reductive metabolism in the liver endoplasmic reticulum can occur through the mediation of both
hemoprotein (cytochrome P450) and flavoproteins. Reductions of azo and nitro groups are the most
commonly encountered (Figure 3.10), but reduction of disulfides, sulfoxides, epoxides, and N-oxides
can also occur. In many instances, the products of reductive metabolism can be reoxidized under
aerobic conditions.
Phase II; Glucuronidation
Glucuronidations are catalyzed by a group of closely related 55,000-dalton isozymes, termed UDPglucuronosyltransferases,
located within the endoplasmic reticulum. They catalyze the transfer of
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′-phosphoadenosine-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
cytosolic 64,000-dalton enzymes, termed sulfotransferases, and are one of the exceptions to the major
groups of drug metabolizing enzymes in that they appear to not be induced by xenobiotic compounds
(see Table 3.6). The sulfates are completely ionized at physiological pH and easily eliminated. Much
like glucuronides, enzymes exist (termed sulfatases) that can break the conjugate and return the
xenobiotic, if it is phenolic, or the phase I metabolite of a xenobiotic, if it was oxidized or hydrolyzed
to that functional group.