tion results in substantial increase in xenobiotic hydrophilicity, thus promoting
the excretion of xenobiotics. Many xenobiotics are lipophilic and undergo
Phase I and Phase II reactions sequentially, whereas others may participate in
only one phase. In this case, a toxicant may combine directly with an
endogenous substance, forming a conjugate. Endogenous substances known to
participate only in Phase II reactions include glycine, cysteine, GSH,
glucuronic acid, sulfates, and some other water-soluble substances. Several
representative Phase II reactions are shown in Figure 6.3.
6.4 CHARACTERISTICS OF BIOTRANSFORMATION
The NADPH-cytochrome P450 system, commonly known as the mixedfunction oxygenase system (MFO system), is the most important enzyme system
involved in Phase I biotransformation. The cytochrome P450 system, localized
in the smooth endoplasmic reticulum of cells of most mammalian tissues, is
particularly abundant in the liver. Contrary to most enzymes, which catalyze
the metabolism of one substrate with one mechanism, quickly and efficiently,
the cytochrome P450 system contains a number of isozymes – multiforms of an
enzyme that are structurally equivalent but catalytically distinct from one
another – that can catalyze a variety of substrates with multiple mechanisms,
slowly and inefficiently (average turnover rate is one per minute). The reactions
that the isozymes catalyze include aliphatic or aromatic hydroxylation,
epoxidation of a double bond, N-oxidation, sulfoxidation, dealkylation,
deamination, dehydrogenation, dehalogenations, oxidative group transfer,
and cleavage of esters (Figure 6.2).
2
During the catalytic reaction, the oxidized form of an iron atom (Fe
3þ ) at
the active site of cytochrome P450 binds directly to the substrate (XH) (Figure
6.4). Reduction of this enzyme–substrate complex follows, with an electron
being transferred from NADPH via NADPH cytochrome P450 reductase. The
reduced (Fe
2þ
) enzyme–substrate complex binds molecular oxygen (O 2 ), and is
reduced further by a second electron (presumably donated by NADH via
cytochrome b 5 and NADH cytochrome b 5 reductase). The enzyme–substrate–
oxygen complex splits into oxidized substrate, water, and the oxidized form of
the enzyme. The overall reaction by which a substrate or an environmental
chemical, XH, is oxidized by the cytochrome P450 system is shown in Reaction
6.1:
XH ðsubstrateÞ þ O 2 þ NADPH þ H
þ ! XOH ðproductÞ þ H 2 O þ NADP
þ
ð6:1Þ
As shown in Reaction 6.1, one atom from O 2 is reduced to water and the
other is incorporated into the substrate, producing ROH, a hydroxylated
metabolite. The constituents required in this enzyme system are O 2 , NADPH,
and magnesium ions (Mg
2þ ).
Biotransformation – Metabolism of Xenobiotics
89
[16:54 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-006.3d]
Ref: 4365 MING-HO YU Chap-006 Page: 89 85-98
the excretion of xenobiotics. Many xenobiotics are lipophilic and undergo
Phase I and Phase II reactions sequentially, whereas others may participate in
only one phase. In this case, a toxicant may combine directly with an
endogenous substance, forming a conjugate. Endogenous substances known to
participate only in Phase II reactions include glycine, cysteine, GSH,
glucuronic acid, sulfates, and some other water-soluble substances. Several
representative Phase II reactions are shown in Figure 6.3.
6.4 CHARACTERISTICS OF BIOTRANSFORMATION
The NADPH-cytochrome P450 system, commonly known as the mixedfunction oxygenase system (MFO system), is the most important enzyme system
involved in Phase I biotransformation. The cytochrome P450 system, localized
in the smooth endoplasmic reticulum of cells of most mammalian tissues, is
particularly abundant in the liver. Contrary to most enzymes, which catalyze
the metabolism of one substrate with one mechanism, quickly and efficiently,
the cytochrome P450 system contains a number of isozymes – multiforms of an
enzyme that are structurally equivalent but catalytically distinct from one
another – that can catalyze a variety of substrates with multiple mechanisms,
slowly and inefficiently (average turnover rate is one per minute). The reactions
that the isozymes catalyze include aliphatic or aromatic hydroxylation,
epoxidation of a double bond, N-oxidation, sulfoxidation, dealkylation,
deamination, dehydrogenation, dehalogenations, oxidative group transfer,
and cleavage of esters (Figure 6.2).
2
During the catalytic reaction, the oxidized form of an iron atom (Fe
3þ ) at
the active site of cytochrome P450 binds directly to the substrate (XH) (Figure
6.4). Reduction of this enzyme–substrate complex follows, with an electron
being transferred from NADPH via NADPH cytochrome P450 reductase. The
reduced (Fe
2þ
) enzyme–substrate complex binds molecular oxygen (O 2 ), and is
reduced further by a second electron (presumably donated by NADH via
cytochrome b 5 and NADH cytochrome b 5 reductase). The enzyme–substrate–
oxygen complex splits into oxidized substrate, water, and the oxidized form of
the enzyme. The overall reaction by which a substrate or an environmental
chemical, XH, is oxidized by the cytochrome P450 system is shown in Reaction
6.1:
XH ðsubstrateÞ þ O 2 þ NADPH þ H
þ ! XOH ðproductÞ þ H 2 O þ NADP
þ
ð6:1Þ
As shown in Reaction 6.1, one atom from O 2 is reduced to water and the
other is incorporated into the substrate, producing ROH, a hydroxylated
metabolite. The constituents required in this enzyme system are O 2 , NADPH,
and magnesium ions (Mg
2þ ).
Biotransformation – Metabolism of Xenobiotics
89
[16:54 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-006.3d]
Ref: 4365 MING-HO YU Chap-006 Page: 89 85-98