FROM:
Alternative Medicine Review 1998 (Jun); 3 (3): 187–198 ~ FULL TEXT
DeAnn J. Liska, Ph.D.
Introduction
We are exposed to a great number of xenobiotics during
the course of our lifetime, including a variety of pharmaceuticals and
food components. Many of these compounds show little relationship to previously
encountered compounds or metabolites, and yet our bodies are capable of
managing environmental exposure by detoxifying them. To accomplish this
task, our bodies have evolved complex systems of detoxification enzymes.
These enzyme systems generally function adequately to minimize the potential
of damage from xenobiotics. However, much literature suggests an association
between impaired detoxification and disease, such as cancer, Parkinson's
disease, fibromyalgia, and chronic fatigue/immune dysfunction syndrome.
Therefore, accumulated data suggests an individual's ability to remove
toxins from the body may play a role in etiology or exacerbation of a range
of chronic conditions and diseases.
The detoxification systems are highly complex, show a
great amount of individual variability, and are extremely responsive to
an individual's environment, lifestyle, and genetic uniqueness. This review
of the detoxification systems is meant to whet the appetite for a more
in-depth look at detoxification and, as such, it may raise more questions
than it answers.
Discovery of Detoxification Reactions
The hypothesis that xenobiotics consumed by animals are
transformed to water-soluble substances and excreted through the urine
was first put forth in the late 18th century. For years, scientists collected
urine from various animals, purifying and then chemically characterizing
the compounds present in an attempt to understand how the body managed
to remove various xenobiotics. Hippuric acid, discovered in 1773, was one
of the first metabolites identified in these early studies and, from chemical
characterization, was proposed to result from the conjugation of glycine
with benzoic acid (Figure 1). However, it
was not until 1842 that this hypothesis was officially tested. Keller is
attributed with performing the first challenge test, in which he took a
dose of benzoic acid, collected his urine, and showed a direct relationship
between ingestion of benzoic acid and the hippuric acid subsequently excreted. [1]
For more than 100 years after this observation, research
continued in the identification of various metabolites, and a variety of
conjugation reactions were identified. During this time, glucuronic acid,
sulfate, glycine, glutamine, taurine, ornithine, and glutathione were identified
as conjugating substances (Table 1). Although
the conjugation reactions solved the puzzle of how a non-water-soluble
compound can be converted to a substance that could be excreted in urine,
it raised another question. In all these cases of conjugation, the xenobiotic
is required to have the ability to react with the conjugating moiety, i.e.,
to have an active center or "functional" group to react with,
and bind, the conjugating moiety. What happens with compounds that do not
have a reactive site?
In his landmark 1947 monograph, Detoxification Mechanisms,
R.T. Williams defined the field of detoxification. Williams proposed that
these non-reactive compounds could be biotransformed in two phases: functionalization,
which uses oxygen to form a reactive site, and conjugation, which results
in addition of a water-soluble group to the reactive site. [2]
These two steps, functionalization and conjugation, are termed Phase I
and Phase II detoxification, respectively. The result is the biotransformation
of a lipophilic compound, not able to be excreted in urine, to a water-soluble
compound able to be removed in urine (Figure 2). Therefore, detoxification is not one reaction, but rather a process
that involves multiple reactions and multiple players.
Today, the challenge to understand detoxification continues.
The question of how the body can handle such a wide range of compounds
it has never before seen has led to considerable study in an attempt to
understand the protein structure and regulation of various enzymes involved
in detoxification. It is now known one mechanism the body uses is a battery
of enzymes, each with broad specificity, to manage this challenge. Currently,
over 10 families of Phase I enzymes have been described, which include
at least 35 different genes. Phase II reactions are equally complex, and
involve multiple gene families as well.
Enzyme Systems Involved in Detoxification
The Phase I System: The Phase I detoxification
system, composed mainly of the cytochrome P450 supergene family of enzymes,
is generally the first enzymatic defense against foreign compounds. Most
pharmaceuticals are metabolized through Phase I biotransformation. In a
typical Phase I reaction, a cytochrome P450 enzyme (CypP450) uses oxygen
and, as a cofactor, NADH, to add a reactive group, such as a hydroxyl radical.
As a consequence of this step in detoxification, reactive molecules, which
may be more toxic than the parent molecule, are produced. If these reactive
molecules are not further metabolized by Phase II conjugation, they may
cause damage to proteins, RNA, and DNA within the cell. [4]
Several studies have shown evidence of associations between induced Phase
I and/or decreased Phase II activities and an increased risk of disease,
such as cancer, systemic lupus erythematosus, and Parkinson's disease. [5-10]
Compromised Phase I and/or Phase II activity has also been implicated in
adverse drug responses. [5,11,12]
As stated, at least 10 families of Phase I activities
have been described in humans(Table 2). The
major P450 enzymes involved in metabolism of drugs or exogenous toxins
are the Cyp3A4, Cyp1A1, Cyp1A2, Cyp2D6, and the Cyp2C enzymes (Figure
3). The amount of each of these enzymes present in the liver reflects
their importance in drug metabolism. [11,13]
Most information on the Phase I activities has been derived from studies
with drug metabolism; however, Phase I activities are also involved in
detoxifying endogenous molecules, such as steroids.
The Phase II System: Phase II conjugation reactions
generally follow Phase I activation, resulting in a xenobiotic that has
been transformed into a water-soluble compound that can be excreted through
urine or bile. Several types of conjugation reactions are present in the
body, including glucuronidation, sulfation, and glutathione and amino acid
conjugation (Table 3). These reactions require
cofactors which must be replenished through dietary sources.
Much is known about the role of Phase I enzyme systems
in metabolism of pharmaceuticals as well as their activation by environmental
toxins and specific food components. However, the role of Phase I detoxification
in clinical practice has received less consideration. The contribution
of the Phase II system has received lesser attention both in academic research
circles and in clinical practice. And, little is currently known about
the role of the detoxification systems in metabolism of endogenous compounds.
Is There a Phase III?: Recently, antiporter activity
(p-glycoprotein or multi-drug resistance) has been defined as the Phase
III detoxification system. [14] Antiporter
activity is an important factor in the first pass metabolism of pharmaceuticals
and other xenobiotics. The antiporter is an energy-dependent efflux pump,
which pumps xenobiotics out of a cell, thereby decreasing the intracellular
concentration of xenobiotics. [15]
Antiporter activity in the intestine appears to be co-regulated
with intestinal Phase I Cyp3A4 enzyme. [16]
This observation suggests the antiporter may support and promote detoxification.
Possibly, its function of pumping non-metabolized xenobiotics out of the
cell and back into the intestinal lumen may allow more opportunities for
Phase I activity to metabolize the xenobiotic before it is taken into circulation
(Figure 4).
Two genes encoding antiporter activity have been described:
the multi-drug resistance gene 1 (MDR1) and multi-drug resistance gene
2 (MDR2). [15] The MDR1 gene product is responsible
for drug resistance of many cancer cells, and is normally found in epithelial
cells in the liver, kidney, pancreas, small and large intestine, brain,
and testes. MDR2 activity is expressed primarily in the liver, and may
play a role similar to that of intestinal MDR1 for liver detoxification
enzymes; however, its function is currently undefined.
