bio transforming enzymes
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Biotransforming Enzymes, Substrates and End Products
Biotransformation is the process within our body whereby a substance is changed, mainly
transformed, from one chemical to another by a chemical reaction. It is also known as
Metabolism or metabolic transformations.
Biotransformation is vital to survival in that it transforms absorbed nutrients (food, oxygen,
etc.) into substances required for normal body functions. For some pharmaceuticals, it is a
metabolite that is therapeutic and not the absorbed drug. Like, phenoxybenzamine, a drug given
to relieve hypertension, is biotransformed into a metabolite, which is the active agent. It also
serves as an important defense mechanism in those toxic xenobiotics and body wastes are
converted into less harmful substances and that can be excreted from the body.
Biotransforming Enzyme:
Chemical reactions are continually taking place in the body. Most of these chemical reactions
occur at significant rates only because specific proteins, known as enzymes, are present to
catalyze them, that is, accelerate the reaction.
These enzymatic reactions are not always simple biochemical reactions. Some enzymes require
the presence of cofactors or co-enzymes in addition to the substrate before their catalyticactivity can be exerted. Substrate is the substance that will be catalyzed. These co-factors exist
as a normal component in most cells and are frequently involved in common reactions to
convert nutrients into energy; Vitamins are an example of co-factors. It is the drug or chemical
transforming enzymes that hold the key to xenobiotic transformation. The relationship of
substrate, enzyme, co-enzyme, and transformed product is illustrated below:
Figure 01: Relationship of substrate, enzyme, co-enzyme, and transformed product
Most biotransforming enzymes are high molecular weight proteins, composed of chains of
amino acids linked together by peptide bonds. While an enzyme may encounter many different
chemicals, only those chemicals (substrates) that fit within the enzymes convoluted structure
and spatial arrangement will be locked on and affected. This is sometimes referred to as the
"lock and key" relationship. As shown in Figure 1, when a substrate fits into the enzyme's
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Figure 03: Metabolism of Procaine
Figure 05: Metabolism of Serotonin
The reactions catalyzed by xenobiotic biotransforming enzymes generally are divided into two
groups:
1. Phase I reactions Non-synthetic reactions
2. Phase II reactions Synthetic reactions
Phase I reaction: Phase I reactions are generally reactions which modify the chemical by adding
a functional structure. This allows the substance to "fit" into the Phase II enzyme so that it can
become conjugated (joined together) with another substance.
In Phase I reactions, a small polar group (containing both positive and negative charges) is
either exposed on the toxicant or added to the toxicant. The main Phase I reactions are:
- Oxidation,
- Reduction,
- Hydrolysis.
Table 01: Example of type of reaction and enzymes participate in phase I reaction
Reaction Enzyme
Procaine p-Aminobenzoic
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1. Oxidation: Oxidation is a chemical reaction in which a substrate loses electrons. There
are a number of reactions that can achieve the removal of electrons from the substrate. The
specific oxidizing reactions and oxidizing enzymes are numerous. Here are several of these
oxidation reactions.
Alcohol dehydrogenation
Aldehyde dehydrogenation
Alkyl/acyclic hydroxylation
Aromatic hydroxylation
Deamination
Desulfuration
N-dealkylation
N-hydroxylation
N-oxidation
O-dealkylation
Sulphoxidation
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Table 02: An overview of possible types of phase I biotransformation reaction (oxidations)
Oxidation of Drugs by Cytochrome P450: The cytochrome P450 superfamily (officiallyabbreviated as CYP) is a large and diverse group ofenzymes. The function of most CYP
enzymes is to catalyze the oxidation oforganic substances. The substrates of CYP enzymes
include metabolic intermediates such as lipids and steroidal hormones, as well as xenobiotic
substances such as drugs and othertoxic chemicals. CYPs are the major enzymes involved in
drug metabolism and bioactivation, accounting for about 75% of the total number of different
metabolic reactions.
The most common reaction catalyzed by cytochromes P450 is a monooxygenase reaction, e.g.,
insertion of one atom of oxygen into an organic substrate (RH) while the other oxygen atom is
reduced to water:
http://en.wikipedia.org/wiki/Enzymehttp://en.wikipedia.org/wiki/Catalysishttp://en.wikipedia.org/wiki/Redoxhttp://en.wikipedia.org/wiki/Organic_compoundhttp://en.wikipedia.org/wiki/Enzyme_substratehttp://en.wikipedia.org/wiki/Metabolismhttp://en.wikipedia.org/wiki/Lipidhttp://en.wikipedia.org/wiki/Steroidhttp://en.wikipedia.org/wiki/Xenobiotichttp://en.wikipedia.org/wiki/Toxichttp://en.wikipedia.org/wiki/Drug_metabolismhttp://en.wikipedia.org/wiki/Monooxygenasehttp://en.wikipedia.org/wiki/Redoxhttp://en.wikipedia.org/wiki/Enzymehttp://en.wikipedia.org/wiki/Catalysishttp://en.wikipedia.org/wiki/Redoxhttp://en.wikipedia.org/wiki/Organic_compoundhttp://en.wikipedia.org/wiki/Enzyme_substratehttp://en.wikipedia.org/wiki/Metabolismhttp://en.wikipedia.org/wiki/Lipidhttp://en.wikipedia.org/wiki/Steroidhttp://en.wikipedia.org/wiki/Xenobiotichttp://en.wikipedia.org/wiki/Toxichttp://en.wikipedia.org/wiki/Drug_metabolismhttp://en.wikipedia.org/wiki/Monooxygenasehttp://en.wikipedia.org/wiki/Redox -
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RH + O2 + 2H+ + 2e ROH + H2O
Aliphatic Oxidation:
Aromatic Hydroxylation:
N-Dealkylation:
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N-Hydroxylation:
Desulfuration:
2. Reduction: Reduction is a chemical reaction in which the substrate gains electrons.
Reductions are most likely to occur with xenobiotics in which oxygen content is low. There
are fewer specific reduction reactions than oxidizing reactions. Here are several of these
reduction reactions.
Azo reduction
Dehalogenation
Disulfide reduction
Nitro reduction
N-oxide reduction
Sulfoxide reduction
Table 03: An overview of possible types of phase I biotransformation reaction (reductions)
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Azo Reduction:
Nitro Reduction:
Dehalogenation: There are three major mechanisms of dehalogenation:
i. Reductive dehalogenation: Halogen replaced with a hydrogen atom.
ii. Oxidative dehalogenation: Halogen & hydrogen replaced with oxygen
iii. Double dehalogenation: 2 halogens replaced from two adjacent Cs to form a C-Cdouble bond
3. Hydrolysis: Hydrolysis is a chemical reaction in which the addition of water splits the
toxicant into two fragments or smaller molecules. The hydroxyl group (OH-) is
incorporated into one fragment and the hydrogen atom is incorporated into the other. Larger
chemicals such as esters, amines, hydrazines, and carbamates are generally biotransformed
by hydrolysis.
Table 04: An overview of possible types of phase I biotransformation reaction (Hydrolysis)
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Ester Hydrolysis:
Epoxide Hydrolase:
Phase II reaction: Phase II reactions consist of those enzymatic reactions that conjugate the
modified xenobiotic with another substance. A xenobiotic that has undergone a Phase I reaction
is now a new intermediate metabolite that contains a reactive chemical group, e.g., hydroxyl (-
OH), amino (-NH2), and carboxyl (-COOH). Many of these intermediate metabolites do not
possess sufficient hydrophilicity to permit elimination from the body. These metabolites must
undergo additional biotransformation as a Phase II reaction.
Phase II reactions are conjugation reactions, that is, a molecule normally present in the body is
added to the reactive site of the Phase I metabolite. The result is a conjugated metabolite that is
more water-soluble than the original xenobiotic or Phase I metabolite. Usually the Phase II
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metabolite is quite hydrophilic and can be readily eliminated from the body. The primary Phase
II reactions are:
Glucuronide conjugation - most
important reaction
Sulfate conjugation - important
reaction
Acetylation
Amino acid conjugation
Glutathione conjugation
Methylation
Table 01: Example of type of reaction and enzymes participate in phase I reaction
Reaction Enzyme
Methylation S-Methyltransferases
O- Methyltransferases
N- Methyltransferases
Acetylation N-Acetyltransferases
Acetyltransferases
Glucuronide conjugation Glucuronosyltransferases
Glutathione conjugation Glutathione S-transferase
Glucuronide conjugation:
Amino acid conjugation:
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Methylation:
Acetylation: