**4. Metabolism of drug and other xenobiotics**

Metabolism of drugs and other xenobiotics involves activities that modify the chemical structure of the substances which are foreign to the body's internal mileu. These reactions often act to detoxify poisonous compounds; however in some cases, the intermediate metabolite can themselves be toxic

The purpose of biotransformation is to convert lypophilic compounds to hydrophilic ones which will facilitate their excretion. The consequences of biotransformation is changes in pharmacokinetic characteristics.

Pharmacogenetics: The Scientific Basis 187

enzyme activities. In addition, there is a distinct racial diversity in the frequency of the

classes. Examples of CYP450 catalyzed drug metabolic reactions include

ix. **Dehydrogenation:** Acetaminophen CYP2E1 N-Aacetyl benzoquinoneimine x. **Ester Cleavage**: Loratidine CYP3A4, CYP2D6 Desacetylated Loratidine xi. **Reduction**: Paraquat FLAVOPROTEIN REDUCTASE paraquat radicals

ii. **Reduction**: Chloral Hydrate ALC. DEHYDROGENASE Trichloroethanol

iii. **Oxidoreduction**: Alcohol ALC.DEHYDROGENASE Aldehyde

Besides the CYP 450 enzymes, other enzymes that participate in drug biotransformations include; monoamineoxidases, peroxidases, lactoperoxidases myeloperoxidases, prostaglandin-H-synthetase and flavin-containing monooxygenases.(FMO).Examples of the reactions they

In phase II reactions, the activated xenobiotic metabolites are conjugated with charged species such as glutathione (GSH), sulfate, glycine or glycuronic acid and increased risk of early renal complications in type 2 diabetes mellitus (Banjoko & Akinlade 2010). These reactions are catalysed by substrate specific transferases which in total can metabolize almost any hydrophobic compound that contains nucleophilic or electrophilic group.

One of the most important of this group is the glutathione S-transferase (GSTs). The addition of large anionic groups such as glutathione detoxifies reactive electrophiles and produces more polar metabolites that cannot diffuse across membranes and may therefore

Glutathione is a tripeptide of glycine, cysteine and glutamic acid formed by the action of glutamylcysteine synthetase (glutathione synthetase).The enzyme glutathione transferase catalyses the conjugation of modified xenobiotic with glutathione. A large number of drugs are conjugated by glutathione during metabolism. Inhibitors of the enzyme include Buthione – S – Sulfoxine. Two types of reactions are common with glutathione. The first is displacement of halogen, sulfate, sulfonate or phosphonitro group. The second is the addition of glutathione to activated double bond or strained ring system. Some of the

i. **Hydroxylation**: S-mephenytoin CYP3A4 4-OH-S-mephenytoin ii. **Epoxidation:** Carbamazepine CYP3A4/5 10,11 Epoxide iii. **Oxygenation**: Amines CYP 2D6 Hydroxylamines

iv. **O-dealkylation**: Dextromethorphan CYP2D6 Dextrophan

v. **N-demethylation**: Caffeine CYP2E1 Theobromine vi. **N-demethylation:** Caffeine CYP1A2 Paraxanthine vii. **N-demethylation** Caffeine CYP2E1 Theophylline viii. **Oxidative Group Transfer**: Parathion CYP2B6 Paraoxon

i. **Hydrolysis:** hydrolysis of peptide bond of Insulin

**4.1.1 Non P450 enzyme catalysis** 

catalyse include:

**4.2 Phase II reactions** 

be actively transported.

**4.2.1 Glutathione conjugation** 

conjugation reactions include:

Xenobiotics metabolism can be divided into three phases. In phase 1, enzymes such as cytochrome p450 oxidases introduce an active or polar group into the xenobiotics. These modified compounds are then conjugated to polar compounds in phase II reactions. The main enzyme that catalyses the reactions in phase II is glutathione S-transferase since it acts on a wide range of substrates.

The final phase; phase III may involve further metabolism of conjugates of phase II reactions like the processing of glutathione conjugates to acetylcysteine (mercapturic acic) conjugates before being recognized by efflux transporters and pumped out of the cells (Boyland & Chassaud 1969, Thomalley 1990)

Peculiar to all organisms is the possession of cell membranes which serve as hydrophobic permeability barriers to control access to their internal environment. Polar compounds cannot diffuse across these cell membranes, and the uptake of useful molecules is mediated through transport proteins that specifically select substrates from the extracellular mixture. The implication of this structure is that most hydrophilic molecules cannot enter the cells since they need to be recognized by specific transporters (Mizuno et al 2003).

The detoxification of reactive by-products is via a different mechanism. Because these species are derived from normal cellular constituents, they usually share the same polar characteristics therefore, specific designated enzymes can metabolize them. A notable example of these specific detoxification system is the glyoxalase system which catalyses the removal of the reactive aldehyde, methylglyoxal (Thormalley 1990) and the various antioxidant systems that eliminate reactive oxygen species (Sies 1997).

#### **4.1 Phase I reactions**

In Phase I reactions, a variety of enzymes act to introduce reactive and polar groups into their substrates. This is basically a functionalization reaction. One of the most common modifications in this phase is hydroxylation, a reaction catalysed by the cytochrome P-450 dependent mixed function oxidase system. These enzymes complexes act to incorporate an atom of oxygen into nonactivated hydrocarbons, which can result in either the introduction of hydroxyl groups, or Nitrogen, Oxygen.and Sulphate- dealkylation of substrates (Schlichting et al 2000).Of all the enzymes involved in drug metabolism, the cytochrome P450.(CYP450) is regarded as the most important because many drugs are substrates for the enzymes of the group. In all, CYP3A4, CYP2D6, CYP2C9, CYP219, CYP2B6 and CYP1A2 subtypes play the most critical role and account for more than 90% of drugs metabolized by CYP 450 enzymes ( Evans & Relling 1999). These enzymes have proven genetic polymorphism with associated drug responses (Hiratsuka et al 2002, Wong et al 2005, McAlpine et al 2011) and racial variations (Meyer 2004 & Suarez – Kurtz 2005).

Phenotypes of P450 are divided into four groups and these are; the extensive metabolizers (EM)who show low metabolic activities, the poor metabolisers(PM )who carry gene alterations on both alleles which are inherited in an autosomal manner, the intermediate metabolizers (IM) with metabolic capacity in between those of PM and EM and finally the ultra rapid metabolizers (UM )who show higher metabolic capacity than the EM. (Murphy 2001, Hiratsuka et al 2005). Genetic variations have been observed particularly with CYP 2D6, CYP2C9 and CYP2C19 genotypes (Ingelman – Sundberg 1999, Hiratsuka 2006). With regards to CYP2D6,five to ten percent of caucasians are poor metabolizers and have little

Xenobiotics metabolism can be divided into three phases. In phase 1, enzymes such as cytochrome p450 oxidases introduce an active or polar group into the xenobiotics. These modified compounds are then conjugated to polar compounds in phase II reactions. The main enzyme that catalyses the reactions in phase II is glutathione S-transferase since it acts

The final phase; phase III may involve further metabolism of conjugates of phase II reactions like the processing of glutathione conjugates to acetylcysteine (mercapturic acic) conjugates before being recognized by efflux transporters and pumped out of the cells (Boyland &

Peculiar to all organisms is the possession of cell membranes which serve as hydrophobic permeability barriers to control access to their internal environment. Polar compounds cannot diffuse across these cell membranes, and the uptake of useful molecules is mediated through transport proteins that specifically select substrates from the extracellular mixture. The implication of this structure is that most hydrophilic molecules cannot enter the cells

The detoxification of reactive by-products is via a different mechanism. Because these species are derived from normal cellular constituents, they usually share the same polar characteristics therefore, specific designated enzymes can metabolize them. A notable example of these specific detoxification system is the glyoxalase system which catalyses the removal of the reactive aldehyde, methylglyoxal (Thormalley 1990) and the various

In Phase I reactions, a variety of enzymes act to introduce reactive and polar groups into their substrates. This is basically a functionalization reaction. One of the most common modifications in this phase is hydroxylation, a reaction catalysed by the cytochrome P-450 dependent mixed function oxidase system. These enzymes complexes act to incorporate an atom of oxygen into nonactivated hydrocarbons, which can result in either the introduction of hydroxyl groups, or Nitrogen, Oxygen.and Sulphate- dealkylation of substrates (Schlichting et al 2000).Of all the enzymes involved in drug metabolism, the cytochrome P450.(CYP450) is regarded as the most important because many drugs are substrates for the enzymes of the group. In all, CYP3A4, CYP2D6, CYP2C9, CYP219, CYP2B6 and CYP1A2 subtypes play the most critical role and account for more than 90% of drugs metabolized by CYP 450 enzymes ( Evans & Relling 1999). These enzymes have proven genetic polymorphism with associated drug responses (Hiratsuka et al 2002, Wong et al 2005,

Phenotypes of P450 are divided into four groups and these are; the extensive metabolizers (EM)who show low metabolic activities, the poor metabolisers(PM )who carry gene alterations on both alleles which are inherited in an autosomal manner, the intermediate metabolizers (IM) with metabolic capacity in between those of PM and EM and finally the ultra rapid metabolizers (UM )who show higher metabolic capacity than the EM. (Murphy 2001, Hiratsuka et al 2005). Genetic variations have been observed particularly with CYP 2D6, CYP2C9 and CYP2C19 genotypes (Ingelman – Sundberg 1999, Hiratsuka 2006). With regards to CYP2D6,five to ten percent of caucasians are poor metabolizers and have little

since they need to be recognized by specific transporters (Mizuno et al 2003).

antioxidant systems that eliminate reactive oxygen species (Sies 1997).

McAlpine et al 2011) and racial variations (Meyer 2004 & Suarez – Kurtz 2005).

on a wide range of substrates.

Chassaud 1969, Thomalley 1990)

**4.1 Phase I reactions** 

enzyme activities. In addition, there is a distinct racial diversity in the frequency of the classes. Examples of CYP450 catalyzed drug metabolic reactions include


#### **4.1.1 Non P450 enzyme catalysis**

Besides the CYP 450 enzymes, other enzymes that participate in drug biotransformations include; monoamineoxidases, peroxidases, lactoperoxidases myeloperoxidases, prostaglandin-H-synthetase and flavin-containing monooxygenases.(FMO).Examples of the reactions they catalyse include:

