Phases of biotransformation/detoxification:

Detoxification or biotransformation reactions are generally classified into two phases :

1. Phase one : in this phase the compounds are made less toxic with the help of oxidation, hydroxylation etc. reactions.
2. Phase two: in this phase the less toxic compounds formed by phase one are conjugated with othe compounds and they are excreted out.

Phase one reactions:

1. Oxidation : it involves hydroxylation reaction carried out by the enzyme mixed oxidase system.

Example : toluene is hydroxylated to benzyl alcohol.

2. Reduction: it involves the reduction of nitro compounds. The nitro compounds are reduced to their amines where as aldihydes are reduced to the alcohols.

Example: reduction of nitrobenzene to aniline.

3. Hydrolysis: it is the reaction where there is addition of water molecule and this splits the toxic compound into smaller fragmants.

Example: hydrolysis of acetyle choline into choline and acetyl coA

Phase two reactions:

1. Conjugation : phase two reactions are conjugation reactions i.e. new compound is added to the metabolite produced by the phase one reaction. The conjugation reactions can be:
2. Glucuronic acid conjugation: glucuronic acid is conjugated with bilirubin to make the bilirubin water soluble so that it can be easily excreted.
3. Sulfate conjugation: addition of sulfur can reduce the toxicity of some compounds. Some phenol containing compounds can be conjugated with sulfur to make less toxic.
4. Acetylation: conjugation of some drugs like isoniazide with acetic acid is called as acetylation.
5. Conjugation with glycine: benzoic acid is conjugated with glycine to form hippuric acid and this is excreted through the urine.