What is Drug Metabolism Process

Drug Metabolism Process

1.     Metabolism is defined as the larger particle molecules are converted into small particle.

2.     Conversion of one molecule of drug into another suitable molecule of drug is called biotransformation or metabolism.

Purpose of metabolism:-

To convert the drug into more water soluble component.

Primary site of Drug metabolism:-

Liver, kidney, intestine, lungs and plasma.


Metabolic Enzymes:-

Drug metabolizing enzyme is divided into two types:-

·       Microsomal Enzymes.

·       Non Microsomal Enzymes.

Both of them develops partially in first month and completely in 3rd month.

Microsomal Enzyme:-

·       Catalyze most of Oxidation, reduction , hydrolysis and glucouronide conjugation.

·       Example: - Monooxygenase, cytochrome P 450, Glucouronyl transferase.

Non- Microsomal Enzyme:-

·       Located in cytoplasm and mitochondria of hepatic cells as well as in other tissue including plasma.

·       Some oxidation and reduction reactions, many hydrolytic reactions and all conjugating except glucoronidization.

·       Esterase , amidase , flavoprotein oxidases.

Metabolic Processes:-


There are two types of metabolic process.

·       First type of metabolism (phase-I)

·       Second type of metabolism (phase-II)

First type of metabolism Phase – II:-

·       It is also called non-synthetic metabolism /functionalization/catabolic.

·       Functional group is generated -more chemically reactive.

·       Oxidation, reduction and hydrolysis.


·       Addition of oxygen or negatively charged radical to drug.

·       Removal of hydrogen or positively charged radical to drug.

·       Catalyzed by Mono oxygenase(Liver).

·       Example:- hydroxylation, dealkylation.


·       Addition of hydrogen or positively charged molecule or radical to drug.

·       Removal of oxygen or negatively charged molecule or radical to drug.

·       Example:- Nitro and keto reduction. Cortisone →Hydrocortisone.


·       Splitting of drug molecule with the presence of water.

·       Example:-Acetylcholine + H2O → Choline +Acetic Acid

2. Second type of Metabolism Phase -II:-

·       It is also called synthetic or conjugation or anabolic reaction.

·       Involve conjugation of drug or metabolite of phase I with carbohydrates or Amino acid will form more polar.

·       Require high energy.

·       Faster than Phase -I

Types of Phase -II:

1. Glucoronide conjugation:-

·       Carried out by UDP glucoronosyl transferase enzyme.

·       Compound with hydroxyl and carboxylic group conjugate with glucoronic acid then form more water soluble.

·       Example:- Aspirin , paracetamol , diazapam etc.

·       Bilirubin, steroidal hormone follow this path.

2.    Acetylation:-

·       Compound having amino and hydrazine residue conjugates – acetyl CoA.

·       Example sulphonamides, isoniazides, hydralazine, dapsone, clonazepam. 

3. Methylation:-

·       Amine and phenol gets methylated – methaionine and cysteine.

·       Example :- Adrenaline, histamine, methyl dopa , nicotinic acid and captopril.

4. Sulphate conjugation:-

Phenolic and steroid compounds – sulfated by sulphotransferase.

Example:- Chlorampheniol , methyl dopa , sex steroids.

5. Glycine conjugation:-

·       Addition of glycine.

·       Salicylate and other drug having carboxylic acid.

·       Not a major pathway.

6. Ribonucleoside / nucleotide synthesis:-

·       Activation of purines and pyrimidine.

·       Antimetabolites used in chemotherapy (cancer).


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