Microsomal Enzyme Induction

MICROSOMAL ENZYME INDUCTION

Many drugs, insecticides and carcinogens interact with DNA and increase the synthesis of microsomal enzyme protein, specially cytochrome P450 and glucuronyl transferase. As a result rate of metabolism of inducing drug itself and/or other drugs is increased.

Different inducers are relatively selective for certain cytochrome P450 enzyme families, e.g.:

Anticonvulsants including phenobarbitone, rifampin, glucocorticoids induce CYP3A isoenzymes.

Phenobarbitone also induces CYP2B1 and rifampin also induces CYP2D6.

Isoniazid and chronic alcohol consumption induce CYP2E1.

Polycyclic hydrocarbons like 3methylcholanthrene and benzopyrene found in cigarette smoke, charcoalbroiled meat and industrial pollutants induce CYP1A isoenzymes.

Other important enzyme inducers are: chloral hydrate, phenylbutazone, griseofulvin, DDT.

Since different CYP isoenzymes are involved in the metabolism of different drugs, every inducer increases biotransformation of certain drugs but not that of others. However, phenobarbitone like inducers of CYP3A and CYP2D6 affect the metabolism of a large number of drugs, because these isoenzymes act on many drugs. On the other hand induction by polycyclic hydrocarbons is limited to few drugs (like theophylline, phenacetin) because CYP1A isoenzyme metabolizes only few drugs.

Induction involves microsomal enzymes in liver as well as other organs and increases the rate of metabolism by 2–4 fold. Induction takes 4–14 days to reach its peak and is maintained till the inducing agent is being given. Thereafter the enzymes return to their original value over 1–3 weeks.