The cytochrome P450 as it is popularly known as a generic name given to large family of enzyme which can metabolize variety of chemical including toxic substance and therapeutic agents. The enzyme also plays an important role in hormone synthesis and breakdown (including estrogen and testosterone synthesis and metabolism) cholesterol synthesis and vitamin D metabolism. In liver, this substrate includes drug and toxic compound as well as metabolic product such as bilirubin (a break down product of hemoglobin). Cytochrome P450s are also present in blood brain barrier (BBB) in concentration similar to those found in liver. They strengthen the division between blood and brain tissue by metabolizing toxic compound which may somehow cross the BBB.

It is astonishing to know that in human there are 18 families, which in turn are categorized in 44 subfamilies of CYT P450 have been identified. They are coded by more then 6000 CYT P450 gene. Similarly CYT P450 can be divided into three groups via metabolism, steroidogenesis and miscellaneous role.

The cytochrome P450 was coined in 1962 as a stand by name for a coloured substance in cell. This colour substance when reduced and bond with carbon monoxide produce an unusual absorption peak at a 450 nm wavelength. Otherwise cytochrome is a misleading name given to enzyme. Initially it was thought that cytochrome P450 being a single enzyme, but the advances in molecular biology has cleared that cytochrome P450 is a very large family of enzymes with 50 diverse groups in living system. This number is increasing as new enzymes are getting discovered and added to the list. As number of enzymes was causing confusion due to identification of enzymes in various labs. A harmony of naming cytochrome P450 was proposed in the year 1987. Root symbol (CYP), Arabic number for the family and alphabets for subfamily and followed by Arabic numerical for the individual gene. For example CYP1A2 gene codes for Cyp1A2 enzyme. This nomenclature is based on the evolutionary relationship between enzymes. For example, it is estimated that CYP450 families evolved nearly 1.2 billion years ago. Hence in any CYP450 family an enzyme is nearly 40% similar at the amino acid level. In further classification, subfamilies are believed to have diverged 400 million years ago. Hence any 100 enzymes belonging to same subfamily have 55% amino acid similarity. The nomenclature based on the substrate profile was found to be confusing as many overlapping profiles for CYP450 was observed.

Drug metabolism
CYP450s are the major enzymes involved in drug metabolism, amounting for 75% of the total metabolism. The over all generality of strategy of metabolism by CYP450 is to modify molecules to facilitate its excretion by kidneys. It usually involve phase 1 and phase 2 reactions. In phase 1 reaction, the xenobiotics undergo certain chemical reaction such as oxidation, demethylation or reduction so that it is easily conjugated with glucoronic acid and glutathione which happens in the phase 2 reactions. This not only increases the water solubility but also the bulkiness of molecules so as to facilitate easy elimination by nephron. Under the phase 1 reaction monooxygenase reaction is one of the most common reactions catalysed by CYP450

RH + O2 + 2H+ + 2e- ROH + H2O

In general chemical metabolism of the drug or toxicant reduces the pharmacological and toxicological effect of the drug. But some exceptions are there such as codeine and paracetamole. It is mysterious why codeine was producing variable analgesic effect in individual. The mystery was unfolded after the knowledge of generic verbosity in CYT P450 enzyme as therapeutic effects. The codeine is weak analgesic which is converted to morphine (powerful analgesic) by CYP206 enzyme. As this enzyme is expressed differently in individuals the validation of analgesic effect of codeine are accounted for. Paracetamol is metabolized to toxic metabolites which are usually conjugated with glutathione to nontoxic entity. The glutathione content is limited and is exhaustible. In case of paracitamole poisoning, the depletion of all normal recourses of conjugating glutathione leads to accumulation of toxic metabolite which causes hepatotoxicity.

Drug interaction
Drug interaction is one of the major causes of death in hospitalized patient. A slight change in the CYP enzyme activity due to adverse drug interaction may affect metabolism and clearance of various drugs. For example, if one drug inhibits the CYP mediated metabolism of other drug leads to accumulation of second drug within the body to a toxic level. Such drug interaction should be taken to account when using drug of vital importance to patient. For e.g. the critical dose of drug like amiodarone or carbamazepine whose blood plasma concentration may be decrease because of enzyme induction by phenytoin. Bioactive compound such as Bergamottin, Dihydroxybergamottin and Paradisin found in grape fruit juice and some other fruit juice have been found to inhibit CYP3a4 mediated metabolism of certain medication leading to increased bioavailability and these cause overdose.

Other function of cytochrome P450
It is also observed that CYP450 isoforms metabolize the drug at different site leading to a variety of metabolized products. CYP19A (P450 arom, aromatase) in endoplasmic reticulum of gonads, brain, adipose tissue, and elsewhere catalyzes aromatization of androgens to estrogens. A subset of cytochrome P450 enzymes plays important roles in the synthesis of steroid hormones (steroidogenesis) by the adrenals, gonads, and peripheral tissue:

The knowledge about CYP450 has made drug discovery and development programmes to ensure potential drug metabolizing problems which involves application of technologies of 3D QSAR and advanced pk/pd. From the clinical prospective the focus is on understanding and predicting the variety of drug action in individual in a population of patient. This has led to the advent of population pharmacokinetics which tries to rationalize the drug by considering inter individual varicosity.

Conclusion
P450 has great potential to throw light to the metabolism of drugs and their toxicity which largely developed the research interest for many scientists in P450. At present, we are equipped with rich understanding of the biochemical pathways of these systems but still a lot more is yet to be understood. The field has shown high success in predicting about human drug metabolism. However, we are faced with many hurdles in this field which throw a challenge for further development and application of our knowledge of P450s to unresolved problems in chemical toxicity. Nevertheless, dedicated researchers have many opportunities which come along with the challenges, to unravel the unsolved mystery of the biochemistry of P450 and enrich our understanding of P450, thus enlivening their vision in the P450.


(The authors are with Manipal College of Pharmaceutical Sciences, Manipal University, Manipal 576 104)