p1,2,3, cytochromes

• Phase I - modification

  1. Variety of enzymes act to introduce reactive and polar groups into their substrates

  2. One of the most common modifications is hydroxylation catalysed by the cytochrome P-450-dependent mixed-function oxidase system

  3. Enzyme complexes act to incorporate an atom of oxygen into nonactivated hydrocarbons, which can result in either the introduction of hydroxyl groups or N-, O- and S-dealkylation of substrates

  4. Reaction mechanism of the P-450 oxidases proceeds through the reduction of cytochrome-bound oxygen and the generation of a highly-reactive oxyferryl species
• Phase II - conjugation

  1. Activated xenobiotic metabolites are conjugated with charged species such as glutathione (GSH), sulfate, glycine, or glucuronic acid

  2. Reactions are catalyzed by a large group of broad-specificity transferases, which in combination can metabolise almost any hydrophobic compound that contains nucleophilic or electrophilic groups

  3. One of the most important of these groups are the glutathione S-transferases (GSTs)

  4. Addition of large anionic groups (such as GSH) detoxifies reactive electrophiles and produces more polar metabolites that cannot diffuse across membranes, and may, therefore, be actively transported
• Phase III - further modification and excretion

  1. Glutathione conjugates are processed to acetylcysteine (mercapturic acid) conjugates

  2. Gamma-glutamyl transpeptidase and dipeptidases remove the γ-glutamate and glycine residues in the glutathione molecule

  3. Cystine residue in the conjugate is acetylated

  4. Conjugates and their metabolites can be excreted from cells, with the anionic groups acting as affinity tags for a variety of membrane transporters of the multidrug resistance protein (MRP) family

  5. These proteins are members of the family of ATP-binding cassette transporters and can catalyse the ATP-dependent transport of a huge variety of hydrophobic anions, and thus act to remove phase II products to the extracellular medium, where they may be further metabolised or excreted
• Endogenous toxins


  • Detoxification of endogenous reactive metabolites such as peroxides and reactive aldehydes often cannot be achieved by the system described above
  • This is the result of these species' being derived from normal cellular constituents and usually sharing their polar characteristics
  • Since these compounds are few in number, it is possible for enzymatic systems to utilize specific molecular recognition to recognize and remove them
  • The similarity of these molecules to useful metabolites therefore means that different detoxification enzymes are usually required for the metabolism of each group of endogenous toxins
  • Examples of these specific detoxification systems are the glyoxalase system, which acts to dispose of the reactive aldehyde methylglyoxal, and the various antioxidant systems that remove reactive oxygen species
• Cytochrome or CYP


  • Cytochrome P450 (abbreviated CYP, P450, infrequently CYP450) is a very large and diverse superfamily of hemoproteins found in all domains of life
  • Cytochromes P450 use a plethora of both exogenous and endogenous compounds as substrates in enzymatic reactions
  • Usually they form part of multicomponent electron transfer chains, called P450-containing systems
  • The most common reaction catalysed by cytochrome P450 is a monooxygenase reaction, e.g. insertion of one atom of oxygen into an organic substrate (RH) while the other oxygen atom is reduced to water
• • CYP enzymes have been identified from all lineages of life, including mammals, birds, fish, insects, worms, sea squirts, sea urchins, plants, fungi, slime molds, bacteria and archaea
  • More than 7700 distinct CYP sequences are known (as of September 2007; see the web site of the P450 Nomenclature Committee for current counts)
• Name cytochrome P450


  Derived from the fact that these are colored ('chrome') cellular ('cyto') proteins, with a "pigment at 450 nm", so named for the characteristic Soret peak formed by absorbance of light at wavelengths near 450 nm when the heme iron is reduced (often with sodium dithionite) and complexed to carbon monoxide
• Mechanism of cytochrome P450


  1. The active site of cytochrome P450 contains a heme iron center

  2. The iron is tethered to the P450 protein via a thiolate ligand derived from a cysteine residue

  3. This cysteine and several flanking residues are highly conserved in known CYPs and have the formal PROSITE signature consensus pattern

  4. The P450 catalytic cycle proceeds through several steps including substrate binding, electron transfer, oxygen binding, protonation, and product release
• P450-containing systems of proteins

  • CPR/cyb5/P450 systems employed by most eukaryotic microsomal CYPs
  • FR/Fd/P450 systems which are employed by mitochondrial and some bacterial CYPs
  • CYB5R/cyb5/P450 systems in which both electrons required by the CYP come from cytochrome b5
  • FMN/Fd/P450 systems originally found in Rhodococcus sp. in which a FMN-domain-containing reductase is fused to the CYP
  • P450 only systems, which do not require external reducing power
• P450s in humans


  • Human CYPs are primarily membrane-associated proteins, located either in the inner membrane of mitochondria or in the endoplasmic reticulum of cells
  • CYPs metabolize thousands of endogenous and exogenous compounds
  • Most CYPs can metabolize multiple substrates, and many can catalyze multiple reactions, which accounts for their central importance in metabolizing the extremely large number of endogenous and exogenous molecules
  • In the liver, these substrates include drugs and toxic compounds as well as metabolic products such as bilirubin (a breakdown product of hemoglobin)
  • Cytochrome P450 enzymes are present in most other tissues of the body, and play important roles in hormone synthesis and breakdown, cholesterol synthesis, and vitamin D metabolism
• CYP families in humans

  • CYP1 - drug and steroid (especially estrogen) metabolism
  • CYP2 - drug and steroid metabolism
  • CYP3 - drug and steroid (including testosterone) metabolism
  • CYP4 - arachidonic acid or fatty acid metabolism
  • CYP5 - thromboxane A2 synthase
  • CYP7 - bile acid biosynthesis 7-alpha hydroxylase of steroid nucleus
  • CYP8 - varied
  • CYP11 - steroid biosynthesis
  • CYP17 - steroid biosynthesis, 17-alpha hydroxylase
  • CYP19 - steroid biosynthesis: aromatase synthesizes estrogen
  • CYP20 - unknown function
  • CYP21 - steroid biosynthesis
  • CYP24 - vitamin D degradation
  • CYP26 - retinoic acid hydroxylase
  • CYP27 - varied
  • CYP39 - 7-alpha hydroxylation of 24-hydroxycholesterol
  • CYP46 - cholesterol 24-hydroxylase
  • CYP51 - cholesterol biosynthesis
• Sites of drug metabolism


  • Quantitatively, the smooth endoplasmic reticulum of the liver cell is the principal organ of drug metabolism
  • Other sites of drug metabolism include epithelial cells of the gastrointestinal tract, lungs, kidneys, and the skin
  • These sites are usually responsible for localized toxicity reactions