Phase 1 Reaction || Oxidative reactions ||PharmaEasyNotes ||@Pharmacy

Oxidation is the most common reaction in xenobiotics. Oxidation reaction increases the hydrophilicity of the xenobiotics by introducing the polar functional group such as OH.

Since only one oxygen atom from the molecular oxygen is incorporated in the product formed, the mixed-function oxidase is also called mono-oxygenase microsomal hydroxylase.

The multi-enzyme mixed-function oxidase system consists of 3 components.

  1. A heme protein known as cytochrome P450 is actually a family of an enzyme. It is a terminal oxidase and plays an important role in transferring oxygen atoms to the substrate RH and convert into ROH.
  2. A second enzyme, the flavoprotein known a cytochrome 450 reductase which is NADPH dependent. Its functions as an electron carrier, catalyzing the reduction of cytochrome 450 to the ferrous form by transferring an electron from NADPH.
  3. A heat-stable lipid component is known as phosphatidylcholine. Its function is to facilitate electron transfer from NADPH to cytochrome P450.

The mechanism of cytochrome P450 catalyzed metabolism of xenobiotics is depicted in the redox cycle in the following figure.

The various steps in the oxidation of xenobiotics are -

Binding -

Binding of the substrate (RH) to the oxidized form of the cytochrome P450 (Fe+++) to form a complex.

Rate limiting step -

A one-electron transfer form NADPH to the complex by cytochrome P450 reductase, to form reduced (Fe++) P450 substrate complex; this step is considered as the rate-limiting step in the overall oxidation of xenobiotics.

Form a complex and Absorption -

The reduced form (Fe++) of this enzyme binds with carbon monoxide to form a complex that shows maximum absorption at 450nm hence the name cytochrome P450

Form a ternary complex

The reduced enzyme-substrate complex combines with a molecule of oxygen to form a ternary complex.

Form Oxygen P450 substrate complex

The ternary complex combines with a second electron supplied by NADH in presence of enzyme cytochrome b5 reductase to form a ternary activated oxygen P450 substrate complex.

Ready to attach another molecule of substrate

  • One atom of oxygen from the activated oxygen complex is transferred to the substrate to yield the oxidized product and the other atom forms water.
  • The free oxidized form of cytochrome P450 is now ready to attach another molecule of substrate.

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