*11.6.2. Reduction*

**11.6. Mechanisms of phase I reactions**

Is the most important category of the microsomal drug oxidizing systems and requires partici‐ pation of two distinct proteins in endoplasmic reticulum; cytochromes P450 (which functions as a terminal oxidase) and cytochrome P450 reductase. The name Cyt450 is derived from the fact that the reduced form of this hemoprotein complexes with carbon monoxide to form a complex that has a unique absorption spectrum with a maximum at 450nm.Cytochrome P450 reductase serves

The sequence of reactions that transform a drug to its hydroxylated product is shown below

(ROH) Fe3+ O2

oxidase)

RH.Fe3+.O2 2-

NADH - NADH cyt b5 reductase

NADPH

Cofactor donates a proton

NADPH - cyt. P450 reductase

(RH)Fe2+

e-

(Binary complex)

(RH).Fe3+.O2

RH (The drug binds to cyt. P450

(RH) Fe2+.O2


cyt b5

2nd e transfer is required to activate the bound O2 -

to transfer reducing equivalent from NADPH to the cytochrome P450 oxidase:

DrugRH + O2+ NADPH + H<sup>+</sup><sup>→</sup> DrugROH (hydroxylated product) + H2O+ NADP<sup>+</sup>

(RH) Fe3+ ROH

(Ternary complex) Abstraction of

Fe3+ cytP450

(R**.**

H from substrate results in the formation of substrate radical and hydroxyl

The dissociation of ROH and restoration of the ferric form of cyt P450

The O - O bond is split with the uptake of 2 protons. The result is the release of H2Oand generation of an "activated oxygen"

more polar form.

)(Fe - OH)3+

(R)(Fe - OH)3+

H2O

(RH)(Fe-O)3+

2H+

**Figure 7.** Phase 1 drug biotransformation reactions in the liver microsomal fraction in which the drug is converted to a

*11.6.1. Oxidation*

488 Drug Discovery

(Figure 7).

Some drugs with azo-linkages (RN=NR, e.g. prontosil) and nitrogen groups (RNO2, such as chloramphenicol) are transformed by reductive pathways. The Cyt P450 and NADPH–cyt P450reductase enzymes that catalyze oxidation reactions are also involved in reduction reactions for drugs containing quinine moieties. These transformation results in the formation of semiquinone free radicals illustrated in Figure 8. The free radicals that are generated cause oxidative stress, lipid peroxidation, DNA damage, and hence cytotoxicity. These effects are particularly responsible for the antitumor property of a drug like doxorubicin.
