Cytochrome P450 enzymes are enzymes found inplants, animals, and humans. In humans CYP450 mediate the catalysis of endogenousand exogenous compounds during Phase I metabolic reactions.

By convention,CYP450 belong to a large heme-thiolate protein superfamily (Danielson, 2002). Thenomenclature of the enzyme superfamily is attributed to the fact they showmaximum absorbance at 450nm (Shargel,Wu-Pong and Yu, 2004). These enzymes contain an embedded activeheme iron center that is bound to a protein molecule via a cysteine thiolateligand. Members of CYP450 family differ in their amino acid sequence andsensitivity to both enzyme inhibitors and inducers (Mittal et al., 2015).However, the enzyme isotypes may confer an overlapping substrate specificityand may bind to substrates at different rates.

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There has been 74 CYP450 genefamilies identified of which CYP1, CYP2, CYP3 are the most prevalent isoenzymesin phase I metabolic reactions. The CYP450 enzymes are grouped into familiesand subfamilies based on the similarities between their amino acid sequences.Sequences with greater than 40% similarity are grouped into a family. While,sequences with greater than 55% similarity are grouped into a super-family (Dale,Haylett and Rang, 2015). In humans, there are 57 CYP450 genes and around 59pseudogenes divided among 18 families of CYP450 genes and 43 subfamilies.

Furthermore, polymorphisms of CYP450 genes lead to differences in the activityand clinical effects of many drugs (Mittal et al., 2015). The nomenclatureof CYP enzymes is represented in Figure 1.2.

  CYP450 enzymes are most abundant in themicrosomes (Endoplasmic reticulum) and the liver cells (Hepatocytes). Somemight be present in the intestinal epithelial cells and more specifically inthe small intestines (enterocytes). Mitochondrial CYP450 enzymes are essentialfor the synthesis and the  metabolism of endogenous compounds, while themicrosomal CYP450 are implicated with the metabolism of xenobiotics. Xenobioticscan increase or decrease the activity of CYP450 enzymes, which can affect therate at which xenobiotics are metabolized and eliminated from the body. When anexogenous compound increases the activity of CYP450 enzyme the CYP450 metabolicfunction increases which renders the xenobiotic inactive leading to its rapidelimination from the body. Likewise, a xenobiotic can inhibit the activity ofCYP450, which increases its accumulation in the body and slows down itsexcretion inducing toxicity. Enzyme inhibition also leads to toxicity via theaccumulation of substances metabolized by it in the body tissue.

On the otherhand, the enzyme induction reduces the therapeutic effects of xenobiotics dueto the depletion of its plasma concentrations (Saxena et al., 2008).The main feature of CYP enzymes is theirability to activate molecular oxygen and insert it into inert chemical bonds.The insertion of the oxygen molecule into the activated carbon-hydrogen bondsyields alcohol (Reaction 1).  For drugsto undergo oxidation by P450 enzymes several components need to be present: (1)substrate, (2) P450 enzyme, (3) Molecular oxygen, (4) NADPH, (5) Flavoprotein(NADPH-P450 reductase) (Dale, Haylett and Rang, 2015).

 This paper is concerned with CYP2A6 enzyme andits’ inhibition using natural supplements; therefore, this isoform of CYP2Aenzyme will be further explained. CYP2A6 is the only enzyme that is used forcoumarin 7-hydroxylation. It’s responsible for the metabolism of nicotine andits oxidized metabolite cotinine as well as some other xenobiotics. Also,CYP2A6 is capable of forming procarcinogens that predispose humans to lung cancer(Raunio et al., 2001).CYP2A6 catabolizes nicotine in a two-stepreaction, firstly, nicotine is oxidized to an iminium ion then it undergoessubsequent cytosolic oxidation to cotinine by aldehyde oxidase. The first stepin the reaction is the rate limiting step.

Usually 80% of the nicotine found inthe body is metabolized to cotinine (Figure 3). Cotinine undergoes furthermetabolic oxidation by concomitant action of CYP enzymes and CYP2A6 isoforms toform NNN and NNK that are procarcinogens (Raunio et al., 2001).