Cytochrome P450 enzymes are enzymes found in
plants, animals, and humans. In humans CYP450 mediate the catalysis of endogenous
and exogenous compounds during Phase I metabolic reactions. By convention,
CYP450 belong to a large heme-thiolate protein superfamily (Danielson, 2002). The
nomenclature of the enzyme superfamily is attributed to the fact they show
maximum absorbance at 450nm (Shargel,
Wu-Pong and Yu, 2004). These enzymes contain an embedded active
heme iron center that is bound to a protein molecule via a cysteine thiolate
ligand. Members of CYP450 family differ in their amino acid sequence and
sensitivity to both enzyme inhibitors and inducers (Mittal et al., 2015).
However, the enzyme isotypes may confer an overlapping substrate specificity
and may bind to substrates at different rates. There has been 74 CYP450 gene
families identified of which CYP1, CYP2, CYP3 are the most prevalent isoenzymes
in phase I metabolic reactions. The CYP450 enzymes are grouped into families
and 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 59
pseudogenes divided among 18 families of CYP450 genes and 43 subfamilies.
Furthermore, polymorphisms of CYP450 genes lead to differences in the activity
and clinical effects of many drugs (Mittal et al., 2015). The nomenclature
of CYP enzymes is represented in Figure 1.2.  CYP450 enzymes are most abundant in the
microsomes (Endoplasmic reticulum) and the liver cells (Hepatocytes). Some
might be present in the intestinal epithelial cells and more specifically in
the small intestines (enterocytes). Mitochondrial CYP450 enzymes are essential
for the synthesis and the  metabolism of endogenous compounds, while the
microsomal CYP450 are implicated with the metabolism of xenobiotics. Xenobiotics
can increase or decrease the activity of CYP450 enzymes, which can affect the
rate at which xenobiotics are metabolized and eliminated from the body. When an
exogenous compound increases the activity of CYP450 enzyme the CYP450 metabolic
function increases which renders the xenobiotic inactive leading to its rapid
elimination from the body. Likewise, a xenobiotic can inhibit the activity of
CYP450, which increases its accumulation in the body and slows down its
excretion inducing toxicity. Enzyme inhibition also leads to toxicity via the
accumulation of substances metabolized by it in the body tissue. On the other
hand, the enzyme induction reduces the therapeutic effects of xenobiotics due
to the depletion of its plasma concentrations (Saxena et al., 2008).

The main feature of CYP enzymes is their
ability to activate molecular oxygen and insert it into inert chemical bonds.
The insertion of the oxygen molecule into the activated carbon-hydrogen bonds
yields alcohol (Reaction 1).  For drugs
to 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 and
its’ inhibition using natural supplements; therefore, this isoform of CYP2A
enzyme will be further explained. CYP2A6 is the only enzyme that is used for
coumarin 7-hydroxylation. It’s responsible for the metabolism of nicotine and
its 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).

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CYP2A6 catabolizes nicotine in a two-step
reaction, firstly, nicotine is oxidized to an iminium ion then it undergoes
subsequent cytosolic oxidation to cotinine by aldehyde oxidase. The first step
in the reaction is the rate limiting step. Usually 80% of the nicotine found in
the body is metabolized to cotinine (Figure 3). Cotinine undergoes further
metabolic oxidation by concomitant action of CYP enzymes and CYP2A6 isoforms to
form NNN and NNK that are procarcinogens (Raunio et al., 2001).