Cytochrome P450 monooxygenases (CYP) are involved in oxidation
of xenobiotics. Increased solubility by CYP action contributes to excretion of these foreign chemical substances from
the body. Various natural products are known to affect hepatic
expression of cytochrome P450s .
Among them, RES suppresses hepatic CYP1A1 and CYP1B1
(Beedanagari and others 1987) but does not affect intestinal expression (Krizkova and others 2008). Nicotine can be metabolized
via several pathways in humans, including 5-hydroxylation
to yield cotinine and trans-3-hydroxycotinine (Balakrishnan and
Menon 2007; Zhou and others 2010). These reactions of hepatic
metabolism are primarily catalyzed by CYP2A6 (Zhou and others
2010). There are limited studies on interaction of polyphenols
with CYP2A6. Coumarin found in plants served as an alternative
CYP2A6 substrate in an in vitro reaction of nicotine hydroxylation
using hepatic microsome . Woodward
demonstrated that the major polyphenol of caffeine
containing beverages including caffeic acid and quercetin significantly
inhibited CYP2A6 hydroxylation activity in bovine liver
microsomes. The study suggested that polyphenol interaction with
CYP2A6 prolongs the toxicological effects of nicotine by reducing
its rate of elimination. However, these results were not verified
in in vivo studies. On the other hand, RES and OXY induced the
expression of CYP2A6, and the degree of induction by EESC
was more significant in this study. Although the doses of RES and
OXY used in this study was higher than the concentration used
in other studies of efficacy evaluations (Wang and others 2014;
Zhang and others 2013), we found that RES and OXY did not
have CYP2A6 inducing effect below 250 μM (data not shown).
The notion that EESC with much lower concentrations of RES
and OXY induces CYP2A6 indicates that the components in
EESC have the CYP2A6 inducing activity that contributes to the
conversion of nicotine to cotinine in hepatocytes. Despite the fact
that RES and OXY upregulate CYP2A6 expression in high doses,
the additional components in EESC causing CYP2A6 induction
are unclear at this moment and deserve further study