Abstract

Watercress is an exceptionally rich dietary source of ßphenylethyl
isothiocyanate (PEITC). This compound
inhibits phase I enzymes, which are responsible for the
activation of many carcinogens in animals, and induces
phase II enzymes, which are associated with enhanced
excretion of carcinogens. In this study, we show that
watercress extracts are potent inducers of quinone reductase
(QR) in murine hepatoma Hepa 1c1c7 cells, a widely
adopted assay for measuring phase II enzyme induction.
However, contrary to expectations, this induction was
not associated with PEITC (which is rapidly lost to the
atmosphere upon tissue disruption due to its volatility) or
a naturally occurring PEITC–glutathione conjugate, but
with 7-methylsulfinyheptyl and 8-methylsulfinyloctyl isothiocyanates
(ITCs). While it was confirmed that PEITC does
induce QR (5 µM required for a two-fold induction in QR),
7-methylsulfinyheptyl and 8-methylsulfinyloctyl ITCs were
more potent inducers (0.2 µM and 0.5 µM, respectively,
required for a two-fold induction in QR). Thus, while
watercress contains three times more phenylethyl glucosinolate
than methylsulfinylalkyl glucosinolates, ITCs derived
from methylsulfinylalkyl glucosinolates may be more
important phase II enzyme inducers than PEITC, having
10-to 25-fold greater potency. Analysis of urine by liquid
chromatography–mass spectroscopy (LC-MS) following
consumption of watercress demonstrated the presence of
N-acetylcysteine conjugates of 7-methylsulfinylheptyl,
8-methylsulfinyloctyl ITCs and PEITC, indicating that
these ITCs are taken up by the gut and metabolized in the
body. Watercress may have exceptionally good anticarcinogenic
potential, as it combines a potent inhibitor of phase
I enzymes (PEITC) with at least three inducers of phase
II enzymes (PEITC, 7-methylsulfinylheptyl ITC and 8methylsulfinyloctyl
ITC). The study also demonstrates the
application of LC-MS for the detection of complex glucosinolate-
derived metabolites in plant extracts and urine.