D-limonene induces phase I and phase II
carcinogen-metabolizing enzymes (cytochrome p450),
which metabolize carcinogens to less toxic forms and
prevent the interaction of chemical carcinogens with
DNA. D-limonene has been shown to enhance gastrointestinal
UDP-glucuronosyltransferase (UGT) activity
in rats.31 It also inhibits tumor cell proliferation, acceleration
of the rate of tumor cell death and/or induction of
tumor cell differentiation. Furthermore, d-limonene inhibits
protein isoprenylation. Many prenylated proteins
regulate cell growth and/or transformation. Impairment
of prenylation of one or more of these proteins might
account for the antitumor activity of d-limonene.25 It
was found that d-limonene attenuates gastric cancer
through increasing apoptosis, while decreasing DNA
synthesis and ornithine decarboxylase activity of cancer
cells.26,27 D-limonene inhibits hepatocarcinogenesis via
inhibition of cell proliferation, enhancement of apoptosis,
and blockage of oncogene expression