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Chemoprotective glucosinolates and
isothiocyanates of broccoli sprouts: metabolism and excretion in humans.
Cancer Epidemiol Biomarkers Prev 2001 May;10(5):501-8
Shapiro TA, Fahey JW, Wade KL, Stephenson KK, Talalay P.
Department of Pharmacology and Molecular Sciences, The Johns Hopkins University
School of Medicine, Baltimore, Maryland 21205, USA.
Broccoli sprouts are a rich source of glucosinolates and isothiocyanates that
induce phase 2 detoxication enzymes, boost antioxidant status, and protect
animals against chemically induced cancer. Glucosinolates are hydrolyzed by
myrosinase (an enzyme found in plants and bowel microflora) to form
isothiocyanates. In vivo, isothiocyanates are conjugated with glutathione and
then sequentially metabolized to mercapturic acids. These metabolites are
collectively designated dithiocarbamates. We studied the disposition of broccoli
sprout glucosinolates and isothiocyanates in healthy volunteers. Broccoli
sprouts were grown, processed, and analyzed for (a) inducer potency; (b)
glucosinolate and isothiocyanate concentrations; (c) glucosinolate profiles; and
(d) myrosinase activity. Dosing preparations included uncooked fresh sprouts
(with active myrosinase) as well as homogenates of boiled sprouts that were
devoid of myrosinase activity and contained either glucosinolates only or
isothiocyanates only. In a crossover study, urinary dithiocarbamate excretion
increased sharply after administration of broccoli sprout glucosinolates or
isothiocyanates. Cumulative excretion of dithiocarbamates following 111-micromol
doses of isothiocyanates was greater than that after glucosinolates (88.9 +/-
5.5 and 13.1 +/- 1.9 micromol, respectively; P < 0.0003). In subjects fed four
repeated 50-micromol doses of isothiocyanates, the intra- and intersubject
variation in dithiocarbamate excretion was very small (coefficient of variation,
9%), and after escalating doses, excretion was linear over a 25- to 200-micromol
dose range. Dithiocarbamate excretion was higher when intact sprouts were chewed
thoroughly rather than swallowed whole (42.4 +/- 7.5 and 28.8 +/- 2.6 micromol;
P = 0.049). These studies indicate that isothiocyanates are about six times more
bioavailable than glucosinolates, which must first be hydrolyzed. Thorough
chewing of fresh sprouts exposes the glucosinolates to plant myrosinase and
significantly increases dithiocarbamate excretion. These findings will assist in
the design of dosing regimens for clinical studies of broccoli sprout efficacy.
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