Sulforaphane [1-isothiocyanato-4-(methylsulfinyl)
butane], a naturally occurring
isothiocyanate derived from cruciferous vegetables, is a highly potent inducer of phase 2 cytoprotective
enzymes and can protect against electrophiles including
carcinogens, oxidative stress, and
inflammation. The mechanism of action of
sulforaphane is believed to involve modifications of critical
cysteine residues of Keap1, which lead to stabilization of Nrf2 to activate the antioxidant response element of phase 2
enzymes. However, the dithiocarbamate functional group formed by a reversible reaction between
isothiocyanate of
sulforaphane and sulfhydryl nucleophiles of Keap1 is kinetically labile, and such modification in intact cells has not yet been demonstrated. Here we designed
sulforaphane analogs with replacement of the reactive
isothiocyanate by the more gentle electrophilic sulfoxythiocarbamate group that also selectively targets
cysteine residues in
proteins but forms stable
thiocarbamate adducts. Twenty-four sulfoxythiocarbamate analogs were synthesized that retain the structural features important for high potency in
sulforaphane analogs: the
sulfoxide or keto group and its appropriate distance to electrophilic functional group. Evaluation in various cell lines including
hepatoma cells,
retinal pigment epithelial cells, and keratinocytes as well as in mouse skin shows that these analogs maintain high potency and efficacy for phase 2 enzyme induction as well as the inhibitory effect on
lipopolysaccharide-induced
nitric oxide formation like
sulforaphane. We further show in living cells that a sulfoxythiocarbamate analog can label Keap1 on several key
cysteine residues as well as other cellular
proteins offering new insights into the mechanism of chemoprotection.