A major inducer of anticarcinogenic protective enzymes from broccoli: isolation and elucidation of structure.

TLDR

Consumption of cruciferous vegetables reduces cancer risk, likely through induction of phase II detoxification enzymes such as quinone reductase and glutathione S‑transferases, although the precise mechanisms remain unclear. The study aimed to isolate the major enzyme inducer from broccoli and to determine the structural features that confer its high potency in activating phase II detoxification pathways. Using cultured murine hepatoma cells to monitor quinone reductase induction, the authors isolated sulforaphane from SAGA broccoli, synthesized racemic sulforaphane and analogues with varying sulfur oxidation and methylene length, and quantified their inducer potencies. Sulforaphane proved to be the most potent, selective phase II enzyme inducer, with sulfur oxidation enhancing activity; it induced quinone reductase and glutathione transferase in multiple mouse tissues, suggesting a key role in broccoli’s anticarcinogenic effect.

Abstract

Consumption of vegetables, especially crucifers, reduces the risk of developing cancer. Although the mechanisms of this protection are unclear, feeding of vegetables induces enzymes of xenobiotic metabolism and thereby accelerates the metabolic disposal of xenobiotics. Induction of phase II detoxication enzymes, such as quinone reductase [NAD(P)H:(quinone-acceptor) oxidoreductase, EC 1.6.99.2] and glutathione S-transferases (EC 2.5.1.18) in rodent tissues affords protection against carcinogens and other toxic electrophiles. To determine whether enzyme induction is responsible for the protective properties of vegetables in humans requires isolation of enzyme inducers from these sources. By monitoring quinone reductase induction in cultured murine hepatoma cells as the biological assay, we have isolated and identified (-)-1-isothiocyanato-(4R)-(methylsulfinyl)butane [CH3-SO-(CH2)4-NCS, sulforaphane] as a major and very potent phase II enzyme inducer in SAGA broccoli (Brassica oleracea italica). Sulforaphane is a monofunctional inducer, like other anticarcinogenic isothiocyanates, and induces phase II enzymes selectively without the induction of aryl hydrocarbon receptor-dependent cytochromes P-450 (phase I enzymes). To elucidate the structural features responsible for the high inducer potency of sulforaphane, we synthesized racemic sulforaphane and analogues differing in the oxidation state of sulfur and the number of methylene groups: CH3-SOm-(CH2)n-NCS, where m = 0, 1, or 2 and n = 3, 4, or 5, and measured their inducer potencies in murine hepatoma cells. Sulforaphane is the most potent inducer, and the presence of oxygen on sulfur enhances potency. Sulforaphane and its sulfide and sulfone analogues induced both quinone reductase and glutathione transferase activities in several mouse tissues. The induction of detoxication enzymes by sulforaphane may be a significant component of the anticarcinogenic action of broccoli.