No de novo sulforaphane biosynthesis in broccoli seedlings

The isothiocyanate sulforaphane, present in significant amounts in broccoli (Brassica oleracea L.) seedlings in the form of its precursor glucoraphanin, has been identified as an inducer of quinine reductase, a phase-II detoxification enzyme known for its anticarcinogenic properties. Its concentration in broccoli seedlings usually decreases during the first 7–14 days after germination. No conclusive data on sulforaphane metabolism in seedlings are available in the literature. Here, we unambiguously demonstrate in 12C/13C-cross experiments that sulforaphane is not biosynthesised de novo during the first week of seedling development. Both 12C (99 atom% 12C) and 13C (98 atom% 13C) broccoli seeds were produced and subsequently germinated and grown either in a 13CO2 or a 12CO2 environment. Afterwards, the labelling degree of sulforaphane in seeds and in seedlings was analysed by HPLC–MS. We conclude that sulforaphane exclusively originates from seed reserves and that de novo biosynthesis is not detectable (<1%) in broccoli seedlings.