Glucosinolate and Desulfo-glucosinolate Metabolism by a Selection of Human Gut Bacteria

Abstract

Glucosinolate (GSL) hydrolysis is mediated by the enzyme myrosinase which together with specifier proteins can give rise to isothiocyanates (ITCs), thiocyanates, and nitriles (NITs) in cruciferous plants. However, little is known about the metabolism of GSLs by the human gut flora. The aim of the work was to investigate the metabolic fates of sinigrin (SNG), glucotropaeolin (GTP), gluconasturtiin (GNT), and their corresponding desulfo-GSLs (DS-GSLs). Three human gut bacterial strains, Enterococcus casseliflavus CP1, Lactobacillus agilis R16, and Escherichia coli VL8, were chosen for this study. GNT was metabolized to completion within 24 h to phenethyl ITC and phenethyl NIT (PNIT) by all bacteria, except for L. agilis R16 which produced only PNIT. At least 80 % of GTP and SNG were metabolized by all bacteria within 24 h to the corresponding ITCs and NITs. The pH of media over time gradually became acidic for both L. agilis R16 and E. coli VL8, while for E. casseliflavus CP1 the media became slightly alkaline with NIT and ITC production occurring between pH 3.0 and 7.5. ITC production peaked between 4 and 10 h in most cases and gradually declined while NIT production increased and remained relatively constant over time. The total percentage products accounted for 3-53 % of the initial GSL. NITs were produced from DS-GSLs suggesting an alternative metabolism via desulfation for the food based GSLs. The metal ion dependency for NIT production for GNT and its DS form was investigated where it was shown that Fe(2+) increased NIT production, while Mg(2+) stimulated the formation of ITC.

Gut Glucosinolate Metabolism and Isothiocyanate Production

“…The authors could demonstrate myrosinase activity with intact cells but not with cell-free protein extracts. Subsequent studies [23,24] with L. agilis R16 showed similar results with sinigrin except that allylnitrile was also a product. This study was expanded to include glucotropaeolin, gluconasturtiin, glucoraphanin, glucoerucin, and glucoiberin.…”

“…Glucoerucin-and glucotropaoelin-like sinigrin were converted to ITCs and nitriles. [23,24] It was not possible to demonstrate myrosinase activity with cell-free protein extracts. Further studies were carried out with two bacterial strains Enterococcus casseliflavus CP1 and Escherichia coli VL8 that originated from human fecal material.…”

“…The activity of L. agilis R16 with these GSL substrates showed a degree of substrate specificity as neither glucoraphanin nor glucoiberin was metabolized while gluconasturtiin produced only an ITC. Glucoerucin‐ and glucotropaoelin‐like sinigrin were converted to ITCs and nitriles . It was not possible to demonstrate myrosinase activity with cell‐free protein extracts.…”

“…It was not possible to demonstrate myrosinase activity with cell‐free protein extracts. Further studies were carried out with two bacterial strains Enterococcus casseliflavus CP1 and Escherichia coli VL8 that originated from human fecal material . E. coli VL8 was able to metabolize all the GSLs tested (Table ) to both nitriles and ITCs while E. casseliflavus CP1 was able to metabolize all GSLs tested to nitriles and ITCs with the exception of glucoraphanin and glucoiberin where only trace amounts of ITCs and nitriles were observed.…”

Dietary Broccoli Alters Rat Cecal Microbiota to Improve Glucoraphanin Hydrolysis to Bioactive Isothiocyanates

“…In our study, a three-day withdrawal from the broccoli diet decreased GRP hydrolysis ex vivo, suggesting that frequent broccoli ingestion may be essential for this improved microbial GRP hydrolysis. Recent studies have shown that desulfoglucosinolates can be substrates for beta hydrolase, resulting in formation of nitriles [40]. It will be interesting to see if there are bacterial N-sulfatase enzymes that are able to generate desulfoglucosinolates and whether these, by forming inactive nitriles and not reactive isothiocyanates, can protect the bacterium from glucosinolate toxicity.…”