Abstract
Broccoli is rich in glucosinolates, which can be converted upon chewing and processing into Aryl hydrocarbon Receptor (AhR) ligands. Activation of AhR plays an important role in overall gut homeostasis but the role of broccoli processing on the generation of AhR ligands is still largely unknown. In this study, the effects of temperature, cooking method (steaming versus boiling), gastric pH and further digestion of broccoli on AhR activation were investigated in vitro and in ileostomy subjects. For the in vitro study, raw, steamed (t = 3 min and t = 6 min) and boiled (t = 3 min and t = 6 min) broccoli were digested in vitro with different gastric pH. In the in vivo ileostomy study, 8 subjects received a broccoli soup or a broccoli soup plus an exogenous myrosinase source. AhR activation was measured in both in vitro and in vivo samples by using HepG2-Lucia™ AhR reporter cells. Cooking broccoli reduced the AhR activation measured after gastric digestion in vitro, but no effect of gastric pH was found. Indole AhR ligands were not detected or detected at very low levels both after intestinal in vitro digestion and in the ileostomy patient samples, which resulted in no AhR activation. This suggests that the evaluation of the relevance of glucosinolates for AhR modulation in the gut cannot prescind from the way broccoli is processed, and that broccoli consumption does not necessarily produce substantial amounts of AhR ligands in the large intestine.
Highlights
Brassica vegetables are rich in glucosinolates, which can be converted into Aryl hydrocarbon Receptor (AhR) ligands by myrosinase, a plant enzyme that can enzymatically hydrolyse the chemically stable glucosinolates.[4,7,8] Myrosinase and glucosinolates are physically separated from each other in intact vegetable tissues, but mastication in the oral cavity allows them to come into contact with each other, promoting enzymatic hydrolysis of glucosinolates into a variety of breakdown products (BP).[9]
Glucobrassicin is the only glucosinolate responsible for AhR activation as it can be converted into AhR ligands DIM and ICZ via I3C.3,16,28,29
The glucobrassicin concentration in the 3 min steamed broccoli was the highest, with 466 μmol per 100 g, which was significantly higher than the raw broccoli (280 μmol per 100 g, p < 0.05) and boiled broccoli samples (293 and 199 μmol per 100 g after 3 and 6 min boiling, p < 0.05 and p < 0.01 respectively)
Summary
Brassica vegetables are rich in glucosinolates, which can be converted into AhR ligands by myrosinase, a plant enzyme that can enzymatically hydrolyse the chemically stable glucosinolates.[4,7,8] Myrosinase and glucosinolates are physically separated from each other in intact vegetable tissues, but mastication in the oral cavity allows them to come into contact with each other, promoting enzymatic hydrolysis of glucosinolates into a variety of breakdown products (BP).[9]. We investigated the effect of cooking method (boiling versus steaming) and gastric pH on AhR activation by glucosinolate derivatives from broccoli in vitro. To investigate AhR activation in humans and the effect of absorption, which is lacking in in vitro systems, we measured AhR activity in the ileal fluid samples collected from ileostomy subjects after consumption of a broccoli soup with or without supplementation of an exogenous source of active myrosinase
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