Lactase-phlorizin hydrolase (LPH, EC 3.2.123/62) is an apically-sorted glycoprotein of the small intestine. One of the two catalytic sites of LPH is responsible for hydrolyzing lactose, the main carbohydrate in milk. The second active site exhibits a broad specificity against substrates like phlorizin and glycosyl-N-acylsphingosines [1]. Phlorizin, a dihydrochalcone, belongs to the group of flavonoids often present as β-glycosides in food and herbal medicinal plant products. Bioavailability of flavonoids may depend on intestinal hydrolysis before absorption and delivery to systemic circulation. The ability of purified LPH to hydrolyze various flavonoid glycosides was investigated. Isolation of human LPH from stably transfected CHO-cells by immunoprecipitation with monoclonal anti-LPH antibodies allows specific activity measurements with known substrates, individual flavonoids and plant extracts. Deglycosylation of approximately 20 putative substrates was tested by determination of the released products by a photometric method (glucose) and by HPLC-DAD (flavonoid aglyca). Flavonoid glycosides with terminal rhamnose (e.g. rutin, naringin and quercitrin) and isoflavone glycosides (e.g. sophoricoside and genistin) were not hydrolyzed by human LPH. A cleavage of flavonoid-mono-/di-β-glycosides with glucose residues in different positions (3, 4', 3', 7) of the aglycone could be observed (e.g. kaempferol-3-O-glucoside, quercetin-4'-O-glucoside, apigenin-7-O-glucoside and luteolin-3'-7-O-diglucoside). Hydrolysis of specific flavonoids could also be shown for plant extracts of onion and curly kale. As an example about 40% of quercetin-4'-O-glucoside, the main component of an onion extract, was deglycosylated by LPH demonstrating its importance in the metabolisation of biologically active compounds.
Acknowledgement:
We thank the Federal Ministry of Education and Research for financial support, Project-No. 315371E.
References: 1. Behrendt M et al. (2009) Gastroenterology 136: 2295–2303
