Deconjugation of polyphenol-glucuronides by a factor with β-glucuronidase activity from human monocytes (Mono Mac 6 cells)

Plant polyphenols are present in the human body predominantly as their phase II metabolites. Recent results demonstrate that β-glucuronidase from human neutrophils is able to deconjugate and thus activate glucuronide conjugates during inflammation (in vitro). In the present study, the ability of Mono Mac 6 cells (human acute monocytic leukemia cells) to deconjugate polyphenol-glucuronides was investigated. Baicalin (5,6,7-trihydroxyflavone-7-O-glucuronide (1) and 4-methylumbelliferone-glucuronide (2) were selected as test substances. The cells were seeded in 24-well plates (0.3 × 10⁶ cells/ml) and grown for 48h. After removing cells by centrifugation, the supernatant's pH was adjusted to 4 – 5 and was incubated with the substances (50µM) over a period of 2, 4, 6, and 8 hours. The reaction was stopped by precipitating the proteins with MeOH, and the samples were analyzed by HPLC-DAD. The same procedure was performed without cells to ensure the stability of tested substances. Both glucuronides were deconjugated to a different extent: while 71% of 1 were deconjugated after 8 hours, only 53% of 2 were cleaved. The addition of TNF-alpha (110 U/ml) had no influence on the deconjugation, but the specific β-glucuronidase inhibitor D-saccharic acid-1,4-lactone (10µM) completely inhibited this effect. No deconjugation could be observed at pH = 7.4.

These results indicate the presence of a factor with β-glucuronidase activity, which seems to be secreted constitutively by the Mono Mac 6 cells and shows pH dependency. The varying extent of deconjugation is presumably based on different substrate structure.