Hydrolysis of glycyrrhizin to 18β-glycyrrhetyl monoglucuronide by lysosomal β- D-glucuronidase of animal livers

Abstract

Glycyrrhizin (GL), a main constituent of liquorice, was hydrolysed to 18β-glycyrrhetic acid mono-β- d-glucuronide (GAMG, glycyrrhetyl monoglucuronide) by rat liver homogenate, and the hydrolytic activity was localized in the lysosomes among the same subcellular fractions as acid β- d-glucuronidase activity ( p-nitrophenyl β- d-glucuronide (pNPG)-hydrolysing activity). Rat liver lysosomeshydrolysed GAMG to 18β-glycyrrhetic acid (GA) at only 30% rate compared with the rate of GL to GAMG. GA was also produced slowly from GL after time lag by the lysosomes. Thus, GL seems to be first hydrolysed to GAMG, which was successively hydrolysed slowly to GA. GL-hydrolysing activity was released together with acid β- d-glucuronidase activity from the lysosomes by sonication. Both activities from the sonicated lysosomes were eluted coincidentally on Sephacryl S-300 and butyl-Toyopearl 650M column chromatography, indicating that both activities are exhibited by the same enzyme. Moreover, GL-hydrolysing activity was inhibited strongly with d-saccharic acid 1,4-lactone, a specific inhibitor of β- d-glucuronidases of various origins. pH optimum of GL-hydrolysing activity was found to be 5.6, different from that (less than 4.0) of pNPG-hydrolysing activity. K m for GL was found to be 2 × 10 −5 M. Although hepatic lysosomes from mouse and cattle hydrolysed GAMG to GA similarly to those from rat, the hydrolysis of GAMG was not detected in lysosomes of human and porcine livers. Accordingly, lysosomal β- d-glucuronidases from human and porcine livers converted GL to GAMG only.