Human bile contains a considerable amount of endogenous
beta-glucuronidase. The effects of pH and
bile acids on its activity have been studied in regard to its role in the pathogenesis of
cholelithiasis.
beta-Glucuronidase, purified from human liver to homogeneity, was structurally stable between pH 4 and 10, but was active only over a much narrower range of pH, with a pH optimum of 5.2. The inactivation below pH 4 was due to its irreversible denaturation, whereas the inactivation at higher pH was due to a true reversible pH effect on the
enzyme velocity. Kinetic studies revealed that
hydrogen ion acted as a substrate-directed activator of the free
enzyme, but not the
enzyme-substrate complex, with a molecular dissociation constant of 4 X 10(-6). The
enzyme activity was not affected by unconjugated
bile acids, primarily due to their extremely low water solubility. Conjugated
bile acids, on the other hand, exerted heterogeneous and pH-dependent effects on the
enzyme. At pH 5.2,
taurocholic acid and
glycocholic acid were substrate-directed activators of the
enzyme;
taurochenodeoxycholic acid and
taurodeoxycholic acid, competitive inhibitors; and
glycochenodeoxycholic acid and
glycodeoxycholic acid, mixed inhibitors. At pH 7.0 all
taurine and
glycine conjugates behaved as substrate-directed activators. Though
beta-glucuronidase activity at pH 7 was only 23% of its maximal activity at pH 5.2, conjugated
bile acids tended to restore its activity to a certain extent at pH 7. Thus, endogenous
beta-glucuronidase could play a significant role in pigment
cholelithiasis.