Effect of Glucuronides on Glucuronide Biosnthesis

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Abstract
1‐Naphthyl and phenolphthalein glucuronides were found to be potent inhibitors of p‐nitro‐phenyl glucuronide synthesis catalyzed by rat liver microsomal fraction. They did not compete with theaglycone, p‐nitrophenol, but with glucuronyl donor, UDPglucuronic acid. The free aglycones, 1‐naphthol and phenolphthalein, were, however, more powerful inhibitors. They competed with p‐nitrophenol andnot with UDPglucuronic acid.Also the o‐aminophenyl glucuronide synthesis in rat liver slices was inhibited by 1‐naphthyl and phenolphthalein glucuronides and free 1‐naphthol.The results indicate that the glucuronyl residue transfer to the aglycone totally block the affinity of the enzyme for the compound, although it is reduced. Glucuronides and their aglycones are apparently bound by different sites in the enzyme and glucuronides attach to UDPglucuronic acid binding site.