pH-Dependent rearrangement of the biosynthetic ester glucuronide of valproic acid to $-glucuronidase-resistant forms. Drug Metab. Dispos

A major metabolite of the antiepileptic drug valproic acid (VPA) in animals and man is the glucuronic acid conjugate, which is cleaved by incubation with fl-glucuronidase (specific for 1-O-substituted-$-o-glucopyranosiduronic acids) or hydrolysis in strong acid or alkali. Previous studies revealed that an often substantial proportion of the alkali-labile conjugated VPA in stored urine or bile samples was not hydrolyzed by fi-glucuronidase, suggesting the presence of nonglu-curonide conjugates. In the present study, bile from a NaVPA-treated rat was preincubated at 37#{176}Cfor 3 hr at pH values from-0.8 to 12.9, and then analyzed for nonconjugated VPA, VPA released by hydrol-ysis with g-glucuronidase, and VPA released by hydrolysis with alkali. At pH 3-7, all alkali-labile conjugated VPA remained suscep-tible to i9-glucuronidase hydrolysis, whereas at pH 0-3 and 7-1 1, a proportion became resistant to the enzyme. GLC and GC/MS analy-sis of trimethylsilyl derivatives of the intact conjugates revealed th