Benzoic acid glycine conjugation in the isolated perfused rat kidney

The fate of varying input concentrations (0.002-372 microM) of benzoic acid was examined in the single-pass isolated perfused rat kidney preparation under constant flow rate (8 ml min-1.organ-1). With an increasing concentration of benzoate, the steady-state renal extraction ratio decreased from 0.24 to 0.1. Little unchanged drug was found in the urine; the urinary clearance of benzoate was low (0.018 ml-1 min-1.g-1) and concentration-independent, yielding a rather constant fractional excretion of approximately 0.2. Metabolic clearance, due primarily to conjugation with glycine to form hippuric acid, constituted the majority of total renal clearance, and this decreased with concentration. These divergent trends for the metabolic and urinary clearance with concentration suggest that benzoate net influx across the basolateral membrane has not been saturated. Upon fitting of the hippurate formation rates vs. the plasma unbound logarithmic average concentrations of benzoate, overall kinetic constants (KM = 5.3 microM and Vmax = 195 nmol min-1.g-1) that likely reflect glycine conjugation were obtained. The formed hippurate either returned to the venous circulation or was excreted into urine without further biotransformation; the apparent renal extraction ratio (excretion rate/formation rate of hippurate) was quite high (approximately 0.48). Avid glycine conjugation and hippurate excretion thus occurred with administration of benzoic acid to the isolated perfused rat kidney.