BENZOIC-ACID GLYCINE CONJUGATION IN THE ISOLATED-PERFUSED RAT-KIDNEY

The fate of varying input concentrations (0.002-372 mu M) of benzoic a cid was examined in the single-pass isolated perfused rat kidney prepa ration under constant flow rate (8 ml min(-1).organ(-1)). With an incr easing concentration of benzoate, the steady-state renal extraction ra tio 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 fra ctional excretion of similar to 0.2. Metabolic clearance, due primaril y to conjugation with glycine to form hippuric acid, constituted the m ajority of total renal clearance, and this decreased with concentratio n. These divergent trends for the metabolic and urinary clearances wit h concentration suggest that benzoate net influx across the basolatera l membrane has not been saturated. Upon fitting of the hippurate forma tion rates vs. the plasma unbound logarithmic average concentrations o f benzoate, overall kinetic constants (K-M = 5.3 mu M and V-max = 195 nmol min(-1).g(-1)) that likely reflect glycine conjugation were obtai ned. The formed hippurate either returned to the venous circulation or was excreted into urine without further biotransformation; the appare nt renal extraction ratio (excretion rate/formation rate of hippurate) was quite high (similar to 0.48). Avid glycine conjugation and hippur ate excretion thus occurred with administration of benzoic acid to the isolated perfused rat kidney.