Hepatic uptake and metabolism of benzoate: a multiple indicator dilution, perfused rat liver study

Multiple, noneliminated references (Cr-51-labeled erythrocytes, I-125-album in, [C-14]- or [H-3]sucrose, and [H-2](2)O), together with [H-3]hippurate o r [C-14]benzoate, were injected simultaneously into the portal vein of the perfused rat liver during single-pass delivery of benzoate (5-1,000 muM) an d hippurate (5 muM) to investigate hippurate formation kinetics and transpo rt. The outflow dilution data best fit a space-distributed model comprising vascular and cellular pools for benzoate and hippurate; there was further need to segregate the cellular pool of benzoate into shallow (cytosolic) an d deep (mitochondrial) pools. Fitted values of the membrane permeability-su rface area products for sinusoidal entry of unbound benzoate were fast and concentration independent (0.89 +/- 0.17 ml.s(-1).g(-1)) and greatly exceed ed the plasma flow rate (0.0169 +/- 0.0018 ml.s(-1).g(-1)), whereas both th e influx of benzoate into the deep pool and the formation of hippurate occu rring therein appeared to be saturable. Results of the fit to the dilution data suggest rapid uptake of benzoate, with glycination occurring within th e deep and not the shallow pool as the rate-determining step.