Evaluation of Liver Function by Co-administration Methodology Using 13C-Labelled Benzoic Acid and Hippuric Acid Coupled with Nuclear Magnetic Resonance Spectroscopy

The amount of hippuric acid synthesized and excreted in the urine after benzoic acid loading (hippuric acid test) is a useful index of liver function. However, the hippuric acid test gives erroneous results in the event of failure of renal excretory function. A new stable isotope co-administration methodology using nuclear magnetic resonance (NMR) spectroscopy has been developed to overcome this defect. [7-¹³C]Benzoic acid and [glycine carbonyl-¹³C]hippuric acid ([gly-¹³C]hippuric acid), each 0.4–0.6 mmol kg^?1 were simultaneously administered intravenously as probes to normal or liver-injured rats and the urine was analysed by 100 MHz ¹³C NMR spectroscopy. Consequently, urinary excretion of [7-¹³C]hippuric acid formed from [7-¹³C]benzoic acid and [gly-¹³C]hippuric acid was successfully traced with very simple and convenient procedures. The urinary excretion of [7-¹³C]hippuric acid indicated the combined functions of hippuric acid synthesis and renal excretion, whereas that of [gly-¹³C]hippuric acid was indicative of renal excretion of hippuric acid only. The heights of resonances for C7 of [7-¹³C]hippuric acid and the glycine carbonyl carbon of [gly-¹³C]hippuric acid were used to calculate the concentrations of labelled hippuric acids. [7-¹³C]Hippuric acid was excreted more slowly than [gly-¹³C]hippuric acid by both normal and liver-injured rats. The liver-injured rats excreted the labelled hippuric acids more slowly than the normal rats. The kinetic parameters were computed for the individual rats on the basis of Michaelis-Menten elimination for benzoic acid and first-order elimination for hippuric acid. The maximum rates of metabolism (V_max) (4.8–5.8 μmol min^?1 kg^?1) and the renal elimination rate constants of hippuric acid (K_re) (0.010–0.021 min^?1) in the liver-injured rats were lower than those (V_max 6.7–11.8 μmol min^?1 kg^?1; K_re 0.026–0.045 min^?1) in the normal rats. These results have demonstrated that liver function can be evaluated from the V_max value even though the renal function of hippuric acid excretion (K_re) is impaired. Thus the co-administration methodology is feasible and can remove the defect of the previous hippuric acid test. These results could form the basis for a more convenient and reliable hippuric acid test in man.