Although the authors recently reported that nafamostat, a clinically used serine protease inhibitor, was mainly hydrolysed by carboxylesterase in human liver microsomes, the involvement of human liver cytosol has not been elucidated. The current study examined the in vitro metabolism of nafamostat with human liver cytosols. Kinetic analysis indicated that the Vmax and Km values in the liver cytosols were 9.82 nmolmin(-1) mg(-1) protein and 197 microM for a liver sample HL-1, and 15.1 nmolmin(-1) mg(-1) protein and 157 microM for HL-2, respectively. The Vmax/Km values in both cytosols were at least threefold higher than those in the corresponding microsomes. The liver cytosolic activity for nafamostat hydrolysis was inhibited by phenylmethylsulfonyl fluoride (PMSF) (43% inhibition at 100 microM), whereas diisopropyl fluorophosphate (DFP) and bis(p-nitrophenyl)phosphate (BNPP) failed to inhibit the activity. Furthermore, the hydrolytic activity was also reduced by palmitoyl-CoA (67% inhibition at 100 microM) but not by acetyl-CoA. Effects of PMSF, DFP and BNPP on cytosolic palmitoyl-CoA hydrolytic activity were comparable with those of the cytosolic nafamostat hydrolytic activity. In addition, the palmitoyl-CoA hydrolytic activity was competitively inhibited by nafamostat with the apparent Ki value of 164 microM for the liver cytosol from HL-2. These results suggest that an isoform of long-chain acyl-CoA hydrolase may be responsible for the nafamostat hydrolysis in human liver cytosol.