2006 Volume 21 Issue 2 Pages 147-155
Metabolism of nafamostat, a clinically used serine protease inhibitor, was investigated with human blood and liver enzyme sources. All the enzyme sources examined (whole blood, erythrocytes, plasma and liver microsomes) showed nafamostat hydrolytic activity. Vmax and Km values for the nafamostat hydrolysis in erythrocytes were 278 nmol/min/mL blood fraction and 628 μM; those in plasma were 160 nmol/min/mL blood fraction and 8890 μM, respectively. Human liver microsomes exhibited a Vmax value of 26.9 nmol/min/mg protein and a Km value of 1790 μM. Hydrolytic activity of the erythrocytes and plasma was inhibited by 5, 5′-dithiobis(2-nitrobenzoic acid), an arylesterase inhibitor, in a concentration-dependent manner. In contrast, little or no suppression of these activities was seen with phenylmethylsulfonyl fluoride (PMSF), diisopropyl fluorophosphate (DFP), bis(p-nitrophenyl)phosphate (BNPP), BW284C51 and ethopropazine. The liver microsomal activity was markedly inhibited by PMSF, DFP and BNPP, indicating that carboxylesterase was involved in the nafamostat hydrolysis. Human carboxylesterase 2 expressed in COS-1 cells was capable of hydrolyzing nafamostat at 10 and 100 μM, whereas recombinant carboxylesterase 1 showed significant activity only at a higher substrate concentration (100 μM). The nafamostat hydrolysis in 18 human liver microsomes correlated with aspirin hydrolytic activity specific for carboxylesterase 2 (r=0.815, p<0.01) but not with imidapril hydrolysis catalyzed by carboxylesterase 1 (r=0.156, p=0.54). These results suggest that human arylesterases and carboxylesterase 2 may be predominantly responsible for the metabolism of nafamostat in the blood and liver, respectively.
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