Although the enantioselective kinetic resolution of ester racemates of the non-steroidal anti-inflammatory drug naproxen ([2-(6-methoxy-2-naphthyl)propionic acid]) is a common demonstration for biocatalysis, the enantiomeric excess of the reactions is usually insufficient to warrant commercialisation. However, optimised reactions using heterologously expressed Carboxylesterase NP provided highly enantioselective hydrolysis of racemic naproxen methyl ester. Up to 46.9% conversion was achieved in 5 h in the presence Of 10 units enzyme/g ester with a product ee of 99% and an E of approximately 500. The final optimised reaction conditions were 150 g/l of substrate Slurry in 0.01 M sodium phosphate buffer pH 8.75 (maintained with 2.5 M NaOH) at 45 C and in the presence of 1% surfactant (Tween 80). Stabilisation of the enzyme with >2000 ppm formaldehyde prior to use resulted in an optimal volumetric productivity of 22.2 g/l/h Substrate at an enzyme loading of 18 units enzyme/g ester. Reaction kinetics revealed that the product S-naproxen formed causes product inhibition, but not enzyme toxicity, and resulted in the decrease in reaction rate with conversion. The R-NME (naproxen methyl ester) (unwanted enantiomer) did not have a significant influence on the reaction rate. DBU (diazobicycloundecene), used for the racemisation of the unwanted enantiomer, was recycled with the substrate but did not influence the conversion rate. (c) 2008 Elsevier Ltd. All rights reserved.