To examine nitrilase-mediated hydrolysis of nitriles to produce optically pure alpha-hydroxycarboxylic acids. A novel nitrilase, GPnor51, from Luminiphilus syltensis NOR5-1B was discovered by genomic data mining. It could hydrolyze racemic o-chloromandelonitrile to (R)-o-chloromandelic acid with high enantioselectivity (ee 98.2 %). GPnor51 was overexpressed in Escherichia coli BL21 (DE3), purified, and its catalytic properties studied. GPnor51 had a broad substrate acceptance toward various nitriles with structure diversity. It was an arylacetonitrilase that uses arylacetonitriles as optimal substrates. The V (max) and K (m) of GPnor51 towards o-chloromandelonitrile were 1.9 mu mol min(-1) mg(-1) protein and 0.38 mM, respectively. GPnor51 also demonstrated high enantioselectivity toward mandelonitrile and other substituted mandelonitrile. This enzyme has a great potential for commercial production of optically pure (R)-mandelic acid and its derivatives.