D-Galacturonic acid reductase was purified from a psychrophilic yeast strain of Cryptococcus diffluens, which was isolated from Satho, a traditional alcohol drink in Thailand. This enzyme, named Cd-GalUAR, assimilates D-galacturonic acid and requires NADPH as a cofactor. Cd-GalUAR is about 45 kDa and stable from pH 6.5 to 7.5 and up to 35 degrees C. Its optimum pH and temperature are pH 7.0 and 40 degrees C, respectively. However, 80% of its maximum activity remained at 4 degrees C. The reaction of Cd-GalUAR from D-galacturonic acid produces L-galactonic acid, which was identified by (13)C NMR and LC-MS. Three amino acid sequences were determined from trypsin-digested peptides of Cd-GalUAR. Similar sequences are found in many NAD or NADP oxidoreductases, including some D-galacturonate reductases. Our results suggest that Cd-GalUAR is the first D-galacturonate reductase identified in yeast. (C) 2010, The Society for Biotechnology, Japan. All rights reserved.