An enzyme of Ralstonia/ Burkholderia strain DSM 6920 catalyzing the initial hydroxylation of 6-methylnicotinic acid at position 2 was purified to apparent homogeneity. It also catalyzed the unusual conversion of nicotinic acid to 2-hydroxynicotinic acid and was therefore designated as nicotinic acid dehydrogenase (NDH). Native NDH had a molecular mass of 280 kDa and was composed of subunits of 75, 30 and 16 kDa. It contained molybdenum, iron, acid-labile sulfur and FAD in a ratio of 1.6:7.3:8.0:0.6 mol(-1) of native enzyme. The molybdenum cofactor was characterized as molybdopterin cytosine dinucleotide. Zinc was identified as an additional metal ion in a molar ratio of 1.8 mol mol(-1) of native enzyme. Purified NDH exhibited a maximal specific activity of 22.6 micromol nitro blue tetrazoliumchloride reduced min(-1) mg(-1) of protein, using nicotinic acid as electron donor. The apparent K(m) value for nicotinic acid was determined to be 154 microM. Pyridine-3,5-dicarboxylic acid and quinoline-3-carboxylic acid were further substrates, but exhibited significantly different activity pH optima. Several artificial electron acceptors were reduced by NDH, but no activity was detected with NAD or O(2). NDH was inactivated upon incubation with cyanide, but no loss of activity was obtained in the presence of arsenite.