Purification and partial characterization of a chromate reductase from Bacillus

A soluble NADH-dependent enzyme capable of reducing hexavalent chromium [Cr(VI)] to the trivalent form [Cr(III)] was purified from chromate-resistant Bacillus QC1-2. An enriched single protein band of 24 kDa was observed by SDS-PAGE following HPLC ion-exchange and size-exclusion procedures. In the latter step, the chromate reductase showed a molecular mass of 44 kDa, which suggested that the enzyme consists of two subunits of about 24 kDa. Purified chromate reductase displayed optimal activity at a temperature and pH of 37 degrees C and 7.0, respectively. The enzyme showed a Km of 0.35 mM for chromate and a Vmax of 50 nmol Cr(VI) reduced per minute per mg protein.