Chromate reduction by cell-free extract of Bacillus firmus KUCr1

Microbial enzymatic reduction of a toxic form of chromium [Cr(VI)] has been considered as an effective method for bioremediation of this metal. This study reports on the in vitro reduction of Cr(VI) using cell-free extracts from a Cr(VI) reducing Bacillusfirmus KUCr1 strain. Chromium reductase was found to be constitutive and its activity was observed both in soluble cell fractions (S₁₂and S₁₅₀) and membrane cell fraction (P₁₅₀). The reductase activity of S₁₂ fraction was found to be optimal at 40 μM Cr(VI) with enzyme concentration equivalent to 0.493 mg protein/ml. Enzyme activity was dependent on NADH or NADPH as electron donor; optimal temperature and pH for better enzyme activity were 70°C and 5.6, respectively. The Km value of the reductase was 58.33 μM chromate having a Vmax of 11.42 μM/min/mg protein. The metabolic inhibitor like sodium azide inhibited reductase activity of membrane fraction of the cell-free extract. Metal ions like Cu²⁺, Co²⁺, Ni²⁺ and As³⁺ stimulated the enzyme but others, such as Ag⁺, Hg²⁺, Zn²⁺, Mn²⁺, Cd²⁺ and Pb²⁺, inhibited Cr(VI) reductase activity.