Pranee Pattanapipitpaisal and Tipmontha Reakyai

pp. 407 - 414

Abstract

Residual chromium compounds in discharged effluents is a serious problem, due to hexavalent chromium or chromate
[Cr(VI)] being extremely toxic and showing mutagenic and carcinogenic effects on biological systems. The bacterial enzymatic
Cr(VI) reduction can occur and this could be an effective method of detoxifying Cr(VI) polluted effluent. The present study
characterized Cr(VI) reductase activity of cell-free extracts (CFE) of thermophilic chromate-reducing bacteria, Bacillus
fusiformis NTR9. Results showed that the optimum temperature and pH for Cr(VI) reductase activity of CFE was 80°C and
pH 7, respectively. The reductase activity remained at 60.34% and 26.44% after 30 minutes of exposure to 70 and 90°C,
respectively, suggesting a heat stable enzyme. Moreover, the enzyme was resistant under acidic and neutral condition but its
stability was decreased under alkaline condition. The Cr(VI) reductase activity of CFE was enhanced when exposed in Cu²⁺
and Fe³⁺ by 188.19% and 180.38%, respectively. The Cr(VI) reductase activity could be reduced to 72.19% and 8.95% in the
presence of Mn²⁺ and Ag⁺
, respectively. Mg²⁺, Zn²⁺, As³⁺ and electron acceptors like sulfate and nitrate had no affect on Cr(VI)
reductase activity. The external electron donors (glucose, glycerol, citrate, malate, succinate, and acetate, but not NADH)
were essential to improve the chromate reductase activity of NTR9 strain. The chromate reductase was mainly associated
with the soluble fraction in the cytoplasm of the bacterial cell. The molecular weight of the enzyme was 20 KDa. The results
showed that Cr(VI) reductase could be a good candidate for detoxification of Cr(VI) in industrial effluents.