Development of a free beta-galactosidase in vitro test for the assessment of heavy metal toxicity

The effect of heavy metals and organic compounds on the activity of the enzyme beta-galactosidase in a standardized bioassay has been evaluated, considering future applications in environmental monitoring. The tests were done using a commercial extract of a hydrolase from the eukaryote yeast Kluyveromyces lactis and o-nitrophenyl-beta-D-galactopyranoside (ONPG) as substrate. The enzyme was exposed to Cr(VI), Cd(II), Cu(II), Ni(II), Pb(II), Hg(II), phenol, sodium dodecyl sulfate, methanol and pentachlorophenol for 5, 15, 30, and 60 min. According to the results, a 15 min exposure time was considered optimum for the performance of the assay. Results of tests with metals showed IC50 values ranging between 9.25mg/L for Cd(II) and 0.015mg/L for Hg(II), with an order of sensitivity of: Cd(II) < Ni(II) < Cr(VI) = Pb(II) < Cu(II) < Hg(II). Sensitivity to organic compounds ranged from 200 to 4,000 mg/L, showing a higher specificity to heavy metals. The present in vitro free enzyme test showed a similar behavior to other tests based on beta-galactosidase such as the MetPlate. Furthermore, when compared to data from the literature on acute toxicity assays currently used in environmental assessment, test results show good agreement regarding the sensitivity to metals. After standardization, the proposed test could be used as a rapid and low-cost assay when evaluating biological effects of heavy metals in monitoring programs.