Abstract

Inhibition of α-glucosidase (EC 3.2.1.20) and β-galactosidase (EC 3.2.1.23) biosynthesis by zinc and cadmium in Escherichia coli, Bacillus and Pseudomonas species isolated from petroleum refinery wastewater was assessed. At sufficient concentrations, exposure of the cells to zinc and cadmium resulted in repression of α-glucosidase and β-galactosidase induction. The extent of inhibition varied among the test bacteria, metal and enzyme. Bacillus with IC₅₀ of 0.125 ± 0.004 and 0.120 ± 0.007 mM is most sensitive to cadmium inhibition of α-glucosidase and β-galactosidase respectively. Biosynthesis of β-galactosidase in Bacillus species is most sensitive to toxicity of zinc. The patterns of these toxic effects can be mathematically described with logistic or sigmoid dose-response models. The coefficients of inhibition Ki correlated with the IC₅₀ and indicate that inhibition coefficient can be used in addition to the EC₅₀ values as a measure of toxicity. The information could have a bearing on the development of bacterial models for acute toxicity assays.

Key words: Zinc, cadmium, α-glucosidase, β-galactosidase, toxicity, inhibition coefficient.