Heavy metal effects on beta-glucosidase activity influenced by pH and buffer systems

Inhibition of beta-glucosidase activity by Cu(II); Zn(II) and Ni(II) was in vestigated as a function of pH and buffer type. Both factors were found to exert a strong effect on the activity of the enzyme. All three of the inves tigated heavy metals inhibited the enzyme activity in acetate buffer. At me tal concentrations of 0.6 mM, Zn and Ni reduced the enzyme activity by 25-3 0% under optimal pH conditions (pH 5-5.2). Under the same conditions, Cu sh owed an even more pronounced inhibitory effect than Zn and Ni. In presence of 0.6 mM Cu, the enzyme activity was lowered by more than 90% in compariso n to metal free systems. In contrast to these results, no enzyme inhibition was observed in citrate buffer, even in the presence of 1 mM Cu. The inhibition of beta-glucosidase activity by Cu increased with increasing pH. Inhibition by Zn and Ni was less pH-dependent in the observed pH range (pH 4-5.5). Copper caused a distinct shift in the pH optimum of enzyme act ivity, whereas this was not the case for Zn or Ni. The effects of buffer an d pH on enzyme inhibition by Cu, Zn and Ni were successfully described usin g a chemical speciation model, based on the assumption that enzyme activity depends on the protonation of the amino acids at the reactive site and tha t enzyme activity is inhibited by complexation of the reactive sites by the heavy metal cations. The results show the importance of taking chemical co nditions and speciation into account when investigating the effect of heavy metal cations on biological systems.