Optimum immobilization of urease on modified acrylonitrile copolymer membranes: Inactivation by heavy metal ions

Abstract

Poly(acrylonitrile-methylmethacrylate-sodium vinylsulfonate) membranes were subjected to chemical modification with hydroxyl ammonium sulfate (NH 2OH·H 2SO 4) and the amount of functional amidoxime groups was determined in the modified membranes. Urease was covalently immobilized on the modified membranes. The relationship between immobilization factors and enzyme activity was examined by a series of contour plots. The selections of the immobilization variable range were extremely precise in the 3-level-3-factor fractional design. The results indicated that the optimal conditions for urease immobilization were: 0.1% enzyme solution, immobilization temperature – 4 °C and immobilization time – 20 h. The inhibitory effect (IC 50) of Cu(II), Cd(II), Zn(II), Ni(II) and Pb(II) was studied on free and immobilized urease. The behavior of the immobilized urease in model solutions, containing different mixtures of heavy metals was represented in a 3D model. By studying the inhibition effect of two different mixtures Cu(II) and Cd(II); Cu(II) and Zn(II) it was found that the linear range of urease inhibition by Cu(II) ions for the first model mixture was from 0 to 1 mg l −1 and by Cd(II) ions from 0 to 1.5 mg l −1, as for the second model mixture the linear range of urease inhibition by Cu(II) ions was from 0 to 1 mg l −1 and by Zn(II) ions from 0 to 3.0 mg l −1. This linear dependence is very important from a practical point of view, regarding the application of immobilized urease in the construction of a biosensor for the detection of low concentrations of heavy metal ions.