Characterization of immobilized β-Glucuronidase in aqueous and mixed solvent systems

β-Glucuronidase (β-d-glucuronide glucuronosohydrolase, EC 3.2.1.31) was covalently attached to alkylamine-controlled pore glass via a glutaraldehyde immobilization scheme. The activity of this immobilized β-glucuronidase was studied with respect to several kinetic parameters in comparison with the behavior of the soluble enzyme. K_m values for p-nitrophenyl glucuronide, estriol-3-glucuronide, and estriol-16β-glucuronide were determined. For each substrate the K_m was essentially the same, 0.2 mM, and this value did not change when the enzyme was immobilized. The soluble and immobilized enzyme both displayed a relatively broad pH maximum centered at pH 6.8 for all substrates. Several organic-aqueous mixtures including methanol, ethanol, acetonitrile and ethylene glycol were tested, and their effects on the activity of immobilized β-glucuronidase were similar to those found for the soluble enzyme. Long-term (1 year) storage stability tests of the immobilized enzyme were carried out. The immobilized enzyme retained 40% of its initial activity after 1 year and was very robust towards most of the organic solvents tested.