Enhancement of the thermal and storage stability of urease by covalent attachment to phospholipid-bound silica

Urease was immobilized directly on silanized silica surfaces carrying alkyl moieties with terminal carboxylic groups. The enzyme was also covalently attached to phospholipid-bound silanized silica surfaces through the terminal carboxyl moiety on the sn-2 acyl chain of the lipid. The surfaces were characterized by X-ray photoelectron spectroscopy and ellipsometry. The activity of the immobilized urease was determined by UV spectrophotometry using a urea/bromocresol purple substrate. The enzymic activity decreases exponentially upon storage under dry solid conditions for 1 week or upon heating to 100 degrees C in the case of the silane/enzyme surfaces. On the other hand, the enzyme immobilized on phospholipid-carrying silica surfaces retained its entire original activity under dry storage or heat treatment conditions. Such immobilized urease systems could find extensive applicability in the design of in vivo dialysis equipment or for on-line monitoring of urea.