Immobilization of soybean (Glycine max) urease on alginate and chitosan beads showing improved stability: Analytical applications

The soybean (Glycine max) urease was immobilized on alginate and chitosan beads and various parameters were optimized and compared. The best immobilization obtained were 77% and 54% for chitosan and alginate, respectively. A 2% chitosan solution (w/v) was used to form beads in 1N KOH. The beads were activated with 1% glutaraldehyde and 0.5 mg protein was immobilized per ml of chitosan gel for optimum results. The activation and coupling time were 6 h and 12 h. respectively. Further, alginate and soluble urease were mixed to form beads and final concentrations of alginate and protein in beads were 3.5% (w/v) and 0.5 mg/5 ml gel. From steady-state kinetics, the optimum temperature for urease was 65 degrees C(soluble), 75 degrees C (chitosan) and 80 degrees C (alginate). The activation energies were found to be 3.68 kcal mol(-1), 5.02 kcal mol(-1), 6.45 kcal mol(-1) for the soluble, chitosan- and alginate-immobilized ureases, respectively. With time-dependent thermal inactivation studies, the immobilized urease showed improved stability at 75 degrees C and the t(1/2) of decay in urease activity was 12 min, 43 min and 58 min for soluble, alginate and chitosan, respectively. The optimum pH of urease was 7, 6.2 and 7.9 for soluble, alginate and chitosan, respectively. A significant change in K-m Value was noticed for alginate-immobilized urease (5.88 mM), almost twice that of soluble urease (2.70 mM), while chitosan showed little change (3.92 mM). The values of V-max for alginate-, chitosan-immobilized ureases and soluble urease were 2.82 x 10(2) mu mol NH3 min(-1) mg(-1) protein, 2.65 x 10(2) mu mol NH3 min(-1) mg(-1) protein and 2.85 x 10(2) mu mol NH3 min(-1) mg(-1) protein, respectively. By contrast, reusability studies showed that chitosan-urease beads can be used almost 14 times with only 20% loss in original activity while alginate-urease beads lost 45% of activity after same number of uses. Immobilized urease showed improved stability when stored at 4 degrees C and t(1/2) of urease was found to be 19 days, 80 days and 121 days, respectively for soluble, alginate and chitosan ureases. The immobilized urease was used to estimate the blood urea in clinical samples. The results obtained with the immobilized urease were quite similar to those obtained with the autoanalyzer (R). The immobilization studies have a potential role in haemodialysis machines. (C) 2008 Elsevier B.V. All rights reserved.