Isolation and characterization of urease from Aspergillus niger

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Abstract
~~~~~~~~~~ ~~~~~~~~ Urease was purified (4126-fold) from Aspergillus niger (NRRL 003) to a homologous enzyme preparation with a specific activity of 1341 pmol min-' (mg protein)? One species of urease was detected in A. niger, with K, = 3-0 mM, native molecular mass 250000 Da, pH optimum of 8.0 and a high specificity for urea. Hydroxyurea was a strong competitive inhibitor of urease activity, while N-methylurea acted as a weak uncompetitive inhibitor, based on Lineweaver-Burk and EadieHoftstee plots. The activity of urease was enhanced by, but not dependent on, the presence of Na,EDTA, DL-dithiothreitol (< 0.1 to 5.0 mM), CaZ+, Ba2+ and citrate (2 to 20 mM). Urease activity was not affected by Na+, K+, C1-, Br-, acetate or nitrate (2 to 20 mM), but was significantly decreased in the presence of Li+, Ni2+, Mgz+, Zn2+ or I-. Urease activity decreased 26.0 % after 30 min at 65 "C, and 86-5 % and 100.0% after 5 and 1 min at 80 and 100 "C, respectively. Urease activity decreased 30.5% after 90 d at 4 "C and 21.0% after 28 d at -20 or -80 OC.

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