The effect of phosphate buffer in the range of pH 5.80-8.07 on jack bean urease activity

The effect of phosphate buffer on the activity of jack bean urease was stud ied in the range of pH 5.80-8.07. The inhibition constants of phosphate buf fer were determined by measuring initial reaction rates at each pH for a se ries of buffer concentrations at a series of urea concentrations. It was sh own that: (1) at pH 5.80-7.49 the buffer is a competitive inhibitor of the enzyme with K-i,K-buffer increasing from 0.54 mM for pH 5.80 to 362 mM for pH 7.49, (2) the values of pK(i,buffer) are pH-dependent exhibiting a slope of -1 at pH 5.80-6.5 and a slope of -2 at pH 6.5-7.49, (3) from pH 7.62 as the pH is further raised the competitive inhibition of urease by the buffe r was not observed, (4) the true competitive inhibitor of urease is H2PO4- ion, and (5) pH 6.5 and 7.6 correspond to the ionization constants of the a ctive site groups of urease responsible for the inhibitory strength of H2PO 4- ion. (C) 1999 Elsevier Science B.V. All rights reserved.