Kinetics of urea hydrolysis in wheat residue

Abstract

When urea granules dissolve on crop residue, the concentration near the point of dissolution may be high enough to slow the hydrolysis rate. Our objective was to determine urease activity in wheat residue at a wide range of urea concentrations (0.007-5 M urea). Urease was assayed in the laboratory at 35°C with l g ground wheat residue (1 mm size), toluene, THAM-buffer (pH = 8.0) and urea solutions. The rate of urea hydrolysis was measured as the rate of ammonium appearance. A Michaelis-Menten model modified for uncompetitive substrate inhibition was fitted to the hydrolysis rate data using a non-linear regression procedure Rate = V max(1 + ( K m/[ U]) + ([ U]/ K i) where Rate is the rate of urea hydrolysis, [ U] = urea concentration (m); V max, K m and K i, are the maximum rate (mgNH 4 +-Nkg residue −1 h −1), the concentration at 1 2 V max (M urea), and inhibition constant [or dissociation constant of enzyme substrate complex (m −1) which does not form product], respectively. The values of V max, K m and K i, for the wheat residue urease were 792, 0.003 and 5.94, respectively. The urea hydrolysis rate increased with increasing concentrations up to 0.05 M urea, remained steady between 0.05 and 0.5 m urea, and then declined above 0.5 m urea. The model adequately described the hydrolysis rate up to 2.0 m urea, but overestimated it at higher concentrations. Thus, the decreased rate of urea hydrolysis above 2.0 M urea could not be attributed to uncompetitive inhibition alone but to a combination of factors. Since high urea concentrations cause denaturation of proteins, the rate declines not explained by the model may be due to denaturation of the urease enzyme.