Inhibition of activity of urease in native form and immobilized on chitosan membrane by sodium fluoride

The influence of sodium fluoride inhibitor on the activity of urease in native form and immobilized covalently on glutaraldehyde-pretreated chitosan membrane was studied. Initially, in the presence of fluoride ions (I), a complex EI is formed which undergoes slow isomerization into a secondary form EI*. Both stages are represented on the progress curves. The dissociation constant of EI, K_i, for native and bonded urease were determined. The apparent rate constant as a function of the initial urea concentration for a given fluoride ion concentration was calculated, k = k ([I], [S]). It was found that sodium fluoride is a competitive slow-binding inhibitor of urease. Hydrolysis of urea in the standard conditions, at pH = 7.0, at 25° and a constant ionic strength and variable concentrations of substrate and of inhibitor has been studied. The reaction was initiated by addition of enzyme and was observed over a period of 10 min. The rate of hydrolysis at any point of time for both forms of urease, free and bound, may be calculated from the inhibition constants K_i, K*_i and apparent rate constant, k. It was found that immobilized urease is more resistant to the action of the inhibitor than the native one. This property offers potential for practical application.