Soybean urease (EC 3.5.1.5) is a homohexameric enzyme (containing six subunits with molecular weights of 90770 Da and two nickel atoms each), which catalyzes urea hydrolysis by water, resulting in the formation of ammonium cations and carbonate anions [1]. Urease is widely used for determination of urea by bioanalytical methods and in immunoassays as an enzyme label for many antigens [2,3]. Despite the great importance of urease in biotechnology and medicine, the detailed mechanism of urea hydrolysis by urease is still not clarified, and the structure of the metal site, containing two nickel atoms, is not precisely known [1,4,5].An important tool for the investigation of the catalytic mechanism of urease is inhibition and activation of this enzyme. The main groups of urease inhibitors, specificity and efficiency of urease activity were discussed in our earlier review [6]. It was found that urease was produced by many microorganisms and was associated with such pathologies as duodenal and gastric ulcers as well as a number of human and animal urinary diseases [5,7]. It was also demonstrated that there was a clear correlation between the inhibiting properties of the same compounds with respect to various ureases of microbial and plant origin. Therefore, the primary screening of potential inhibitors for urease can be performed more easily and quickly using catalytically active and thermally stable soybean urease [4,5,8].In earlier studies, we investigated the following urease inhibitors: polydisulfides of urea and thiourea, which compete with the substrate for binding to the urease active sites; amides of thiophosphoric acid, which were analogues of the substituted phosphorodiamidates (these compounds were the most effective urease inhibitors [4-6, 8]); cyclic β -triketones (CTKs); and fluoride anions. These inhibitors are organic and inorganic ligand chelators of nickel [9]. An important advantage of CTKs and fluoride anions is that they are nontoxic. It is obvious that the range of polycarbonyl compounds (PCCs), potential chelators of nickel ions, should be widened, and nickel ligands of polycarbonyl structure should be tested as urease inhibitors within a broad pH range, because the value of the inhibition constant for urease inhibition by fluoride anions significantly increases due to pH variation within the range from 3.85 to 6.45 [9]. This factor is particularly important for practical purposes because of the acidity of gastric medium in humans and animals.The practical aspect of this work is associated with selection of effective urease inhibitors for use as chemotherapeutical agents in gastroenterology and stopreagents in biochemical assay of urea (enzyme immunoassay and biosensors).The goal of this work was a comparative kinetic study of soybean urease inhibition by such organic chelators of nickel as polycarbonyl compounds, including oxalyldihydrazide (ODH), with two carbonyl groups, cyclic triketones I -VII with three carbonyl groups, and poly(disulfide-oxalyldihydrazide (poly(DSODH)) with 28 carbonyl gro...

Inactivation of soybean urease in aqueous solution at pH 5.4, 36 degrees C, and high-frequency sonication (2.64 MHz, 1.0 W/cm2) is substantially reduced in the presence of seven structurally different flavonoids. A comparative kinetic study of the effect of these flavonoids on the effective first-order rate constants that characterize the total (thermal and ultrasonic) inactivation k(i), thermal inactivation k(i)*, and ultrasonic inactivation k(i)(US) of 25 nM enzyme solution was carried out. The dependences of the three inactivation rate constants of the urease on the concentrations of flavonoids within the range from 10(-11) to 10(-4) M were obtained. The following order of the efficiency of the flavonoids used in respect of the urease protection from ultrasonic inactivation was found: astragalin > silybin > naringin > hesperidin > quercetin > kaempferol > morin. The results confirm a significant role in the inactivation of the urease of HO* and HO2*, free radicals, which are formed in the ultrasonic cavitation field.