Hargreaves A B, Marcondes N, Elias C A
An Acad Bras Cienc. 1976;48(3):567-76.
Urease obtained from seeds of Citrullus vulgaris fruits has been studied under three points of view: a) the effect of the urea analogs acetamide and hydroxi-urea on the enzyme kinetic b) the action of the sulfhydryl reagents and the reactivation agents on the enzyme c) the effect of X-rays and the protective action of the cysteamine. The Berthelot reaction for the determination of the liberated NH3 was used enzyme activity. Acetamide has no effect on urease kinetic. Hidroxy-urea which produces a typical green color when it is mixed with the Berthelot reagents at high concentrations, when properly diluted acts a aompetitive inhibitor of urease. Spectrophotometric experiments suggest that the studied urease decomposes hydroxi-urea with liberation of hydroxilamine. The sulphydril reagent, p-hydroxi-mercuribenzoate inhibits the enzime. Cysteine and dithiotreitol reactivate the enzyme activity in no more then 50% even when excess of the substances is used. Probably only in the first step of the urea hydrolysis, the enzyme behaves as a typical SH-enzyme. Urease is very sensitive to X-rays. Cysteamine acts as a protective agent of the enzyme. Dithiotreitol reinforces this protective action. This effect is clearly observed when the Fisbein catalytic method for urease is employed.
a）尿素类似物乙酰胺和羟基脲对酶动力学的影响；b）巯基试剂和复活剂对酶的作用；c）X射线的影响以及半胱胺的保护作用。采用伯索罗反应测定释放的NH₃来检测酶活性。乙酰胺对脲酶动力学无影响。高浓度时与伯索罗试剂混合会产生典型绿色的羟基脲，适当稀释后可作为脲酶的竞争性抑制剂。分光光度实验表明，所研究的脲酶分解羟基脲并释放出羟胺。巯基试剂对羟基汞苯甲酸抑制该酶。即使使用过量的半胱氨酸和二硫苏糖醇，它们对酶活性的复活作用也不超过50%。可能仅在尿素水解的第一步，该酶表现为典型的巯基酶。脲酶对X射线非常敏感。半胱胺可作为该酶的保护剂。二硫苏糖醇可增强这种保护作用。采用菲斯贝恩脲酶催化法时能清楚观察到这种效果。
