Influence of Yucca schidigera preparations on the activity of urease from Bacillus pasteurii

Abstract

AbstractExtracts and preparations of the desert plant Yucca schidigera Roezl ex Ortgies (Mohave yucca), family Lillaceae, have a variety of beneficial effects when included in the diet of humans and domestic animals. Such effects include reduced gastrointestinal and faecal ammonia levels. A proposed mode of action is inhibition of microfloral urease (urea amidohydrolase; EC 3.5.1.5). We describe a rigorous method of in vitro urcase assay, in the presence of potential effectors such as Y schidigera preparations, using the phenolindophenol reaction to measure the ammonia product. The urease of Bacillus pasteurii was characterised in order to determine its suitability as a model bacterial urease. K‐phosphate, but not K‐HEPES or K‐citrate, was found to inhibit this bacterial urease, particularly at low pH, as previously reported for other plant and bacterial ureases. B pasteurii urease was found to have a Michaelis‐Menten constant, Km, of 10.5 ± 3.2 mM in 150 mM K‐HEPES, pH 65 and 0.89 M KCl, total ionic strength = 0.90 M at 30°C. It was therefore concluded that B pasteuriiurease is indeed a typical bacterial urease, suitable for studying the influence of Y schidigera preparations. For comparison, the effects of Y schidigera preparations on the activity of β‐galactosidase (β‐D‐galactoside galactohydrolase; EC 3.2.1.23) from Aspergillus or yzae, an unrelated hydrolase, were also determined. Urease and β‐galactosidase were both weakly and non‐specifically inhibited, in a fashion linearly related to the concentration of γ schidigera preparation. Linear regression of the relationship between γ schidigera preparation and enzyme activity yielded inhibition ratios of 3.2 ± 0.4 and 5.4 ± 1.6 nkat ml−1 preparation for urease and β‐galactosidase respectively. By comparing with reported in vivo rates of urea degradation in mammals it was concluded that the observed inhibitory properties of Y schidigera preparations are much too low to account for their in vivo effects at feed inclusion levels of as little as 100 g tonne−1.