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

Extracts 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.