INFLUENCE OF YUCCA-SCHIDIGERA PREPARATIONS ON THE ACTIVITY OF UREASE FROM BACILLUS-PASTEURII

Extracts and preparations of the desert plant Yucca schidigera Roezl e x Ortgies (Mohave yucca), family Lillaceae, have a variety of benefici al effects when included in the diet of humans and domestic animals. S uch 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 urease assay, in the presence of potential effecters such as Y schidi gera 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-pho sphate, but not K-HEPES or K-citrate, was found to inhibit this bacter ial urease, particularly at low pH, as previously reported for other p lant and bacterial ureases. B pasteurii urease was found to have a Mic haelis-Menten constant, K-m, of 10.5 +/- 3.2 mM in 150 mM K-HEPES, pH 6.5 and 0.89 M KCl, total ionic strength = 0.90 M at 30 degrees C. It was therefore concluded that B pasteurii urease is indeed a typical ba cterial urease, suitable for studying the influence of Y schidigera pr eparations. For comparison, the effects of P schidigera preparations o n the activity of beta-galactosidase (beta-D-galactoside galactohydrol ase; EC 3.2.1.23) from Aspergillus oryzae, an unrelated hydrolase, wer e also determined. Urease and beta-galactosidase were both weakly and non-specifically inhibited, in a fashion linearly related to the conce ntration of Y schidigera preparation. Linear regression of the relatio nship between Y schidigera preparation and enzyme activity yielded inh ibition ratios of 3.2 +/- 0.4 and 5.4 +/- 1.6 nkat ml(-1) preparation for urease and beta-galactosidase respectively. By comparing with repo rted in vivo rates of urea degradation in mammals it was concluded tha t the observed inhibitory properties of Y schidigera preparations are much too low to account for their in vivo effects at feed inclusion le vels of as little as 100 g tonne(-1).