163 Studies on bovine and ovine rumen fluid urease A. Moharrery 1 Animal Science Department, Agricultural College, Shahrekord University, Iran ABSTRACT Rumen fluids were collected from a slaughterhouse. Mixed rumen contents of sheep and Holstein cows were transported to the laboratory in thermos flasks, separately for each species. Ammonium sulphate and acetone were used for precipitation of protein in cell free rumen fluid. The precipitate that was observed after freezing and thawing of cell free rumen fluid and precipitate, obtained with 25% saturated ammonium sulphate showed no urease activity. Urease activity in precipitates from 80% saturated ammonium sulphate for cows and sheep were 3.6l and 6.17 (microgram ammonia- N, mg –1 protein. min –1 ), respectively. These amounts for acetone precipitate for cow and sheep were 1.88 and 0.48, respectively. Sodium dodecyl sulphate (SDS) acrylamide gel electrophoresis indicated the same subunit urease enzyme for sheep and cow. The molecular weight of rumen urease subunit was estimated to 75000 daltons, which is less than jack-bean urease (96000 daltons). Native urease did not separated by 3.2% acrylamide gel electrophoresis. The latter finding may indicate that rumen urease has a hexameric structure with more than 1500 kilodaltons. KEY WORDS: sheep, cow, rumen, urease INTRODUCTION Urease is a nickel-containing enzyme that catalyses the hydrolysis of urea to form ammonia and carbamate. The latter compound spontaneously decomposes to generate a second molecule of ammonia and carbon dioxide. Bacterial urease plays an important role in ruminal nitrogen metabolism. Urea, supplied in the diet or recycled from saliva and the bloodstream, is hydrolysed to yield ammonia. The ammonia is then combined with ketoacids by the microbes to produce amino acids. Urease in the rumen will continue to convert urea to ammonia even if there is a lack of ketoacids. If the rate of ammonia production (i.e. urea via urease breakdown to ammonia) or ingestion of ammonia (i.e. ammoniated feeds) exceeds the microbes ability to utilize it to form amino acids, there is an accumulation of ruminal ammonia. The liver via the urea cycle normally detoxifies ammonia 1 Corresponding author: e-mail: moharrery@yahoo.com Journal of Animal and Feed Sciences, 13, Suppl. 1, 2004, 163−166