Metabolic implications of ammonia production in the ruminant

摘要

NH_3 is generated in the gut of all animal species as a result of two main processes: (1) microbial degradation of nitrogenous compounds such as proteins, pep tides, amino acids and nucleic acids within the gut lumen and (2) microbial hydrolysis of urea passing across the gut wall from the blood and interstitial fluids. Whereas in most species the production of NH.3 and its incorporation into microbial protein in the hindgut is considered of little nutritional benefit to the host (apart from coprophagic species), the pathway of N assimilation into microbial protein in the reticulo-rumen is an essential component of protein flow to the small intestine of ruminant animals. As such it has been demonstrated that ruminants can sustain a modest level of productivity when provided only with non-protein-N in the diet (Virtanen, 1969). Protein rationing for this group of livestock is based on provision of rumen-degradable N for microbial protein synthesis in addition to an undegr adable component calculatedto support required levels of output (Agricultural Research Council, 1980, 1984; Agricultural and Food Research Council, 1993). It is not the purpose of the present review, however, to evaluate rumen N transactions but to identify the pathways by whichNH_3 is generated within the gut and factors which affect its absorption and detoxification in the liver. There are a number of excellent reviews which discuss the broader aspects of N metabolism in the ruminant (MacRae & Reeds, 1980; Chalupa, 1984; Egan et al. 1986; Lobley, 1991).