Urea breakdown
Ammonia is formed extremely rapidly from the breakdown of urea in the rumen and can then be used for amino acid synthesis. This activity enables ruminants to utilize urea entering the rumen either with the feed (Virtanen, 1966; Salter et al., 1979; Roffler and Satter, 1975), or endogenous salivary secretion, or by diffusion across the rumen wall (Kennedy and Milligan, 1980). The enzyme mechanism is a simple hydrolysis by urease, which can be inhibited in vitro by acetohydroxamic acid (Makkar et al., 1981). The ruminal enzyme is probably similar to jackbean urease in its Ni content, judging by the stimulation of ruminal urease activity by dietary Ni (Spears et a I., 1977; Spears and Hatfield, 1978). Urease is associated with the particulate microbial
Table 15.1. 'Hyper-ammonia producing', amino acid-fermenting bacteria isolated from the rumen.
Study Location Source Isolates Properties
Table 15.1. 'Hyper-ammonia producing', amino acid-fermenting bacteria isolated from the rumen.
Study Location Source Isolates Properties
|
Russell etal. (1988) |
Cornell, USA |
Cow consuming timothy hay/concentrate mixture |
Peptostreptococcus anaerobius |
Non-saccharolytic iso-acids producer |
|
Chen and Russell (1989a) |
Cornell, USA |
Cow consuming timothy hay/ concentrate mixture |
Clostridium aminophilum Clostridium sticklandii |
Non-saccharolytic acetate, butyrate producer Non-saccharolytic acetate, iso-acids producer |
|
Attwood etal. (1998) |
New Zealand |
Grazing cow Grazing deer Grazing sheep |
Eubacterium spp. Peptostreptococcus spp. Fusobacterium spp. Peptostreptococcus spp. |
Non-saccharolytic acetate, butyrate producer Non-saccharolytic iso-acids producer Non-saccharolytic acetate, butyrate producer Non-saccharolytic acetate, butyrate producer |
|
McSweeney et al. (1999) |
Queensland, Australia |
Sheep and goats receiving Calliandra |
Clostridium spp. |
Non-saccharolytic, proteolytic, lactate, propionate, butyrate, valerate producer |
|
Eschenlauer etal. (2002) |
Aberdeen, UK |
Sheep consuming mixed grass hay/concentrate diet |
Clostridium/Eubacterium spp. Acidaminococcus spp. Desulfomonas spp. |
Non-saccharolytic butyrate, valerate, caproate producer Non-saccharolytic acetate, butyrate producer Non-saccharolytic lactate producer |
fraction of ruminal fluid, and is predominantly of microbial origin (Gibbons and McCarthy, 1957; Jones et al., 1964; Mahadevan et al., 1976). No urease activity was found in either starved, antibiotic-treated ciliate protozoa (Onodera et al., 1977) or in Piromyces or Neocallimastix fungal isolates (Sakurada et al., 1994). Urea is not hydrolysed in the absence of the microbial population, when its concentration in the rumen is the same as that in blood (Cheng and Wallace, 1979).
Urease is one of the most variable enzyme activities in ruminal contents. Many factors influence urease activity, such as Ni in the diet. Ammonia may suppress activity (Cheng and Wallace, 1979), and urea is an inducer (Czerkawski and Breckenridge, 1982). It is of interest to understand urease regulation, because the principal disadvantage of urea as a source of non-protein nitrogen is that it is broken down too rapidly, resulting in ammonia overflow and inefficient nitrogen retention.
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