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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