Urea Metabolism and Recycling in Ruminants

Abstract

The major sources of nitrogen (N 2 ) in the diet of mammals are amino acids and peptides derived from ingested proteins. The immediate end product of mammalian protein catabolism is ammonia, which is toxic to cells if allowed to accumulate. Therefore, amino acids are broken down in the liver into urea, which is used as non-protein nitrogen in the rations of ruminants as an economical replacement for feed proteins. Urea transferred from the blood to the rumen is also an important source of nitrogen for rumen microbial growth. Mammals cannot break down urea, which is traditionally viewed as a simple waste product passed out in the urine. However, urea from the bloodstream can pass into the gastrointestinal tract, where bacteria expressing urease cleave urea into ammonia and carbon dioxide. The bacteria utilize the ammonia as a nitrogen source, producing amino acids and peptides necessary for growth. Interestingly, these microbial products can be reabsorbed back into the host mammalian circulation and used for synthetic processes; because of this reason, urea has commonly become an accepted ingredient in the diets of ruminants. Therefore, this review focuses on urea metabolism and recycling in ruminants.

Urea transport and hydrolysis in the rumen: A review

“…Urea influxes into the rumen via several routes ( Stewart and Smith, 2015 ; Alemneh, 2019 ). The saliva route accounts for 10% to 40% of urea entry, whereas entry via the gastrointestinal wall is the major entry route, particularly across ruminal epithelium ( Berends et al., 2014 ).…”

1H NMR based metabolic profiling distinguishes the differential impact of capture techniques on wild bighorn sheep

“…The change regarding urea cycle alterations also manifested itself in differential serum urea levels, with fold changes (FC) between the groups decreasing significantly with capture techniques, with a mean FC difference of 1.4 for the dart-captured group, 0.26 for the dropnet-captured group, and − 0.3 for the helicopter-captured animals (Supplementary Table S2 ). As urea recycling is a prominent feature of ruminant metabolism and urea flux can rapidly change, the urea concentration changes observed between the three capture techniques support an impact on urea cycle intermediates 29 . While the trend of an overall decrease in urea cycle intermediates parallels a similar trend in amino acid concentrations, the extent to which amino acid metabolism is linked to changes in urea cycle activity is difficult to evaluate due to the nature of nitrogen recycling in the rumen of these ruminants.…”

1H NMR Based Metabolic Profiling Distinguishes the Differential Impact of Capture Techniques on Wild Bighorn Sheep

“…The change regarding urea cycle alterations also manifested itself in differential serum urea levels, with fold changes (FC) between the groups decreasing signi cantly with capture techniques, with a mean FC difference of 1.4 for the dart-captured group, 0.26 for the dropnet-captured group, and − 0.3 for the helicopter-captured animals (Table 1). As urea recycling is a prominent feature of ruminant metabolism and urea ux can rapidly change, the urea concentration changes observed between the three capture techniques support an impact on urea cycle intermediates 26 . While the trend of an overall decrease in urea cycle intermediates parallels a similar trend in amino acid concentrations, the extent to which amino acid metabolism is linked to changes in urea cycle activity is di cult to evaluate due to the nature of nitrogen recycling in the rumen of these ruminants.…”