Endogenous allantoin excretion in response to changes in protein supply in sheep

Endogenous allantoin derives from the breakdown of tissue nucleic acids. This study examined the effect of changes in protein supply on endogenous allantoin excretion by sheep. The animals were nourished by infusions of volatile fatty acids into the rumen and casein into the abomasum, thus avoiding ruminal microbial fermentation (i.e., no supply of exogenous nucleic acids). While a constant energy supply was maintained, the protein supply was altered in one of two ways: 1) changed from 0 to 3000 mg casein-N/kg metabolic weight (W0.75) per day in progressive steps; or 2) completely removed from an initial constant level [500 mg N/(kg W0.75.d)]. With the first alteration, endogenous allantoin excretion was not directly affected by the daily N input or N retention, but was linearly correlated with the cumulative N retention. With the second alteration, allantoin excretion increased (35-145%) on the first day after removal of the protein supply and then fell to a level equivalent to, or lower than, that before protein removal. The results suggest that the changes in endogenous allantoin excretion may reflect remodeling of the metabolic state of the animal during periods when protein supply fluctuates.