S U M M A R YTwo steers totally nourished by intragastric infusion of volatile fatty acids and casein were given an abomasal infusion of a microbial protein preparation (Pruteen) at eight rates with a purine input ranging from 0 to 170 mmol/day over 11 successive 5-day periods. The urinary excretion of purine derivatives relative to the purine input was measured. Negligible amounts of xanthine plus hypoxanthine were present in the urine. The relative contributions of allantoin and uric acid to the total excretion were not affected by the rate of purine infusion. Total purine derivative excretion (uric acid and allantoin) was linearly correlated with purine input. Recovery in the urine of the infused purines was 0-77. It is suggested that utilization of exogenous purines may only occur in the intestinal mucosa and that the remaining purines may be completely converted, before entering the liver, to uric acid and allantoin, which are subsequently eliminated by the renal and extrarenal routes. The differences between cattle and sheep in excretion of purine derivatives, and the implications of these differences for the use of purine excretion values in order to estimate microbial protein supply to the ruminant, are discussed.

The present study examined the relationship between the supply of exogenous nucleic acid (NA) purines and their recovery as the derivatives hypoxanthine, xanthine, uric acid and allantoin in urine. Six lambs, totally nourished by intragastric infusions of volatile fatty acids (VFA) and casein (i.e. no rumen fermentation), were given by abomasal infusion a microbial NA concentrate at six levels (from zero to 24.5 mmol purines/d). The true digestibility between the abomasum and terminal ileum of the NA purines was measured in a separate experiment using three lambs. The relative proportion of urinary allantoin increased, and that of other derivatives decreased, as the amount of NA infused was increased. The relationship between total excretion of purine derivatives ( Y; mmol/d) and exogenous purines absorbed (X, mmol/d) was Y = 0~84X+0~150W0~7see-0~25x, where W is body-weight (kg). This implies that the endogenous contribution to the total excretion of derivative decreased as the supply of exogenous purines increased, with an associated progressive replacement of de n o w synthesis by exogenous purines. The model also implies that 0.16 of the purines were eliminated through routes other than derivative excretion in urine. Once excretion exceeded 0.6 mmol/kg WO." per d, endogenous excretion was effectively zero and thus Y = 084 X. In normally fed sheep, derivative excretion should therefore relate to the microbial purines and, hence, microbial protein absorbed according to these models. The changing proportions of allantoin and other derivatives in urine were probably due to changes in the relative importance of endogenous and exogenous purines as precursors. Nucleic acids: Purine derivatives: SheepStudies on the urinary excretion of purine derivatives by ruminants have been stimulated by the possible use of their excretion as an estimator of the rumen microbial protein supplied to the host animal. This is because in ruminants nucleic acids (NA) flowing to the small intestines are essentially of rumen microbial origin (McAHan & Smith, 1973). Absorbed purines are degraded to hypoxanthine, xanthine, uric acid and allantoin, which are excreted in urine, and should relate quantitatively to the amount of purines and, hence, microbial protein absorbed.The excretion of allantoin has been shown to be correlated with the amount of NA infused post ruminally to sheep (Antoniewicz et al. 1980;Sibanda et al. 1982;Giesecke et al. 1984;Lindberg, 1985;Fujihara et al. 1987). However, recoveries of NA-nitrogen as allantoin-N reported by these authors varied from 0.12 to 0.47 (i.e. purine recoveries of 0.22-0.79). Since uric acid, xanthine and hypoxanthine were not measured, it is not possible to assess whether the recovery of absorbed purines as the derivatives in the urine was complete in these studies. It is also not clear from the existing information whether, and to what extent, exogenous purines would be retained by the animal body.In the studies to be reported here the intragastric infusion technique (0rskov et al. 1979) was e...