The Use of Urinary Excretion of Purine Metabolites as an Index of Microbial Protein Supply in Ruminants

Tracer studies were undertaken at the University of New England to investigate the kinetics of metabolism and excretion of the purine derivative (PD), allantoin. Its rate of flux through the plasma compartment of sheep was determined using both single injection (Study 1) and continuous infusion (Study 2) of [¹⁴C]-allantoin into the bloodstream of sheep given a roughage diet providing maintenance levels of metabolizable energy and nitrogen. The results indicated that there are two or more kinetically distinct allantoin compartments in sheep. The rate of irreversible loss of labelled allantoin from the primary mixing compartment was delayed by its exchange between this and other compartments. The recovery of ¹⁴C in urinary allantoin in urine was greater than 94% after 3-4 d in both studies, suggesting that excretion of allantoin via the gut (a non-renal route) we negligible. The appearance of low amounts of [¹⁴C]-bicarbonate in the blood confirmed that only a small fraction of the allantoin leaving the bloodstream was degraded in the sheep, probably by gut microorganisms. The results also suggested that the formation of allantoin was inhibited when its concentration in plasma was raised by infusion of unlabelled allantoin, possibly by allosteric regulation of uricase or xanthine oxidase. In Study 3, the rate of excretion of urinary purine derivatives (PD) was determined in 40 Merino sheep from flocks that had been selected over 40 years for high ('fleece-plus') or low ('fleece-minus') greasy wool production. Higher rates of excretion of PD in fleece-plus sheep suggested more microbial protein was synthesised in the rumen of these sheep. This was partly attributable to their higher feed dry matter intake (DMI), but the fleece-plus sheep also had a higher yield of PD per kg DMI, a higher flow of amino acids through the portal blood and their wool growth rate was linearly related to the estimated microbial yield in the rumen. The latter findings suggest that there are heritable traits affecting rumen microbial growth efficiency of Merino sheep that might be detected by the urinary PD excretion method in breeding programs.