Research on urinary excretion of purine derivatives in ruminants: Past, present and future

Research on urinary excretion of purine derivatives (PD), namely allantoin, uric acid, xanthine and hypoxanthine, in ruminants have been carried out with an objective to use the excretion of these purine metabolites as a parameter to estimate the intestinal flow of microbial protein. This paper reviews the published literature, from the first paper in 1931 to the current date. The current status of understanding in some key topics is discussed. The topics include: endogenous excretion, modelling the response of PD excretion to purine absorption, calculation of microbial N supply from PD excretion, use of spot urine measurement, possible use of plasma or milk PD as an alterative index, and applications in ruminant nutrition research. This review also covers the current understanding of PD excretion in different animal species, including sheep, cattle, goats, buffaloes, llamas, camels, yak and deer. Progress in analytical methods for the determination of purine derivatives is also discussed. Finally, areas of future research are highlighted. The paper stresses the need for more studies on metabolism of PD in the tissue, the kinetics of PD in the blood and physiological processes of renal excretion, so as to understand better the mechanism that accounts for the between-species and within species variation in PD excretion. Development of simpler and more rapid methods for defining the endogenous excretion and purine input-output relationship is also an area for future work. (author)