URINARY-EXCRETION OF PURINE DERIVATIVES AS AN INDEX OF MICROBIAL-NITROGEN INTAKE IN GROWING RABBITS

Three experiments were carried out to establish a response model betwe en intake and urinary excretion of purine compounds. In Expt 1 the rel ationship between the intake of purine bases (PB) and the excretion of total purine derivatives (PD) was determined in seven growing rabbits with a mean initial live weight (LW) of 2.03 (SE 0.185) kg, aged 70 d , each fitted with a wooden neck collar to prevent caecotrophagy. They were fed on five experimental diets formulated with different levels of nucleic acids (0.00, 3.75, 7.50, 11.25, 15.00 g yeast-RNA/kg diet). The relationship between intake of purine (x, mu mol/kg W-0.75) and t otal urinary PD excretion (y, mu mol/kg W-0.75), y = 0.56 + 0.67x (r(2 ) 0.86; RSD 0.338), indicated that about 70 % of duodenal PB were reco vered as urinary PD and that the endogenous contribution was constant and independent of dietary PB supply. Endogenous excretion of PD (alla ntoin and uric acid) was measured in a second experiment using six rab bits fed on a purine-free diet and fitted with neck collars to avoid c aecotrophagy. Basal daily urinary excretion Values for allantoin and u ric acid were 532 (SE 33.9) and 55 (SE 7.3) mu mol/kg W-0.75 respectiv ely; xanthine and hypoxanthine were not found in urine samples and the refore the sum of allantoin and uric acid should comprise the total ex cretion of PD (588 (SE 7.3) mu mol/kg W-0.75). The xanthine oxidase (E C 1.2.3.2) activity in plasma, liver, duodenum, jejunum and kidney was measured in a third experiment. The activities of xanthine oxidase in duodenal and jejunal mucosa, liver and kidney were: 0.61 (SE 0.095), 0.37 (SE 0.045), 0.035 (SE 0.001) and 0 units/g fresh tissue respectiv ely and in plasma 2.96 (SE 0.094) units/l. The results show that urina ry excretion of PD may be a useful tool to estimate duodenal PB input and microbial protein intake once the relationship between PB and N ha s been established in caecal micro-organisms.