THE RELATIONSHIP BETWEEN UREA AND PYRIMIDINE DE NOVO SYNTHESIS IN RUMINANT LIVER

¹ The Department of Animal Physiology, Warsaw Agricultural University, Warsaw, Nowoursynowska 166, Poland

The influence of ammonia and inhibition of the urea cycle by norvaline on orotic acid synthesis in five Polish Merino ewes was investigated. The sheep were exposed to 60 min of successive infusions of 0·9% NaCl, NH₄Cl (20 µmol kg^-1 min^-1), and NH₄Cl with DL-norvaline (10 µmol kg^-1 min^-1) solutions administered to the mesenteric vein. The average rate of orotic acid output, urea output and ammonia uptake in the liver in control conditions (infusion of 0·9% NaCl) was: 1·09 nmol g^-1 min^-1 0·97 µmol g^-1 min^-1 and 0·94 µmol g^-1 min^-1, respectively. Ammonia significantly stimulated the orotic acid output, urea output and ammonia uptake. At the same time liver uptake of glutamic acid, alanine and glycine increased. Norvaline as a competitive inhibitor of ornithine transcarbamylase depressed citrulline release and urea output in the liver, and elevated, although not significantly, the orotic acid output. It was concluded that ammonia stimulates the de novo synthesis of pyrimidine in the sheep liver but the participation in this effect of carbamyl phosphate generated in the urea cycle is limited.