EFFECTS OF UREA ON THE ACTIVITY OF GLUTAMATE DEHYDROGENASE, GLUTAMINE SYNTHETASE, CARBAMYL PHOSPHATE SYNTHETASE, AND CARBAMYL PHOSPHOKINASE IN RUMINANT TISSUES

Abstract

The effect of an isonitrogenous (2.7% N) semipurified diet containing 0, 50, or 100% of the added nitrogen (N) as urea (comprising 0, 39, or 76% of the total N in the diet) on the activities of major enzymes involved in nitrogen metabolism was investigated with calves. The activities of glutamate dehydrogenase (L-glutamate:NAD(P) oxidoreductase (deaminating) enzyme code (EC) 1.4.1.3), glutamine synthetase (L-glutamate:ammonia ligase (ADP) EC 6.3.1.2), carbamyl phosphate synthetase (ATP:carbamate phosphotransferase (dephosphorylating) EC 2.7.2.5), and carbamyl phosphokinase (ATPcarbamate phosphotransferase EC 2.7.2.2) were assayed in the liver, rumen epithelial tissue, and rumen microorganisms. No significant differences among diets in the activity of glutamate dehydrogenase from liver or rumen epithelium were found. Rumen fluid (rumen microorganisms) samples from calves fed the 50% urea-N diet exhibited higher glutamate dehydrogenase activity than the 0% urea-N or 100% urea-N diets. Rumen epithelium glutamine synthetase activities were slightly higher for the 50% urea-N diet than the 0 or 100% urea-N diets. Liver glutamine synthetase activities were also slightly higher for the urea-containing diets when compared with the 0% urea-N diet. Rumen microorganisms glutamine synthetase activity was significantly (P < 0.05) higher for animals fed the 50% urea-N diet than those fed 0 or 100% urea-N diets. Liver carbamyl phosphate synthetase was significantly (P < 0.01) higher than rumen epithelium or phosphokinase of rumen microorganisms but there were no significant differences among diets. Rumen fluid from calves fed the 50 or 100% urea-N diets had higher carbamyl phosphokinase activity than animals fed the 0% urea-N diet.