Enzymes of ammonia detoxication after portacaval shunt in the rat. I. Carbamylphosphate synthetase and aspartate transcarbamylase.

At high systemic blood concentrations ammonia may be partially deviated into the pathway of pyrimidine synthesis, as has been observed in different genetic defects of the urea cycle. The portacaval shunt (PCS) rat presents an animal model to study ammonia detoxication without an underlying enzyme defect in the urea cycle. Since ammonia may induce a deviation into the pyrimidine pathway by influencing enzymatic reactions involved in this pathway, the activity of carbamylphosphate synthetase and aspartate transcarbamylase in liver as well as the excretion of orotic acid in the urine were measured in rats 10, 20 and 30 days after PCS. The results suggest that in this experimental model ammonia may be channeled into the pyrimidine pathway leading to a stimulation of the first enzymatic step and to an increased excretion of orotic acid.