Turnover of N-acetylglutamate in isolated rat hepatocytes

Isolated hepatocytes from starved rats were loaded with N-[14C]acetylglutamate by preincubating them with [14C]bicarbonate, oleate, NH3, ornithine and lactate. Turnover of N-acetylglutamate in these cells was subsequently measured in an unlabelled medium under conditions of minimal flux (oleate alone present) and maximal flux (oleate, NH3, ornithine and lactate present) through the urea cycle. 1. Direct measurement of the distribution of N-[14C]acetylglutamate across the mitochondrial membrane in the hepatocytes showed that, under the conditions studied, the rate of degradation of total intracellular N-[14C]acetylglutamate was about equal to the rate of efflux of N-acetylglutamate from the mitochondria. 2. In the presence of oleate alone, intramitochondrial N-acetylglutamate decreased because mitochondrial N-acetylglutamate efflux predominated over the synthesis of N-acetylglutamate in the mitochondria. 3. In the presence of oleate, NH3, ornithine and lactate both the rate of synthesis of N-acetylglutamate and the rate of its transport out of the mitochondria were increased when compared with the condition with oleate alone. However, the intramitochondrial concentration of N-acetylglutamate increased because initially the rate of its synthesis exceeded that of its efflux from the mitochondria. Finally, a steady state was reached in which both rates were equal. 4. The data indicate that in hepatocytes from starved rats N-acetylglutamate transport out of the mitochondria takes place at a rate proportional to its intramitochondrial concentration. It is concluded that transport of N-acetylglutamate either occurs by diffusion or is mediated by a transport system with a high Km for intramitochondrial N-acetylglutamate.