Role of the glutamate dehydrogenase reaction in furnishing aspartate nitrogen for urea synthesis: Studies in perfused rat liver with ¹⁵N

The present study was designed to determine: (i) the role of the reductive amination of α-ketoglutarate via the glutamate dehydrogenase reaction in furnishing mitochondrial glutamate and its transamination into aspartate; (ii) the relative incorporation of perfusate ¹⁵NH₄Cl, [2- ¹⁵N]glutamine or [5-¹⁵N]glutamine into carbamoyl phosphate and aspartate-N and, thereby, [¹⁵N]urea isotopomers; and (iii) the extent to which perfusate [¹⁵N]aspartate is taken up by the liver and incorporated into [¹⁵N]urea. We used a liver-perfusion system containing a physiological mixture of amino acids and ammonia similar to concentrations in vivo, with ¹⁵N label only in glutamine, ammonia or aspartate. The results demonstrate that in perfusions with a physiological mixture of amino acids, approx. 45 and 30% of total urea-N output was derived from perfusate ammonia and glutamine-N respectively. Approximately two-thirds of the ammonia utilized for carbamoyl phosphate synthesis was derived from perfusate ammonia and one-third from glutamine. Perfusate [2- ¹⁵N]glutamine, [5-¹⁵N]glutamine or [¹⁵N] aspartate provided 24,10 and 10% respectively of the hepatic aspartate-N pool, whereas perfusate ¹⁵NH₄Cl provided approx. 37% of aspartate-N utilized for urea synthesis, secondary to the net formation of [¹⁵N]glutamate via the glutamate dehydrogenase reaction. The results suggest that the mitochondrial glutamate formed via the reductive animation of α-ketoglutarate may have a key role in ammonia detoxification by the following processes: (i) furnishing aspartate-N for urea-genesis; (ii) serving as a scavenger for excess ammonia; and (iii) improving the availability of the mitochondrial [glutamate] for synthesis of N-acetylglutamate. In addition, the current findings suggest that the formation of aspartate via the mitochondrial aspartate aminotransferase reaction may play an important role in the synthesis of cytosolic argininosuccinate.