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 alpha-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 amination of alpha-ketoglutarate may have a key role in ammonia detoxification by the following processes: (i) furnishing aspartate-N for ureagenesis; (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.
Nissim
Itzhak
I
Division of Child Development and Rehabilitation Medicine, Department of Pediatrics, Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, PA 19104, USA. ssitz@mail.med.upenn.edu
Horyn
Oksana
O
Luhovyy
Bohdan
B
Lazarow
Adam
A
Daikhin
Yevgeny
Y
Nissim
Ilana
I
Yudkoff
Marc
M
D000596:Amino Acids / Q000737:chemistry / Q000378:metabolism; D000641:Ammonia / Q000737:chemistry / Q000378:metabolism; D000818:Animals; D001224:Aspartic Acid / Q000737:chemistry / Q000378:metabolism*; D003250:Constriction; D005615:Freezing; D005969:Glutamate Dehydrogenase / Q000378:metabolism* / Q000502:physiology; D005971:Glutamates / Q000378:metabolism; D005973:Glutamine / Q000737:chemistry / Q000378:metabolism; D008099:Liver / Q000201:enzymology* / Q000378:metabolism; D008297:Male; D009584:Nitrogen / Q000378:metabolism*; D009587:Nitrogen Isotopes; D010477:Perfusion; D051381:Rats; D017207:Rats, Sprague-Dawley; D014508:Urea / Q000378:metabolism*
D000596:Amino Acids; D005971:Glutamates; D009587:Nitrogen Isotopes; D005973:Glutamine; D001224:Aspartic Acid; D000641:Ammonia; D014508:Urea; D005969:Glutamate Dehydrogenase; C016195:N-acetylglutamic acid; D009584:Nitrogen