Participation of ornithine aminotransferase in the synthesis and catabolism of ornithine in mice. Studies using gabaculine and arginine deprivation

Abstract

Gabaculine, a potent suicide inhibitor of ornithine aminotransferase (OAT), at a dose of 50 mg/kg inhibited this enzyme in mouse tissues and dramatically increased tissue ornithine concentrations, whether or not arginine was present in the diet. Thus even under arginine deprivation there is catabolism of ornithine which involves OAT. This was confirmed by administration of [14C]ornithine to arginine-deprived mice. Gabaculine (3-amino-2,3-dihydrobenzoic acid) drastically decreased the release of 14CO2 and increased the radioactivity in the basic amino acids in the tissues. When [1-14C]glutamate was injected into mice deprived of arginine, a significant amount of radioactivity was recovered in tissue ornithine and arginine, and gabaculine decreased this labelling by about two-thirds, indicating that ornithine was synthesized in vivo from glutamate via OAT. In addition, we failed to detect in liver and small intestine alpha-N-acetylornithine, N-acetylglutamate kinase or N-acetylornithine aminotransferase, which are obligatory components of a potential route of ornithine synthesis from N-acetylglutamate. Our results indicate that at least 45 mumol of ornithine was synthesized and catabolized daily via OAT in the mouse deprived of arginine.

A Novel Two-domain Architecture Within the Amino Acid Kinase Enzyme Family Revealed by the Crystal Structure of Escherichia coli Glutamate 5-kinase

“…[6][7][8] Mammals also use G5K and G5PR to synthesize ornithine ( Figure 1). [9][10] This latter function is crucial for proper ammonia detoxification in humans, as highlighted in  1 -pyrroline 5-carboxylate synthetase deficiency (OMIM 138250; http://www.ncbi.nlm.nih.gov), an inborn error in which a G5K-inactivating mutation causes clinical hyperammonaemia, with hypoornithininemia and hypoprolinemia. 11 G5K plays a key role in proline or ornithine biosynthesis since it catalyses the first step of these biosynthetic routes and since it is the subject of feedback inhibition by the final products, these being proline in microorganisms and plants, 6,7 and ornithine in animals.…”

Basis of Arginine Sensitivity of Microbial N -Acetyl- l -Glutamate Kinases: Mutagenesis and Protein Engineering Study with the Pseudomonas aeruginosa and Escherichia coli Enzymes

“…The absence of steps 2 to 5 in animals (2,14) renders the enzymes catalyzing these steps potential candidates for developing antibacterials and biocides. In some microorganisms, such as Escherichia coli, the arginine biosynthetic route is linear because ornithine is produced by the hydrolysis of N-acetylornithine (5).…”

Amino Acid Biosynthesis

“…Subsequently, in other bacteria, a different family of smaller DITTOs were identified; these are sometimes fused to a second domain catalyzing the last step of arginine biosynthesis, argininosuccinate lyase -a hint of higher-level order in the biosynthetic apparatus. Other bacteria use a completely different enzyme for the acetylation chemistry: an ornithine acetyltransferase [138]. This enzyme acts via a sequential Bi Bi kinetic mechanism whereby it accepts an acetyl group from ornithine onto an active-site threonine, generating an acetyl-enzyme intermediate, and then, in a second step, transfers it to glutamate [139].…”