Ornithine aminotransferase (OAT) is a reversible enzyme expressed mainly in the liver, kidney and intestine. OAT controls the interconversion of ornithine into glutamate semi-aldehyde, and is therefore involved in the metabolism of arginine and glutamine which play a major role in N homeostasis. We hypothesised that OAT could be a limiting step in glutamine -arginine interconversion. To study the contribution of the OAT enzyme in amino acid metabolism, transgenic mice that specifically overexpress human OAT in the liver, kidneys and intestine were generated. The transgene expression was analysed by in situ hybridisation and real-time PCR. Tissue (liver, jejunum and kidney) OAT activity, and plasma and tissue (liver and jejunum) amino acid concentrations were measured. Transgenic male mice exhibited higher OAT activity in the liver (25 (SEM 4) v. 11 (SEM 1) nmol/min per mg protein for wild-type (WT) mice; P, 0·05) but there were no differences in kinetic parameters (i.e. K m and maximum rate of reaction (V max )) between WT and transgenic animals. OAT overexpression decreased plasma and liver ornithine concentrations but did not affect glutamine or arginine homeostasis. There was an inverse relationship between ornithine levels and OAT activity. We conclude that OAT overexpression has only limited metabolic effects, probably due to the reversible nature of the enzyme. Moreover, these metabolic modifications had no effect on phenotype.
Ornithine: Arginine: GlutamineOrnithine aminotransferase (OAT; L-ornithine 2-oxo acid aminotransferase; EC 2.6.1.13) is a pyridoxal-5 0 -phosphate-dependent mitochondrial matrix aminotransferase that catalyses the interconversion of ornithine into glutamate semi-aldehyde, which is in spontaneous equilibrium with its cyclic tautomer, pyrroline-5-carboxylate (P5C). In contrast to other aminotransferases, this reaction enables the reversible transfer of the ornithine v-amino group. It depends on two tyrosine residues leading to the interaction with ornithine and the protection of the v-amino group against hydrolysis (1) .OAT is localised at a crossing between two important metabolisms, with arginine and polyamine metabolism on one side and glutamate and proline metabolism on the other (Fig. 1).Although OAT is expressed constitutively in many tissues, it is mainly found in the liver, kidney (at higher levels in females) and the small intestine (2,3) where its response to hormones and variations in dietary protein intake is subject to complex regulation mechanisms (2,4,5) . The reaction is directed towards glutamate semi-aldehyde synthesis in the liver and kidneys and towards ornithine synthesis in the small intestine. Hepatic OAT expression is restricted to perivenous hepatocytes (4) and co-localised with glutamine synthetase expression (6) , which suggests that OAT plays a major role in N homeostasis. In the kidney, Levillain et al. (7) showed that OAT is distributed along the whole nephron, but its activity is higher in proximal tubules than in distal tubules. In the intestine, OAT is...
Arginine (Arg) and glutamine (Gln) utilization is greatly increased during catabolic stress. While the supply of both amino acids has been advocated in this situation, arginine administration is possibly associated with deleterious effects. From a metabolic point of view, these two amino acids are reciprocal precursors via ornithine aminotransferase (OAT). We hypothesized that OAT plays a key role in the interconversion between Arg and Gln. To test this hypothesis, we evaluated the influence of OAT activity in a model of septic shock induced by intraperitoneal injection of lipopolysaccharide (LPS) in wild-type (WT) and transgenic mice overexpressing OAT (OAT) in the liver, kidney and intestine, i.e. the three main organs of OAT expression. Plasma and tissue amino acid concentrations and tissue OAT expression and activity were measured. Five hours after LPS injection, WT and OAT mice showed a similar response to LPS in terms of inflammatory cytokine production and protein catabolism, suggesting that the interconversion between Arg and Gln through this pathway remains limited. Endotoxemia led to a significant decrease in plasma Orn levels and an increase in liver Orn levels. Of note, Orn levels were always lower in OAT mice. While only plasma Arg and Gln remained unaffected by LPS treatment, hepatic Gln was significantly increased without any difference between the two genotypes. In this model of early endotoxemia, arginine and glutamine maintained their metabolic homeostasis. Our results show an inhibition of OAT activity and expression in the liver following LPS treatment. These data highlight the importance of OAT in ornithine metabolism, especially in the liver, and suggest a post-transcriptional regulation of OAT by LPS in the liver.
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