Functional properties of ornithine-ketoacid aminotransferase from rat liver

Determination of the pH dependence, substrate specificity, and turnovers of alternative substrates for human ornithine aminotransferase

Hepatocellular carcinoma (HCC) is the most common primary cancer of the liver and occurs predominantly in patients with underlying chronic liver diseases. Over the past decade, human ornithine aminotransferase (hOAT), which is an enzyme that catalyzes the metabolic conversion of ornithine into an intermediate for proline or glutamate synthesis, has been found to be overexpressed in HCC cells. hOAT has since emerged as a promising target for novel anticancer therapies, especially for the ongoing rational design effort to discover mechanism-based inactivators (MBIs). Despite the significance of hOAT in human metabolism and its clinical potential as a drug target against HCC, there are significant knowledge deficits with regard to its catalytic mechanism and structural characteristics. Ongoing MBI design efforts require in-depth knowledge of the enzyme active site, in particular, pKa values of potential nucleophiles and residues necessary for the molecular recognition of ligands. Here, we conducted a study detailing the fundamental active-site properties of hOAT using stopped-flow spectrophotometry and X-ray crystallography. Our results quantitatively revealed the pH dependence of the multistep reaction mechanism and illuminated the roles of ornithine α-amino and δ-amino groups in substrate recognition and in facilitating catalytic turnover. These findings provided insights of the catalytic mechanism that could benefit the rational design of MBIs against hOAT. In addition, substrate recognition and turnover of several fragment-sized alternative substrates of hOATs, which could serve as structural templates for MBI design, were also elucidated.

Ornithine Aminotransferase, an Important Glutamate-Metabolizing Enzyme at the Crossroads of Multiple Metabolic Pathways

Ornithine δ-aminotransferase (OAT, E.C. 2.6.1.13) catalyzes the transfer of the δ-amino group from ornithine (Orn) to α-ketoglutarate (aKG), yielding glutamate-5-semialdehyde and glutamate (Glu), and vice versa. In mammals, OAT is a mitochondrial enzyme, mainly located in the liver, intestine, brain, and kidney. In general, OAT serves to form glutamate from ornithine, with the notable exception of the intestine, where citrulline (Cit) or arginine (Arg) are end products. Its main function is to control the production of signaling molecules and mediators, such as Glu itself, Cit, GABA, and aliphatic polyamines. It is also involved in proline (Pro) synthesis. Deficiency in OAT causes gyrate atrophy, a rare but serious inherited disease, a further measure of the importance of this enzyme.

A sensitive assay for ornithine amino transferase in rat brain mitochondria by ninhydrin method

To establish/develop an assay method for measuring Ornithine Aminotransferase (EC.2.6.1.13) activity using rat brain mitochondria as a source of enzyme in presence and absence of Pyridoxal Phosphate (PLP). The modified method, with the improved sensitivity, is adopted for the assay of ornithine amino transferase activity in rat brain mitochondria. The enzyme activity was measured at 620 nm, the study showed that reaction was optimum at 37°C for 30 minutes. The assay is sensitive enough to detect activity at the order of nanomoles pyrroline-5-carboxylate/mg protein/minute and can be compared as an alternative to the radio isotopic method which is more cumbersome and aminobenzaldehyde method which is less sensitive. The K(m) & V(max) shows maximum activity in the presence of Pyridoxal Phosphate (Coenzyme) concentration at 0.05mM when compared with absence of Pyridoxal Phosphate as higher the concentration of Pyridoxal Phosphate affects the affinity of the enzyme to substrate. The OAT activity in different tissues of the rat was also studied and highest activity was found in liver and kidney.

Molecular basis of ornithine aminotransferase deficiency in B-6-responsive and -nonresponsive forms of gyrate atrophy

Gyrate atrophy (GA), a recessive eye disease involving progressive loss of vision due to chorioretinal degeneration, is associated with a deficiency of the mitochondrial enzyme ornithine aminotransferase (OATase; ornithine-oxo-acid aminotransferase; L-ornithine:2-oxo-acid aminotransferase, EC 2.6.1.13) with consequent hyperornithinemia. Genetic heterogeneity of GA has been suggested by the demonstration that administration of pyridoxine to increase the level of pyridoxal phosphate, a cofactor of OATase, reduces hyperornithinemia in a subset of patients. We have cloned and sequenced cDNAs for OATase from two GA patients, one responsive and one nonresponsive to pyridoxine treatment. The respective cDNAs contained different single missense mutations, which were sufficient to eliminate OATase activity when each cDNA was tested in a eukaryotic expression system. However, like the enzyme in fibroblasts from the pyridoxine-responsive patient, OATase encoded by the corresponding cDNA from this individual showed a significant increase in activity when assayed in the presence of an increased pyridoxal phosphate concentration. These data firmly establish that both pyridoxine responsive and nonresponsive forms of GA result from mutations in the OATase structural gene. Moreover, they provide a molecular characterization of the primary lesion in a pyridoxine-responsive genetic disorder.

Comparison of ornithine aminotransferase activities in the pigment epithelium and retina of vertebrates

1. The specific activities of ornithine aminotransferase (OAT) in the pigment epithelia, retinas, and livers from several classes of vertebrates were assayed. 2. The specific activities of OAT were much higher in the pigment epithelia from mammals and birds than in their respective retinas or livers. 3. Pigment epithelium from porcine eyes had the highest specific activity measured. The specific activity of OAT in the pigment epithelium from the pig was five times higher than the OAT activity in its retina and 13 times higher than the OAT activity in its liver.

Effect of the mouse mutants testicular feminization and sex reversal on hormone-mediated induction and repression of enzymes

The mouse mutants testicular feminization and sex reversal have been used to investigate hormone-mediated induction and repression of enzymes. Tfm/Y animals were already known to be androgen insensitive, rendering the androgen-inducible enzymes ADH and beta-glucuronidase noninducible because of an inherited deficiency of a cytosol androgen-receptor complex. The animals display female secondary sexual characteristics. Sxr/+,XX animals display male primary and secondary sexual characteristics with small testes. We demonstrate (1) that the Tfm mutation is pleiotropic, preventing repression of an androgen-repressible enzyme (ornithine aminotransferase) as well as induction of androgen-inducible enzymes, (2) that an estrogen-inducible enzyme (histidine decarboxylase) is not affected by the Tfm mutation, and (3) that Sxr/+,XX animals produce enough androgen for malelike activities of androgen-sensitive enzymes. It was also discovered that histidine decarboxylase repressed by androgen in normal animals, rather than being unaffected by it in Tfm/Y animals, is in fact induced. This unexpected phenomenon is discussed and an explanation is suggested for it.

Factors influencing the activity of ornithine aminotransferase in isolated rat liver mitochondria

1. The characteristics of ornithine catabolism by the aminotransferase pathway in isolated mitochondria were determined. 2. Ornithine synthesis from glutamate and glutamate gamma-semialdehyde produced by the oxidation of proline was studied. No ornithine was formed in the absence of rotenone. 3. The mechanism of ornithine transport was reinvestigated, and the existence of an ornithine+/H+ exchange system postulated. 4. The kinetics of ornithine transport, ornithine catabolism in intact mitochondria and ornithine aminotransferase activity in solubilized mitochondria were compared. It is concluded that ornithine aminotransferase activity in liver mitochondria is rate-limited by the transport of ornithine across the mitochondrial membrane, and that this enzyme is involved primarily in ornithine degradation rather than ornithine synthesis.

Preparation and properties of ornithine-oxo-acid aminotransferase of rat kidney. Comparison with the liver enzyme

Ornithine-oxo-acid aminotransferase (EC 2.6.1.13) from rat kidney was prepared as a single homogeneous protein as judged by polyacrylamide gel electrophoresis, ultracentrifuge analysis and double diffusion precipitin test. Content of pyridoxal phosphate, light absorption spectra, circular dicroism spectra, Km values, inhibitors, and electrophoretic mobilities of the proteins after reactions with group modifying reagents were similar for the ornithine-oxo-acid aminotransferases of rat kidney and liver. Rates of reaction with group modifying reagents, stabilities to storage at -15 degrees C, and stabilities to temperatures above 55 degrees C differed significantly for the two enzymes. The liver enzyme contained two more cysteine residues than the kidney enzyme as determined by three different methods. Heating the liver enzyme at 66-67 degrees C at pH 5.9 for 1 h decreased the thiol groups titratable by 5,5'-dithio-bis(2-nitrobenzoic acid) (Nbs2). Uncer the same conditions titratable thiol groups of the kidney enzyme were not decreased. Amino acid analysis revealed probably significant differences in tyrosine and isoleucine content in addition to cysteine. It was concluded that the primary structures of ornithine-oxo-acid aminotransferases of rat liver and kidney are not fully identical.