Effects of kainic acid injection and cortical lesion on ornithine and aspartate aminotransferases in rat striatum

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.

Phenelzine causes an increase in brain ornithine that is prevented by prior monoamine oxidase inhibition

Phenelzine (PLZ), a nonselective irreversible inhibitor of monoamine oxidase (MAO), also inhibits GABA-transaminase (GABA-T), markedly increasing brain GABA levels. PLZ is also a substrate for MAO, and studies suggest that a metabolite formed by the action of this enzyme on PLZ may be responsible for the increase in GABA observed. We have recently found that PLZ also elevates brain ornithine (ORN), an amino acid precursor to both glutamate (and GABA) and the polyamines, and have conducted dose- and time-response studies on this effect. Rats were treated with vehicle or PLZ doses (7.5, 15 or 30 mg/kg i.p.), and brains were collected 3 h later. In the time-response study, animals were treated with vehicle or PLZ (15 mg/kg i.p.) and brains were collected 1-24 h later. To determine whether a metabolite formed by the action of MAO on PLZ may be responsible for the elevation in brain ORN observed, animals were pretreated with vehicle or the MAO inhibitor tranylcypromine (TCP) before vehicle or PLZ (15 mg/kg), and brains collected 3 h later. ORN levels (measured by an HPLC procedure) were dose- and time-dependently increased in PLZ-treated animals, with levels reaching approximately 650% of control at 6 and 12 h. Pretreatment with TCP completely abolished the PLZ-induced increase in brain ORN, suggesting, as with GABA, that a metabolite of PLZ formed by the action of MAO is responsible for the elevation of brain ORN observed. The possible contribution of increased ORN to therapeutic and/or neuroprotective properties of PLZ is discussed.

Endogenous ornithine in search for CNS functions and therapeutic applications

The vertebrate brain has the machinery to transport arginine and ornithine, and to form within nerve endings from these amino acids glutamate and GABA, the major excitatory and inhibitory neurotransmitters. Ornithine aminotransferase is a key enzyme of the Arg-->Orn-->Glu-->GABA pathway; the physiological significance of this pathway is still unclear. With 5-fluoromethylornithine, a selective inactivator of ornithine aminotransferase, a tool is in our hands that allows us to study biochemical and behavioral consequences of elevated tissue ornithine concentrations. Increase of the rate of hepatic urea formation, and of ornithine decarboxylation are the most important changes in vertebrates following inactivation of ornithine aminotransferase. Administration of 5-fluoromethylornithine prevented the accumulation of lethal concentrations of ammonia in brain, and ameliorated pathological consequences of thioacetamide intoxication. Inhibition of ornithine catabolism has, therefore, potentials in the therapy of those hyperammonemic states which are characterized by a conditional deficiency of ornithine. The enhancement of polyamine formation due to elevated ornithine concentrations may allow us to favorably affect tissue regeneration following injury.

Ornithine as a precursor of neurotransmitter glutamate: effect of canaline on ornithine aminotransferase activity and glutamate content in the septum of rat brain

Local injections of L-canaline into the septum produce a rapid and almost complete inhibition of ornithine aminotransferase activity followed by a decrease in glutamate content in this region. The time-course of canaline action shows the existence of two glutamate pools with different sizes and half-life values. Surgical lesions of the hippocampal-septal glutamatergic pathway affected the site and kinetics of the small pool of glutamate in the septum, suggesting the participation of ornithine aminotransferase in the synthesis of this pool. This indicates a possible role of ornithine as a precursor of the transmitter glutamate. The localization of ornithine aminotransferase does not seem, however, to be specific for the nerve-terminal compartment. The data obtained allow estimation of the turnover rate of the specific pool of neurotransmitter glutamate.

Stimulatory effect of neonatal beta-estradiol treatment on aspartate-aminotransferase activity within rat cerebellar cortex

The effect of neonatal beta-estradiol treatment on the Aspartate-aminotransferase (Asp-T) activity within rat cerebellar cortex has been studied using a semi-quantitative histochemical technique. The Asp-T, localized chiefly at the level of nerve fiber-like structures in the molecular layer as well as around the perikaryon of Purkinje neurons, is significantly stimulated after estrogenization. The nerve fibers localized in the molecular layer were the Asp-T cerebellar structures more remarkably influenced by beta-estradiol treatment.

Proline, glutamate and glutamine metabolism in mouse brain synaptosomes

In nerve terminals, glutamate (Glu) may serve as precursor of the inhibitory neurotransmitter, GABA, and the putative excitatory transmitter, aspartate (Asp), in addition to exerting its own excitatory neurotransmitter role in brain. Glu carbon can originate from glucose through glycolysis and the Krebs cycle, from glutamine (Gln) subsequent to uptake, and from proline (Pro) and ornithine (Orn). Orn, but not Glu, is an effective precursor in nerve terminals of Pro, a putative inhibitory neurotransmitter. [3H]Arg can be converted in mouse brain nerve terminals to Orn, which in turn gives rise to Glu, Pro and GABA. In the present study, the conversion subsequent to uptake of labeled Glu, Gln and Pro to other amino acids was studied in unfractionated and subfractionated synaptosomal particles which layered, respectively, on 1.0 M, 1.2 M, 1.3 M and 1.5 M sucrose after centrifugation in a discontinuous gradient (fractions 1-4, respectively). Fraction 1 contained small synaptosomal fragments with vesicles and almost no mitochondria. Fractions 2 and 3 showed numerous normal-appearing mitochondria-containing synaptosomes, and fraction 4 contained large synaptosomes and more free mitochondria than the other fractions. Glu was readily taken up in all fractions and converted to Asp, Gln and GABA, the greatest formation of Asp from Glu occurring in fractions 2 and 3 and of Gln in fraction 4. In contrast, Gln was taken up poorly in fraction 1 and not metabolized, converted extensively to Glu and GABA in fractions 2-4, giving rise only to very small amounts of Asp in fractions 2 and 3. Although Pro was taken up to the greatest extent in fraction 2, it was by far most readily converted to Glu, Gln and GABA in fraction 1, showing only small amounts of Asp formation in fractions 1-3 and none in 4. There was no significant production of Pro from Glu or Gln or of Arg and Orn from any of the 3 precursors studied. The above results suggest that Glu, Gln and Pro may be taken up largely in different classes of synaptosomes which are distributed among the centrifugally separated fractions and which possess differing transport and metabolic characteristics. Determination of glutamate decarboxylase activity (GAD) indicated that GABA-forming nerve terminals were present in all synaptosomal fractions studied. Amino acid determinations by HPLC in the subfractionated synaptosomes showed a similar distribution for Glu, Asp and GABA contents, peaking in fraction 2, and an inverse relationship of the latter 3 with Arg contents.(ABSTRACT TRUNCATED AT 400 WORDS)

Alanine aminotransferase in the rat nervous system during the postnatal development referring to the glutamate transmitter metabolism

Alanine aminotransferase has been studied in various nervous tissues during the postnatal development of the rat. At birth the enzyme activity was low and showed similar levels in all tissues studied. In the hippocampal formation and in the cerebellum which are supposed to be endowed with glutamatergic structures, the enzyme activity increased significantly during the postnatal development. These results contrast markedly with dorsal root ganglia and superior cervical ganglia, in which glutamatergic transmission processes are obviously absent. In these peripheral ganglia the time course of the enzyme activity persisted on a very low level after birth. The participation of alanine aminotransferase in forming of transmitter glutamate is discussed.