Immunocytochemical localizations of cytosolic and mitochondrial glutamic oxaloacetic transaminase isozymes in rat brain

Emerging Moonlighting Functions of the Branched-Chain Aminotransferase Proteins

Significance: Unique to the branched-chain aminotransferase (BCAT) proteins is their redox-active CXXC motif. Subjected to post-translational modification by reactive oxygen species and reactive nitrogen species, these proteins have the potential to adopt numerous cellular roles, which may be fundamental to their role in oncogenesis and neurodegenerative diseases. An understanding of the interplay of the redox regulation of BCAT with important cell signaling mechanisms will identify new targets for future therapeutics. Recent Advances: The BCAT proteins have been assigned novel thiol oxidoreductase activity that can accelerate the refolding of proteins, in particular when S-glutathionylated, supporting a chaperone role for BCAT in protein folding. Other metabolic proteins were also shown to have peroxide-mediated redox associations with BCAT, indicating that the cellular function of BCAT is more diverse. Critical Issues: While the role of branched-chain amino acid metabolism and its metabolites has dominated aspects of cancer research, less is known about the role of BCAT. The importance of the CXXC motif in regulating the BCAT activity under hypoxic conditions, a characteristic of tumors, has not been addressed. Understanding how these proteins operate under various cellular redox conditions will become important, in particular with respect to their moonlighting roles. Future Directions: Advances in the quantification of thiols, their measurement, and the manipulation of metabolons that rely on redox-based interactions should accelerate the investigation of the cellular role of moonlighting proteins such as BCAT. Given the importance of cross talk between signaling pathways, research should focus more on these "housekeeping" proteins paying attention to their wider application. Antioxid. Redox Signal. 34, 1048-1067.

Emerging role of hydrogen sulfide in health and disease: critical appraisal of biomarkers and pharmacological tools

H2S (hydrogen sulfide) is a well known and pungent gas recently discovered to be synthesized enzymatically in mammalian and human tissues. In a relatively short period of time, H2S has attracted substantial interest as an endogenous gaseous mediator and potential target for pharmacological manipulation. Studies in animals and humans have shown H2S to be involved in diverse physiological and pathophysiological processes, such as learning and memory, neurodegeneration, regulation of inflammation and blood pressure, and metabolism. However, research is limited by the lack of specific analytical and pharmacological tools which has led to considerable controversy in the literature. Commonly used inhibitors of endogenous H2S synthesis have been well known for decades to interact with other metabolic pathways or even generate NO (nitric oxide). Similarly, commonly used H2S donors release H2S far too quickly to be physiologically relevant, but may have therapeutic applications. In the present review, we discuss the enzymatic synthesis of H2S and its emerging importance as a mediator in physiology and pathology. We also critically discuss the suitability of proposed 'biomarkers' of H2S synthesis and metabolism, and highlight the complexities of the currently used pharmacological H2S 'donor' molecules and 'specific' H2S synthesis inhibitors in their application to studying the role of H2S in human disease.

3-Mercaptopyruvate sulfurtransferase produces hydrogen sulfide and bound sulfane sulfur in the brain

Hydrogen sulfide (H(2)S) is a synaptic modulator as well as a neuroprotectant. Currently, pyridoxal-5'-phosphate (PLP)-dependent cystathionine beta-synthase (CBS) is thought to be the major H(2)S-producing enzyme in the brain. We recently found that brain homogenates of CBS-knockout mice, even in the absence of PLP, produce H(2)S at levels similar to those of wild-type mice, suggesting the presence of another H(2)S-producing enzyme. Here we show that 3-mercaptopyruvate sulfurtransferase (3MST) in combination with cysteine aminotransferase (CAT) produces H(2)S from cysteine. In addition, 3MST is localized to neurons, and the levels of bound sulfane sulfur, the precursor of H(2)S, are greatly increased in the cells expressing 3MST and CAT but not increased in cells expressing functionally defective mutant enzymes. These data present a new perspective on H(2)S production and storage in the brain.

Possible mechanism for the decrease of mitochondrial aspartate aminotransferase activity in ischemic and hypoxic rat retinas

Glutamate is believed to be an excitatory amino acid neurotransmitter in the retina. Enzymes for glutamate metabolism, such as glutamate dehydrogenase, ornithine aminotransferase, glutaminase, and aspartate aminotransferase (AAT), exist mainly in the mitochondria. The abnormal increase of intracellular calcium ions in ischemic retinal cells may cause an influx of calcium ions into the mitochondria, subsequently affecting various mitochondrial enzyme activities through the activity of mitochondrial calpain. As AAT has the highest level of activity among enzymes involved in glutamate metabolism, we investigated the change of AAT activity in ischemic and hypoxic rat retinas and the protection against such activity by calpain inhibitors. We used normal RCS (rdy+/rdy+) rats. For the in vivo studies, we clamped the optic nerve of anesthetized rats to induce ischemia. In the in vitro studies, the eye cups were incubated with Locke's solution saturated with 95% N2/5% CO2. The activity of cytosolic AAT (cAAT) was about 20% of total activity, whereas mitochondrial AAT (mAAT) was about 75% in rat retina. Ninety minutes of ischemia or hypoxia caused a 20% decrease in mAAT activity, whereas cAAT activity remained unchanged. To examine the contribution of intracellular calcium ions to the degradation of mAAT, we used Ca2+-free Locke's solution containing 1 mM EGTA, ryanodine (Ca2+ channel blocker), and thapsigargin (Ca2+-ATPase inhibitor). In the present study, thapsigargin in Ca2+-free Locke's solution, but not ryanodine in this solution, was found to prevent AAT degradation. AAT degradation was also prevented by calpain inhibitors (Ca2+-dependent protease inhibitor) such as calpeptin at 1 nM, 10 nM, 0.1 microM, 1 microM and 10 microM, and by calpain inhibitor peptide, but not by other protease inhibitors (10 microM leupeptin, pepstatin, chymostatin). Additionally, we determined the subcellular localization of calpain activity and examined the change of calpain activity in ischemic rat retinas. Our results suggest that decreased activity of mAAT in ischemic and hypoxic rat retinas might be evoked by the degradation by calpain-catalyzed proteolysis in mitochondria.

Chemical anatomy of excitatory endings in the dorsal cochlear nucleus of the rat: differential synaptic distribution of aspartate aminotransferase, glutamate, and vesicular zinc

In order to identify cytochemical traits relevant to understanding excitatory neurotransmission in brainstem auditory nuclei, we have analyzed in the dorsal cochlear nucleus the synaptic distribution of aspartate aminotransferase, glutamate, and vesicular zinc, three molecules probably involved in different steps of excitatory glutamatergic signaling. High levels of glutamate immunolabeling were found in three classes of synaptic endings in the dorsal cochlear nucleus, as determined by quantitation of immunogold labeling. The first type included auditory nerve endings, the second were granule cell endings in the molecular layer, and the third very large endings, better described as "mossy." This finding points to a neurotransmitter role for glutamate in at least three synaptic populations in the dorsal cochlear nucleus. The same three types of endings enriched in glutamate immunoreactivity also contained histochemically detectable levels of aspartate aminotransferase activity, suggesting that this enzyme may be involved in the synaptic handling of glutamate in excitatory endings in the dorsal cochlear nucleus. There was also extrasynaptic localization of the enzyme. Zinc ions were localized exclusively in granule cell endings, as determined by a Danscher-selenite method, suggesting that this ion is involved in the operation of granule cell synapses in the dorsal cochlear nucleus.

Glutamate-synthesizing enzymes in GABAergic neurons of the neocortex: a double immunofluorescence study in the rat

L-Glutamate is the immediate precursor of the inhibitory transmitter GABA, and considered to be supplied from alpha-ketoglutarate through a transamination reaction or from glutamine through a glutaminase reaction. In the present study, the localization of aspartate aminotransferase and glutaminase in GABAergic neurons was investigated in the rat neocortex by a double immunofluorescence method. Immunoreactivities for both soluble and mitochondrial aspartate aminotransferases were detected in more than 90% of GABA-positive neurons, whereas glutaminase immunoreactivity was not found in GABA-positive neurons. All neocortical neurons with soluble aspartate aminotransferase immunoreactivity were immunopositive for GABA, but none for glutaminase. Neurons with mitochondrial aspartate aminotransferase immunoreactivity showed either glutaminase or GABA immunoreactivity. Under confocal laser scan microscopy, immunoreactivity for soluble aspartate aminotransferase was observed in many axons and axon terminals showing immunoreactivity for glutamic acid decarboxylase, whereas immunoreactivity for mitochondrial aspartate aminotransferase was seen in only a few axons displaying immunoreactivity for glutamic acid decarboxylase. The present results indicate that soluble aspartate aminotransferase is selectively localized to cell bodies and axon terminals of GABAergic non-pyramidal neurons in the cerebral neocortex. This suggests that glutamate is supplied from alpha-ketoglutarate via transamination and works as the immediate precursor for GABA in axon terminals of GABAergic neurons. The absence of glutaminase immunoreactivity in GABAergic neurons indicates that glutamine is a "metabolically remote" precursor for GABA. Mitochondrial aspartate aminotransferase was located in perikarya, rather than in axon terminals of GABAergic neurons, suggesting a transmitter-irrelevant role of this enzyme in neurons.

Aspartate aminotransferase and glutaminase activities in rat olfactory bulb and cochlear nucleus; comparisons with retina and with concentrations of substrate and product amino acids

The quantitative distributions of aspartate aminotransferase and glutaminase were mapped in subregions of olfactory bulb and cochlear nucleus of rat, and were compared with similar data for retina and with the distributions of their substrate and product amino acids aspartate, glutamate, and glutamine. The distributions of both enzymes paralleled that of aspartate in the olfactory bulb and that of glutamate in the cochlear nucleus. In retina (excluding inner segments), there were similarities between aspartate aminotransferase and both glutamate and aspartate distributions. The distribution of gamma-aminobutyrate (GABA) was similar to those of both enzymes in olfactory bulb, to aspartate aminotransferase in cochlear nucleus, and to glutaminase in retina (excluding inner segments). The results are consistent with significant involvement of aspartate aminotransferase, especially the cytosolic isoenzyme, and glutaminase in accumulation of the neurotransmitter amino acids glutamate, aspartate, and GABA, although with preferential accumulation of different amino acids in different brain regions.

Cerebellar glutamate metabolizing enzymes in spinocerebellar ataxia type I

We measured the levels of three glutamate metabolizing enzymes, namely, glutamate dehydrogenase (GDH), aspartate aminotransferase (AAT), and glutamine synthetase (GS) in cerebellar and occipital cortices of nine patients with dominantly-inherited olivopontocerebellar atrophy (OPCA; spinocerebellar ataxia type I). As compared with the controls, mean GDH activities in cerebellar cortex of the OPCA patients were normal whereas levels of AAT (-17%) and the glial enzyme GS (-27%) were significantly reduced. No statistically significant changes were observed in occipital cortex, a morphologically unaffected brain area. We suggest that the decreased GS levels could reflect impaired capacity of astrocytes to metabolize glutamate which might contribute to the degenerative processes in OPCA cerebellum.

Clonidine inhibition of potassium-evoked release of glutamate and aspartate from rat cortical synaptosomes

Release of endogenous glutamic acid (Glu), aspartic acid (Asp) and gamma-aminobutyric acid (GABA) has been investigated using synaptosomes prepared from rat cerebral cortex. Exposure in superfusion to a depolarizing concentration of KCl (30 mM) evoked 3-, 2- and 2-fold increases in Glu, Asp and GABA release, respectively. More than 70% of Glu and Asp overflow were calcium-dependent, although 67% of the GABA overflow was calcium-independent. Clonidine inhibited the K(+)-evoked overflow of Glu and Asp in a concentration-dependent manner, but the GABA overflow was not inhibited. Clonidine inhibited K(+)-evoked Glu and Asp overflow to 40 and 30% of the control with a potency (IC50) of 11 and 36 nM, respectively. Similarly, norepinephrine inhibited the K(+)-evoked overflow of Glu and Asp, although phenylephrine and isoproterenol showed no effect. Rauwolscine, yohimbine and idazoxan counteracted the effects of clonidine on Glu and Asp overflow. The data suggest that the depolarization-evoked overflow of excitatory amino acids is regulated in an inhibitory fashion by alpha 2 adrenoceptors, which are located on the nerve terminals of Glu and Asp neurons in rat cortex.

Histochemistry of glutamate metabolizing enzymes in the rat cerebellar cortex

Applying catalytic enzyme histochemistry, glutamate dehydrogenase (GDH) and phosphate activated glutaminase (PAG) were demonstrated at the light microscopic level, and aspartate aminotransferase (AAT) was detected at the electron microscopic level. GDH staining appeared preferentially in glial cells (Bergmann glia and astrocytes), whereas AAT was localized only in neuronal structures. Cytoplasmic AAT was demonstrated in the perikarya and terminal plexus of basket cells, in the perikarya of stellate cells, in about 60% of the granule cells, in mossy fiber boutons, in numerous small boutons in the molecular layer, and in the axoplasm of numerous myelinated and unmyelinated fibers. PAG was observed in both neuronal structures (perikarya of granule and Purkinje cells) and in astrocytes and Bergmann glia.

Distribution of phosphate-activated glutaminase in the human cerebral cortex

Phosphate-activated glutaminase (PAG), which catalyses conversion of glutamine to glutamate, is a potential marker for glutamatergic, and possibly GABA, neurons in the central nervous system. A polyclonal antibody, raised in rabbits against rat brain PAG, was applied to postmortem human brain tissue to reveal the distribution of PAG in the cerebral cortex. PAG immunoreactivity was observed in pyramidal and non-pyramidal neurons but not in glial cells. In the neocortex, large to medium-sized pyramidal neurons in layers III and V were stained most intensely, while the majority of smaller pyramidal cells were labeled either lightly or moderately. Such modified pyramids as the giant Betz cells, the large pyramidal cells of Meynert, and the solitary cells of Ramón y Cajal were also stained intensely. Fusiform cells in layer VI showed moderate to intense labeling. A number of cortical non-pyramidal neurons of various sizes stained moderately to intensely. These included large basket cells which were identified by their characteristic morphology and size in primary cortical areas. Pyramidal cells in the hippocampal formation as well as basket cells of the stratum oriens stained moderately to intensely. Since pyramidal cells are believed to be glutamatergic and large basket cells GABAergic, these results suggest that PAG plays a role in generating not only transmitter glutamate, but also GABA precursor glutamate.

Immunolocalization of cytosolic aspartate aminotransferase (cAAT) in axon terminals that form synapses in the rat cerebellar cortex. A study at the electron microscopic level

This study was conducted to determine the ultrastructural localization of cytosolic aspartate aminotransferase (cAAT)-like immunoreactivity in the cerebellar cortex in the rat. The isoenzyme was found both in excitatory and inhibitory axon terminals, but not in the climbing fibers of the molecular layer. These findings suggest that cAAT may have a different role in the excitatory and inhibitory synapses, and that climbing fibers of the molecular layer do not appear to use aspartate as neurotransmitter.

Immunocytochemical localization of mitochondrial and cytosolic aspartate aminotransferase isozymes in the vestibular end-organs of rats

The localization of mitochondrial (m-) and cytosolic (c-) aspartate aminotransferase (AAT) was examined in the vestibular ganglion neurons and sensory cells in the vestibular end-organs of rats by an indirect immunohistochemical method using antibodies specific for m- and c-AAT. Neurons in the vestibular ganglion were stained by both m- and c-AAT antibodies, but the vestibular sensory cells exhibited only m-AAT-like immunoreactivity and were not labeled by c-AAT. These findings suggested that aspartate is a neurotransmitter in the hair cells of the vestibular end-organs.

Aspartate aminotransferase for synthesis of transmitter glutamate in the medulla oblongata: effect of aminooxyacetic acid and 2-oxoglutarate

The effects of aminooxyacetic acid (AOAA), a transaminase inhibitor, and 2-oxoglutarate, a precursor to glutamate by the activity of aspartate aminotransferase (AAT), on slices of rat medulla oblongata, cerebellum, cerebral cortex, and hippocampus were studied. The slices were superfused and electrically stimulated. There was a Ca2+-dependent stimulus-evoked release of endogenous glutamate, gamma-aminobutyric acid (GABA), and beta-alanine in all regions examined. AOAA (10(-4) and 10(-3) M) decreased the release of glutamate in the medulla oblongata and cerebellum but not in the hippocampus. L-Canaline, a specific inhibitor of ornithine aminotransferase, did not affect the glutamate release in the medulla. 2-Oxoglutarate (10(-3) M) increased the release of glutamate in the medulla oblongata and cerebellum but not in the cerebral cortex and hippocampus. Treatment with AOAA (10(-4) M) almost abolished the activities of AAT in all regions studied. AOAA (10(-4) and 10(-3) M) increased the stimulus-evoked release of GABA in the cerebellum, cerebral cortex, and hippocampus, whereas the stimulus-evoked release of beta-alanine was decreased by this agent in all regions studied. These results suggest the participation of AAT in the synthesis of the transmitter glutamate in the medulla oblongata and cerebellum of the rat.

A radiochemical microassay for aspartate aminotransferase activity in the nervous system

A radiochemical procedure for measuring aspartate aminotransferase activity in the nervous system is described. The method is based on the exchange of tritium atoms at positions 2 and 3 of L-2,3-[3H]aspartate with water when this amino acid is transaminated in the presence of alpha-ketoglutarate to form oxaloacetate. The tritiated water is separated from the radiolabeled aspartate by passing the reaction mixture over a cation exchange column. Confirmation that the radioactivity in the product is associated with water was obtained by separating it by anion exchange HPLC and by evaporation. The product formation is linear with time up to 120 min and with tissue in the 0.05- to 10-micrograms range. The apparent Km for aspartate in the rat brain homogenate is found to be 0.83 mM and that for alpha-ketoglutarate to be 0.12 mM. Methods that further improve the sensitivity of the assay are also discussed.

Immunohistochemical study of glutaminase-containing neurons in the cerebral cortex and thalamus of the rat

In an attempt to identify glutamatergic neurons, the cerebral cortex and thalamus of the rat were examined immunohistochemically by using a monoclonal antibody against phosphate-activated glutaminase (PAG), a major synthetic enzyme of transmitter glutamate in the central nervous system. In both the neocortex and mesocortex, pyramidal cells in layers V and VI showed intense PAG-like immunoreactivity (PAG-LI), whereas neuronal cell bodies in layers I-IV showed weak PAG-LI. At the deep border of layer VI, neurons with horizontally elongated cell bodies showed PAG-LI. In the pyriform and entorhinal cortices, neurons with intense to moderate PAG-LI were seen in layer II as well as in the deeper layers. In the hippocampal formation, pyramidal cells in CA1, CA2, and CA3 and polymorphic cells in CA4 showed PAG-LI; PAG-LI was most intense in pyramidal cells of CA3. Fine granules with weak PAG-LI were also seen on and/or within the cell bodies of granule cells in the dentate gyrus. In the thalamus, neurons with PAG-LI were distributed in all nuclei, although regional differences were observed in the distribution pattern of neurons with PAG-LI and in the intensity of PAG-LI in individual neurons. The largest neurons in each thalamic nucleus showed intense PAG-LI; these were considered to be projection neurons. In addition to perikaryal labeling, many fine, PAG-like immunoreactive granules were distributed in the neuropil of both the cerebral cortex and thalamic nuclei. Some of these fine granules with PAG-LI in the neuropil were assumed to represent fiber terminals with PAG-LI, because the distribution pattern of the deposits in the primary somatosensory and primary visual cortices resembled that of thalamocortical fiber terminals. Glutamate is rather ubiquitous in the mammalian central nervous system, and it is still debatable whether the monoclonal antibody to PAG from brain mitochondria can distinguish transmitter-related glutaminase from the other metabolism-related ones. In the present study, however, large neurons in the thalamic nuclei, as well as pyramidal neurons in the cerebral cortex, showed PAG-LI most intensely, supporting the assumption that projection neurons of the cerebral cortex and thalamus are primarily glutamatergic.

Separate enzymatic microassays for aspartate aminotransferase isoenzymes

The properties of the cytosolic and mitochondrial isoenzymes of aspartate aminotransferase were studied using a commercial preparation of the cytosolic isoenzyme, a mitochondrial preparation, and whole brain homogenate. Based on these properties, microassays were developed and shown to be highly specific and quantitatively accurate for measuring the activity of either the cytosolic or mitochondrial isoenzyme in microgram quantities of tissue. The assays have been successfully applied to homogenates of a wide variety of tissues. They can be used to measure the activities of aspartate aminotransferase isoenzymes in sub-microgram samples of freeze-dried tissue.

Immunocytochemical characterization of glutamate dehydrogenase in the cerebellum of the rat

The immunocytochemical distribution of glutamate dehydrogenase was studied in the cerebellum of the rat using antibodies made in rabbit and guinea pig against antigen purified from bovine liver. Antiserum was found to block partially enzymatic activity both of the purified enzyme and of extracts of the rat cerebellum. Using immunoblots of proteins of rat cerebellum, a major immunoreactive protein and several minor immunoreactive proteins were detected with antiserum. Only a single immunoreactive protein was detected using affinity-purified antibody preparations. This protein migrates with a molecular weight identical to that of the subunit of glutamate dehydrogenase. Further evidence that the antibodies were selective for glutamate dehydrogenase in rat cerebellum was obtained through peptide mapping. Purified glutamate dehydrogenase and the immunoreactive protein from rat cerebellum generated similar patterns of immunoreactive peptides. No significant cross-reaction was observed with glutamine synthetase. Immunocytochemistry was done on cryostat- and Vibratome-cut sections of the cerebellum of rats that had been perfused with cold 4% paraformaldehyde. Glial cells were found to be the most immunoreactive structures throughout the cerebellum. Most apparent was the intense labeling of Bergmann glial cell bodies and fibers. In the granule cell layer, heavy labeling of astrocytes was seen. Purkinje and granule cell bodies were only lightly immunoreactive, whereas stellate, basket, and Golgi cells were unlabeled. Labeling of presynaptic terminals was not apparent. These findings suggest that glutamate dehydrogenase, like glutamine synthetase, is enriched in glia relative to neurons.

Aspartate-like immunoreactivity in mitral cells of rat olfactory bulb

Antisera against aspartate (Asp) were prepared by immunizing rabbits with Asp conjugated to bovine serum albumin by glutaraldehyde, after which the antisera were purified using an Asp-immobilized epoxy-activated affinity column. The purified Asp antiserum showed no cross-reactivity, except for a 3% cross-reactivity against D-Asp. Asp-like immunoreactivity in mitral cells of the rat olfactory bulb was demonstrated, using this purified Asp antiserum.

Immunocytochemical localization of aspartate aminotransferase and glutaminase immunoreactivities in the cerebellum

The distributions of glutaminase and aspartate aminotransferase were studied immunocytochemically in the cerebellum of the guinea pig and the rat. In the granule cell layer, both antibodies gave a similar staining pattern. Granule cell bodies were labeled, but staining was also found to lie outside the cell body, associated with what appear to be synaptic processes. In the molecular and Purkinje cell layers, aspartate aminotransferase was concentrated in stellate and basket cell bodies and in terminal baskets beneath Purkinje cells. Glutaminase, however, was not concentrated in these structures.

Localization of chick retinal visinin-like immunoreactivity in the rat forebrain and diencephalon

The present study is an examination, using an indirect immunofluorescence method, of the distribution of visinin, a 24,000 dalton peptide, in the rat forebrain and diencephalon. Immunoreactive structures were localized in the neuronal elements showing an uneven distribution. Immunoreactive neurons were found in the olfactory bulb, anterior olfactory nucleus, cerebral cortex, amygdaloid complex, ventral portion of the nucleus caudatus putamen, septal area, nucleus accumbens, nucleus paratenialis, nucleus rhomboideus, nucleus reuniens, nucleus paraventricularis hypothalami, nucleus supraopticus, nucleus anterior hypothalami, preoptic area, hypothalamic periventricular nucleus, nucleus mammillaris medialis, medial habenular nucleus, zona incerta, nucleus lateralis thalami, nucleus tractus optici and gyrus dentatus. Immunoreactive fibers were observed in the above areas, particularly near the labelled cells, forming fiber plexuses of varying density. In addition, dense plexuses were also seen in the globus pallidus, anteroventral nucleus of the thalamus, substantia nigra and hippocampus. In the former three structures, no labelled cells were present and in the latter, a few scattered neurons were found, indicating that these fibers originate from extrinsic sources.

Immunocytochemical localizations of cytosolic and mitochondrial glutamic oxaloacetic transaminase isozymes in rat retina as markers for the glutamate-aspartate neuronal system

The localization of cytosolic (s) or mitochondrial (m) glutamic oxaloacetic transaminase (GOT) was examined in the rat retina by means of an indirect immunofluorescence method using antibodies specific for s- and m-GOT. The m-GOT-like immunoreactive structures were seen on the inner segments of the photoreceptor cells and other outer and inner plexiform layers. These structures were dot-like in appearance. Somas were not labeled. In contrast, s-GOT-like structures were found on the inner segments and inner fibers of the photoreceptor cells, numerous cell somas in the inner nuclear layer (horizontal, amacrine and bipolar cells), and ganglion cell layer (displaced amacrine cells) and inner plexiform layer. The difference in distribution between s- and m-GOT isozymes suggests that they may be useful as markers for glutamatergic and/or aspartinergic neurons.

Metabolically derived aspartate--elevated extracellular levels in vivo in lodoacetate poisoning

Glycolysis was blocked in the hippocampus of the otherwise unaffected rabbit by perfusion of a previously implanted thin dialysis tube with a medium containing 0.1 mM iodoacetate. The perfusate from the dialysis tube was analyzed for free amino acids with liquid chromatographic separation of ortophthaldialdehyde derivatives. The data were used to calculate approximate levels of extracellular amino acids. Within 15 min after the beginning of iodoacetate perfusion, extracellular aspartate increased measurably, and the increase was tenfold within less than 2 hr. Although most other amino acids were unaffected, an elevation of taurine, phosphoethanolamine, and ethanolamine was observed with a delay of 30-40 min after the effect on aspartate.