Inactivation of tyrosine aminotransferase in neutral homogenates and rat liver slices

Inactivation of tyrosine aminotransferase induced in vivo by triamcinolone was studied in a homogenate incubated at neutral pH values. The integrity and the presence of subcellular particles together with a compartment of acidic pH are necessary for inactivation of tyrosine aminotransferase. It is suggested that tyrosine aminotransferase is inactivated inside lysosomes. The system responsible for inactivation of tyrosine aminotransferase was partially purified and identified with lysosomal cathepsins B and B(1). Inactivation of tyrosine aminotransferase in liver slices is controlled by the amino acid concentration and strongly stimulated by cysteine. 3,3',5-Tri-iodo-l-thyronine reversibly and strongly decreases the rate of inactivation of tyrosine aminotransferase. The effect is not due to an increased rate of tyrosine aminotransferase synthesis.     

Purification and properties of aromatic amino acid aminotransferase from Klebsiella aerogenes.

We describe the complete purification of aromatic aminotransferase I, the enzyme responsible for the ability of Klebsiella aerogenes to use tryptophan and phenylalanine as sole sources of nitrogen, as well as the partial purification of aromatic aminotransferase IV. An examination of the properties of these enzymes revealed that aminotransferase I had much greater affinity for the aromatic amino acids than aminotransferase IV, explaining the essential role of aminotransferase I in the utilization of exogenously supplied aromatic amino acids. The properties of aminotransferase IV suggest that this enzyme is actually an aspartate aminotransferase (EC 2.6.1.1), corresponding to the product of the aspC gene of Escherichia coli.     

Alpha-aminoadipate and kynurenine aminotransferase activities from rat kidney. Evidence for separate identity

alpha-Aminoadipate aminotransferase and kynurenine aminotransferase activities from rat kidney are reportedly associated with the same protein. We observed that when the supernatant fraction was maintained at pH 4.5 for 75 min, 100% of kynurenine aminotransferase activity was lost, whereas only 40% of aminoadipate aminotransferase activity was lost. We purified alpha-aminoadipate aminotransferase and kynurenine aminotransferase from rat kidney supernatant fraction to electrophoretic homogeneity by ammonium sulfate fractionation, DEAE-Sephacel, and hydroxylapatite chromatography. Kynurenine aminotransferase activity was precipitated by pH treatment. The remaining aminoadipate aminotransferase activity was concentrated and injected into rabbits to raise antibodies that were used to prepare an affinity column. A mixture of aminoadipate aminotransferase and kynurenine aminotransferase activities obtained after hydroxylapatite chromatography was subjected to affinity chromatography. Aminoadipate aminotransferase and kynurenine aminotransferase activities resolved as separate peaks, providing evidence that the two activities are associated with two different proteins.     

Pretranslational control of tyrosine aminotransferase synthesis by 8-bromo-cyclic AMP in H-4 rat hepatoma cells

The H-4 rat hepatoma cell line grown in tissue culture was used as a model system to investigate the action of cAMP in tyrosine aminotransferase induction. An immunoprecipitation technique was used to quantitate the amount and the rate of synthesis of tyrosine aminotransferase; the level of mRNA coding for tyrosine aminotransferase was determined by in vitro translation of poly(A)+ RNA isolated from hepatoma cells. Our results demonstrated that 8-bromo-cAMP gave time-dependent and proportionate increases in the tyrosine aminotransferase activity, the amount of immunoprecipitable tyrosine aminotransferase, the rate of synthesis of tyrosine aminotransferase, and the level of mRNATAT in H-4 hepatoma cells. The time course of increase in mRNATAT preceded the increase in synthesis of tyrosine aminotransferase and was dependent on the continuous production of poly(A)+ RNA. Pretreatment of the cells with cordycepin completely abolished the 8-bromo-cAMP-evoked increase in mRNATAT activity. These results provided evidence that the primary action of cAMP in tyrosine aminotransferase induction is the increase of functional mRNATAT and that this increase can completely account for the increase in tyrosine aminotransferase activity.     

Organ specificity of glucocorticoid-sensitive tyrosine aminotransferase. Separation from aspartate aminotransferase isoenzymes

In order to study whether hormone-sensitive tyrosine aminotransferase exists in tissues other than liver, we have devised means to separate the liver-specific enzyme from other enzymes that transaminate tyrosine and to distinguish between the authentic enzyme and the principal "pseudotyrosine aminotransferases," which are the isoenzymes of aspartate aminotransferase. We accomplish this by suppressing proteolysis of the authentic enzyme using a buffer of pH 8.0 containing 0.1 M potassium chloride; enzyme extracted from liver in this buffer migrates as a single peak during chromatography on hydroxylapatite and represents the undegraded native form. A much smaller peak of tyrosine aminotransferase activity elutes at higher ionic strength and corresponds to a mixture of mitochondrial aspartate aminotransferase and partially degraded tyrosine aminotransferase. Cytosolic aspartate aminotransferase, in contrast, adsorbs weakly to the hydroxylapatite column and transaminates tyrosine very poorly although it readily utilizes monoiodotyrosine. The aspartate aminotransferase isoenzymes separate completely from tyrosine aminotransferase during chromatography on DEAE-Sepharose CL-6B. By combining these techniques with the use of specific antibodies, we show that brain, heart, and kidney do not contain tyrosine aminotransferase. Furthermore, we locate both isoenzymes of aspartate aminotransferase on polyacrylamide gels and show that both react histochemically as tyrosine aminotransferases when monoiodotyrosine is used as substrate. Use of these techniques, therefore, permits unambiguous identification of tyrosine aminotransferase and its separation from the background of nonspecific transamination.     

Subcellular distribution, multiple forms and development of glutamate-pyruvate (glyoxylate) aminotransferase in plant tissues

According to a sucrose density gradient analysis of cell organelles from homogenates of green leaves of rye, wheat and pea seedlings glutamate-pyruvate aminotransferase was predominantly localized in the leaf microbodies (peroxisomes; 90%) and to a minor extent in the mitochondria (10%) but completely absent from chloroplasts. In etiolated rye leaves the distribution of the enzyme was similar. In other non-green tissues glutamate-pyruvate aminotransferase was predominantly associated with the mitochondria but also present in the microbodies of dark-grown pea roots and in the glyoxysomes of Ricinus endosperm. In the microbodies isolated from potato tubers the enzyme was not detectable. Glutamate-pyruvate aminotransferase activity was not associated with the proplastid fractions of the non-green tissues. The distribution of glutamate-oxaloacetate aminotransferase was different from that of glutamate-pyruvate aminotransferase. Glutamate-oxaloacetate aminotransferase was found in chloroplasts, proplastids, mitochondria, microbodies and in the supernatant. Evidence is presented that glutamate-pyruvate and glutamate-glyoxylate aminotransferase activities were catalyzed by the same enzyme. Both activities showed the same organelle distribution on sucrose gradients and both were eluted at the same salt concentration from DEAE-cellulose. By chromatography of preparations from rye leaf extracts on DEAE-cellulose two forms of glutamate-pyruvate (glyoxylate) aminotransferase were separated. The major fraction eluting at a low salt concentration was identified as peroxisomal form and the minor fraction eluting at a higher salt concentration was identified as a mitochondrial form. Both the glutamate-glyoxylate and the glutamate-pyruvate aminotransferase activities of the peroxisomal as well as of the mitochondrial forms of the enzyme were strongly (about 80%) inhibited by the presence of 10 mM glycidate, previously described as an inhibitor of glutamate-glyoxylate aminotransferase in tobacco tissue. Pig heart glutamate-pyruvate aminotransferase exhibited no glutamate-glyoxylate aminotransferase activity and was only slightly inhibited by glycidate. The development of glutamate-pyruvate aminotransferase activity in the leaves of rye seedlings was strongly increased in the light, relative to dark-grown seedlings, and very similar to that of catalase activity while the development of glutamate-oxaloacetate aminotransferase was, in close coincidence with the behavior of leaf growth, only slightly enhanced by light. It is discussed that in green leaves an extrachloroplastic synthesis of alanine is of considerable advantage for the metabolic flow during photosynthesis.     

Tertiary structure of tRNAPhe. A possible correlation between the structural functional unit of this tRNA and its exonic sequence.

By using an antiserum raised against rat liver tyrosine aminotransferase, it was shown that about 60% of tryptophan aminotransferase activity in rat liver extracts is catalysed by this enzyme. Induction of tryptophan aminotransferase activity by intraperitoneal injections of tryptophan or triamcinolone can be entirely attributed to the effects of these agents on tyrosine aminotransferase. The origin of the other 40% of tryptophan aminotransferase activity remains to be established. This activity increases after starvation for 48 h. It is unlikely that tryptophan transamination plays a quantitatively important role in the metabolism of tryptophan by the liver.     

Cloning of tomato (Lycopersicon esculentum Mill.) arginine decarboxylase gene and its expression during fruit ripening.

Extracts of soybean (Glycine max) root nodules and greening etiolated leaves catalyzed radiolabeled delta-aminolevulinic acid (ALA) formation from 3,4-[3H]glutamate but not from 1-[14C]glutamate. Nevertheless, those tissue extracts expressed the activity of glutamate 1-semialdehyde (GSA) aminotransferase, the C5 pathway enzyme that catalyzes ALA synthesis from GSA for tetrapyrrole formation. A soybean nodule cDNA clone that conferred ALA prototrophy, GSA aminotransferase activity, and glutamate-dependent ALA formation activity on an Escherichia coli GSA aminotransferase mutant was isolated. The deduced product of the nodule cDNA shared 79% identity with the GSA aminotransferase expressed in barley leaves, providing, along with the complementation data, strong evidence that the cDNA encodes GSA aminotransferase. GSA aminotransferase mRNA and enzyme activity were expressed in nodules but not in uninfected roots, indicating that the Gsa gene is induced in the symbiotic tissue. The Gsa gene was strongly expressed in leaves of etiolated plantlets independently of light treatment and, to a much lesser extent, in leaves of mature plants. We conclude that GSA aminotransferase, and possibly the C5 pathway, is expressed in a nonphotosynthetic plant organ for nodule heme synthesis and that Gsa is a regulated gene in soybean.     

Nucleus-dependent regulation of tyrosine aminotransferase degradation in hepatoma tissue culture cells.

Upon removal of the nucleus from rat hepatoma tissue culture cells, levels of the enzyme tyrosine aminotransferase no longer change in response to withdrawal of glucocorticoids. The rate of tyrosine aminotransferase degradation is drastically reduced in rat hepatoma tissue culture cytoplasts leading to stabilization of pre-existing levels of tyrosine aminotransferase. Moreover, the rate of synthesis of the enzyme in cytoplasts is very low near that observed in uninduced whole cells. These effects of enucleation occur very rapidly and appear to be specific for tyrosine aminotransferase and a small number of other unstable hepatoma proteins. A nuclear effect is thus directly involved in the control of tyrosine aminotransferase degradation and synthesis.     

In vitro synthesis and processing of a precursor to ornithine aminotransferase

Poly(A)+ RNA was isolated from liver-free polysomes of rats maintained on a 60% casein diet by sodium dodecyl sulfate-phenol-chloroform extraction and oligo(dT)-cellulose chromatography. Poly(A)+ RNA translated in a rabbit reticulocyte lysate system produced a polypeptide of 49,000 daltons that was immunoprecipitated by monospecific, affinity-purified IgG antibodies to ornithine aminotransferase (ornithine-oxo acid aminotransferase, EC 2.6.1.13). This polypeptide is 6,000 daltons larger than mature ornithine aminotransferase when electrophoresed on sodium dodecyl sulfate polyacrylamide gels. One-dimensional peptide mapping demonstrated that this 49,000-dalton polypeptide is structurally related to ornithine aminotransferase. Furthermore, it can be processed to a polypeptide of 43,000 daltons by a rat liver mitochondrial fraction. We have concluded that this polypeptide is a precursor to ornithine aminotransferase.     

Nonidentity of the Aspartate and the Aromatic Aminotransferase Components of Transaminase A in Escherichia coli

Tyrosine, added to the growth medium of a strain of Escherichia coli K-12 lacking transaminase B, repressed the tyrosine, phenylalanine, and tryptophan aminotransferase activities while leaving the aspartate aminotransferase activity unchanged. This suggested that the aspartate and the aromatic aminotransferase activities, previously believed to reside in the same protein, viz. transaminase A, are actually nonidentical. Further experiments showed that, upon incubation at 55 C, the aspartate aminotransferase of crude extracts was almost completely stable, whereas the tyrosine and phenylalanine activities were rapidly inactivated. Apoenzyme formation was faster, and apoenzyme degradation proceeded more slowly with aspartate aminotransferase than with tyrosine aminotransferase. Electrophoresis in polyacrylamide gels separated the aminotransferases. A more rapidly moving band contained tyrosine, phenylalanine, and tryptophan aminotransferases, and a slower band contained aspartate aminotransferase. A mutant of E. coli K-12 with low levels of aspartate aminotransferase exhibited unchanged levels of tyrosine aminotransferase. Thus, transaminase A appears to be made up of at least two proteins: one of broad specificity whose synthesis is repressed by tyrosine and another, specific for aspartate, which is not subject to repression by amino acids. The apparent molecular weights of both the aspartate and the aromatic aminotransferases, determined by gel filtration, were about 100,000.     

The role of transaminases in realizing the protective effect of L-aspartate in hypoxia]

Activation of aspartate aminotransferase and alanine aminotransferase of mitochondria introduced to the incubation medium of pyridoxal-5'-phosphate (40 microM) is approximately 2 times higher than that of the corresponding cytoplasmic forms. At hypoxia aspartate aminotransferase activity in mitochondria and postmitochondrial supernatant tends to an increase while that of alanine aminotransferase decreases (above 2 times). The protection from hypoxic damage when using L-aspartate (100 mg/kg subcutaneously 3-5 min before hypoxia) intensifies an adaptive increase of aspartate aminotransferase activity and removes a decrease of alanine aminotransferase activity. Under these conditions stimulating effect of pyridoxal-5'-phosphate on transaminases activity in vitro weakens. A simultaneous administration of vitamin-coenzyme complex (thiamine pyrophosphate, lipoate, sodium 4-phospho-pantothenate, flavin-mononucleotide, nicotinate) intensifies these metabolic shifts and protective action of L-aspartate.     

Glutamate-glyoxylate aminotransferase in rat liver cytosol. Purification, properties and identity with alanine-2-oxoglutarate aminotransferase

After cortisone injection, virtually identical increases in rat liver cytosol alanine-2-oxoglutarate aminotransferase and glutamate-glyoxylate aminotransferase activities were observed. The two activities were co-purified to homogeneity from rat liver cytosol. The purified enzyme was specific for L-alanine with 2-oxoglutarate as amino acceptor. With glyoxylate, however, the enzyme utilized various L-amino acids as amino donors in the following order of activity: glutamate greater than alanine greater than glutamine greater than methionine. The ratio of alanine-2-oxoglutarate aminotransferase activity to glutamate-glyoxylate aminotransferase activity remained constant during purification and was unchanged by a variety of treatments of the purified enzyme. These results suggest that glutamate-glyoxylate aminotransferase is identical with alanine-2-oxoglutarate aminotransferase. Evidence was obtained that the two enzyme activities in the cytosol of dog, cat and human liver are also properties of the same protein.     

The active site of Sulfolobus solfataricus aspartate aminotransferase

Aspartate aminotransferase from the archaebacterium Sulfolobus solfataricus binds pyridoxal 5' phosphate, via an aldimine bond, with Lys-241. This residue has been identified by reducing the enzyme in the pyridoxal form with sodium cyanoboro[3H]hydride and sequencing the specifically labeled peptic peptides. The amino acid sequence centered around the coenzyme binding site is highly conserved between thermophilic aspartate aminotransferases and differs from that found in mesophilic isoenzymes. An alignment of aspartate aminotransferase from Sulfolobus solfataricus with mesophilic isoenzymes, attempted in spite of the low degree of similarity, was confirmed by the correspondence between pyridoxal 5' phosphate binding residues. Using this alignment it was possible to insert the archaebacterial aspartate aminotransferase into a subclass, subclass I, of pyridoxal 5' phosphate binding enzymes comprising mesophilic aspartate aminotransferases, tyrosine aminotransferases and histidinol phosphate aminotransferases. These enzymes share 12 invariant amino acids most of which interact with the coenzyme or with the substrates. Some enzymes of subclass I and in particular aspartate aminotransferase from Sulfolobus solfataricus, lack a positively charged residue, corresponding to Arg-292, which in pig cytosolic aspartate aminotransferase interacts with the distal carboxylate of the substrates (and determines the specificity towards dicarboxylic acids). It was confirmed that aspartate aminotransferase from Sulfolobus solfataricus does not possess any arginine residue exposed to chemical modifications responsible for the binding of omega-carboxylate of the substrates. Furthermore, it has been found that aspartate aminotransferase from Sulfolobus solfataricus is fairly active when alanine is used as substrate and that this activity is not affected by the presence of formate. The KM value of the thermophilic aspartate aminotransferase towards alanine is at least one order of magnitude lower than that of the mesophilic analogue enzymes.     

Aspartate: 2-oxoglutarate aminotransferase from trichomonas vaginalis. Identity of aspartate aminotransferase and aromatic amino acid aminotransferase.

Aspartate: 2-oxoglutarate aminotransferase from the anaerobic protozoon Trichomonas vaginalis was purified to homogeneity and characterized. It is a dimeric protein of overall Mr approx. 100000. Only a single isoenzyme was found in T. vaginalis. The overall molecular and catalytic properties have features in common with both the vertebrate cytoplasmic and mitochondrial isoenzymes. The purified aspartate aminotransferase from T. vaginalis showed very high rates of activity with aromatic amino acids as donors and 2-oxoglutarate as acceptor. This broad-spectrum activity was restricted to aromatic amino acids and aromatic 2-oxo acids, and no significant activity was seen with other common amino acids, other than with the substrates and products of the aspartate: 2-oxoglutarate aminotransferase reaction. Co-purification and co-inhibition, by the irreversible inhibitor gostatin, of the aromatic amino acid aminotransferase and aspartate aminotransferase activities, in conjunction with competitive substrate experiments, strongly suggest that a single enzyme is responsible for both activities. Such high rates of aromatic amino acid aminotransferase activity have not been reported before in eukaryotic aspartate aminotransferase.     

Enzymatic synthesis of aminocyclitol moieties of aminoglycoside antibiotics from inositol by Streptomyces spp.: detection of glutamine-aminocyclitol aminotransferase and diaminocyclitol aminotransferase activities in a spectinomycin producer.

Extracts of stationary-phase mycelia of the spectinomycin producer Streptomyces flavopersicus ATCC 19756 catalyzed inositol dehydrogenase, L-glutamine:inosose aminotransferase, 2-epi-streptamine:inosose aminotransferase, streptamine:inosose aminotransferase, N3-methyl-2-deoxystreptamine:inosose aminotransferase, and aminodeoxy-scyllo-inositol:inosose aminotransferase reactions, as detected with a new rapid assay procedure. These results suggest that one or both amino groups of the N1,N3-dimethyl-2-epi-streptamine moiety of spectinomycin are derived by transamination from the alpha-amino group of L-glutamine. An enzymatic procedure for distinguishing among N1- and N3-monomethyl diaminocyclitol derivatives and their diaminocyclitol biosynthetic precursors is described. A scheme showing key roles of glutamine-aminocyclitol aminotransferases in biosynthesis of major aminoglycoside antibiotics is presented.