Purification and properties of peroxisomal pyruvate (glyoxylate) aminotransferase from rat liver.

Pyruvate (glyoxylate) aminotransferase from rat liver peroxisomes was highly purified and characterized. The enzyme preparation has a mol.wt. of approx. 80,000 with two identical subunits, and isoelectric point of 8.0 and a pH optimum between 8.0 and 8.5. The enzyme catalysed transamination between a number of L-amino acids and pyruvate or glyoxylate. The effective amino acceptors were pyruvate, phenylpyruvate and glyoxylate with serine, and glyoxylate and phenylpyruvate with alanine as amino donor. These properties and kinetic parameters of the enzyme are remarkably similar to those previously described for mitochondrial alanine-glyoxylate aminotransferase isoenzyme 1 from glucagon-injected rat liver [Noguchi, Okuno, Takada, Minatogawa, Okai & Kido (1978, Biochem. J. 169, 113-122].     

Purification and characterization of aromatic-amino-acid-glyoxylate aminotransferase from monkey and rat liver.

Aromatic-amino-acid-glyoxylate aminotransferase was highly purified from the mitochondrial fraction of livers from monkey and glucagon-injected rats. The two enzyme preparations showed physical and enzymic properties different from a kynurenine aminotransferase previously described. The two enzymes had nearly identical molecular weights (approximate 80 000), isoelectric points (pH 8.0) and pH optima (pH 8.0 - 8.5). However, a difference in substrate specificity was observed between the two enzymes. Both enzymes utilized glyoxylate, pyruvate, hydroxypyruvate and 2-oxo-4-methyl-thiobutyrate as effective amino acceptors. 2-Oxoglutarate was active for rat enzyme but not for monkey enzyme. With glyoxylate, amino donors were effective in the following order of activity; phenylalanine greater than histidine greater than tyrosine greater than tryptophan greater than 5-hydroxytrypotphan greater than kynurenine for the rat enzyme, and phenylalanine greater than kynurenine greater than histidine greater than tryptophan greater than 5-hydroxy-tryptophan for the monkey enzyme.     

Purification, characterization and identification of rat liver histidine-pyruvate aminotransferase isoenzymes.

1. Histidine-pyruvate aminotransferase (isoenzyme 1) was purified to homogeneity from the mitochondrial and supernatant fractions of rat liver, as judged by polyacrylamide-gel electrophoresis and isolectric focusing. Both enzyme preparations were remarkably similar in physical and enzymic properties. Isoenzyme 1 had pI8.0 and a pH optimum of 9.0. The enzyme was active with pyruvate as amino acceptor but not with 2-oxoglutarate, and utilized various aromatic amino acids as amino donors in the following order of activity: phenylalanine greater than tyrosine greater than histidine. Very little activity was found with tryptophan and 5-hydroxytryptophan. The apparent Km values were about 2.6mM for histidine and 2.7 mM for phenylalanine. Km values for pyruvate were about 5.2mM with phenylalanine as amino donor and 1.1mM with histidine. The aminotransferase activity of the enzyme towards phenylalanine was inhibited by the addition of histidine. The mol.wt. determined by gel filtration and sucrose-density-gradient centrifugation was approx. 70000. The mitochondrial and supernatant isoenzyme 1 activities increased approximately 25-fold and 3.2-fold respectively in rats repeatedly injected with glucagon for 2 days. 2. An additional histidine-pyruvate aminotransferase (isoenzyme 2) was partially purified from both the mitochondrial and supernatant fractions of rat liver. Nearly identical properties were observed with both preparations. Isoenzyme 2 had pI5.2 and a pH optimum of 9.3. The enzyme was specific for pyruvate and did not function with 2-oxoglutarate. The order of effectiveness of amino donors was tyrosine = phenylalanine greater than histidine greater than tryptophan greater than 5-hydroxytryptophan. The apparent Km values for histidine and phenylalanine were about 0.51 and 1.8 mM respectively. Km values for pyruvate were about 3.5mM with phenylalanine and 4.7mM with histidine as amino donors. Histidine inhibited phenylalanine aminotransferase activity of the enzyme. Gel filtration and sucrose-density-gradient centrifugation yielded a mol.wt. of approx. 90000. Neither the mitochondrial nor the supernatant isoenzyme 2 activity was elevated by glucagon injection.     

Purification and characterization of the active serine: pyruvate aminotransferase of rat liver mitochondria expressed in Escherichia coli

In the previous study (Oda, T., et al. (1985) Eur. J. Biochem. 150, 415-421), we isolated a cDNA clone which expressed in Escherichia coli a specific size of product having the activity of rat liver serine:pyruvate aminotransferase (SPTm). This specific product (SPT10) was purified to homogeneity through three different column chromatographies. The amino acid composition and N-terminal amino acid sequence of the purified enzyme agreed with those predicted from the nucleotide sequence of cDNA and showed that SPT10 consists of the whole amino acid sequence of mature SPTm and several extra amino acid residues at the N-terminus. The catalytic and physical properties of SPT10, such as substrate specificity, Km for alpha-keto acids, electric charge, and quaternary structure, were all very similar to those of SPTm. Using several cDNA clones which lack a 5'-terminal sequence corresponding to a portion of the N-terminal amino acid sequence of SPTm, we examined the expression profile of the specific product in bacteria transformed with each cDNA clone. The products encoded by these cDNAs were segregated into inclusion bodies and were neither catalytically active nor easily solubilized by sonication. In contrast, the inclusion bodies were not formed in the bacteria transformed with the cDNA clone for SPT10.     

Immunochemical studies on induction of rat liver mitochondrial serine: pyruvate aminotransferase by glucagon.

The 7- to 10-fold increase in the rat liver serine:pyruvate aminotransferase activity after glucagon administration was shown to occur mainly in the mitochondrial matrix of parenchymal cells. The enzyme was purified from glucagon-treated rat liver mitochondria to apparent homogeneity as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A specific rabbit antibody was prepared against the purified enzyme. Upon Ouchterlony double diffusion analysis, the mitochondrial extracts of glucagon-treated rat liver produced a single and fused precipitin line between the purified enzyme against the antibody. The supernatant fraction of glucagon-treated rat liver and the mitochondrial extracts of normal liver were also shown to make a single and fused precipitin line with the purified enzyme, when applied in large quantities. The quantitative immunotitration demonstrated that the glucagon-induced increase in the activity of liver serine:pyruvate aminotransferase were accompanied by the parallel increase in the amount of the enzyme antigen. Isotopic leucine incorporation studies showed that the relative rate of synthesis of the enzyme was increased approximately 10-fold by glucagon administration under the conditions employed. The rate of the degradation of the aminotransferase in the normal rat liver was a relatively slow process with a half-life of approximately 30 h. Thus the accumulation of serine:pyruvate aminotransferase in rat liver mitochondria by glucagon treatment can be ascribed mainly to the rise in the rate of enzyme synthesis.     

Purification and characterization of D-3-aminoisobutyrate-pyruvate aminotransferase from rat liver

D-3-Aminoisobutyrate-pyruvate aminotransferase (EC 2.6.1.40) was purified 1900-fold from rat liver extract. The purified enzyme showed a molecular mass of 180 kDa by gel-permeation HPLC analysis using a TSK gel G3000SW column. Reductive polyacrylamide gel electrophoresis in sodium dodecyl sulfate resulted in identification of a single band of approx. 50 kDa, indicating that the native enzyme is probably a tetrametric protein. The specific activity of the purified enzyme was 1.14 mumol/min per mg protein. D-3-Aminoisobutyrate and beta-alanine were good amino donors. The Km value for L-3-aminoisobutyrate was 100-times larger than that for the D-isomer. The apparent Km values for D-3-aminoisobutyrate and beta-alanine were 35 and 282 microM, respectively. Pyruvate, glyoxylate, oxalacetate, 2-oxo-n-valerate, and 2-oxo-n-butyrate were good amino acceptors. The apparent Km values for pyruvate and glyoxylate were 32 and 44 microM, respectively.     

Purification and biochemical characterization of recombinant rat liver phenylalanine hydroxylase produced in Escherichia coli.

Phenylalanine hydroxylase, important in phenylalanine metabolism in mammals, is regulated through short-term (activation) and long-term (induction) mechanisms. To help elucidate the structure-function relationships involved in the activation of this enzyme, we have isolated and characterized full-length cDNA clones to rat phenylalanine hydroxylase. Recombinant rat phenylalanine hydroxylase was placed into an expression vector in Escherichia coli. The enzyme has been purified to homogeneity and its physical and catalytic properties have been characterized. The molecular weight and the fluorescence emission spectrum of the recombinant enzyme were identical to those of the native enzyme. The recombinant enzyme could be activated by incubation with phenylalanine or lysolecithin or by phosphorylation, as is the rat liver enzyme. The extent of activation is the same as that for the native enzyme in each case except for phenylalanine, which activates the recombinant enzyme only 5- to 10-fold rather than the 15- to 30-fold activation observed with the native enzyme. The kinetic constants determined for the recombinant enzyme are also essentially the same as those reported for the native enzyme. We conclude that this enzyme is essentially identical to the native enzyme and should be very useful in the future study of this important hydroxylase.     

Induction of mitochondrial serine:pyruvate aminotransferase of rat liver by glucagon and insulin through different mechanisms

Studies were performed in the rat liver to examine whether or not insulin as well as glucagon causes the induction of mitochondrial serine:pyruvate aminotransferase (SPTm) [EC 2.6.1.51] and if so, whether the mechanisms of induction are similar or different for the two hormones. Not only glucagon but also insulin induced SPTm. Cell-free translation assaying and RNA blot analysis showed that both hormones cause an increase in the hepatic level of mRNA for the precursor of SPTm. Their effects were virtually additive, and the time course of the increase in the mRNA level differed between the hormones. The maximal increase induced by glucagon was observed 3.5 h after the hormone injection while that by insulin was found after 6 h. The increase in the mRNA due to insulin was completely inhibited by the co-administration of cycloheximide, while that due to glucagon was not. The finding suggests that a newly synthesized, insulin-dependent protein(s) is involved in the regulation of the mRNA level by insulin. On the other hand, hydrocortisone treatment selectively suppressed the increase in the mRNA due to glucagon. These data indicate that the synthesis of the mRNA for SPTm is regulated by glucagon and insulin through different mechanisms. The size of the hormone-induced mRNA for SPTm gradually decreased with time, but the cell-free translation products did not exhibit size alteration. RNase H digestion to remove the poly(A) tail of the mRNA indicated that shortening of the poly(A) sequence might be responsible for the time-dependent size alteration of the mRNA.     

Nucleotide sequence of the cDNA encoding the precursor for mitochondrial serine:pyruvate aminotransferase of rat liver

The nucleotide sequence of the mRNA coding for the precursor of mitochondrial serine:pyruvate aminotransferase of rat liver was determined from those of cDNA clones. The mRNA comprises at least 1533 nucleotides, except the poly(A) tail, and encodes a polypeptide consisting of 414 amino acid residues with a molecular mass of 45,834 Da. Comparison of the N-terminal amino acid sequence of mitochondrial serine:pyruvate aminotransferase with the nucleotide sequence of the mRNA showed that the mature form of the mitochondrial enzyme consisted of 390 amino acid residues of 43,210 Da. The amino acid composition of mitochondrial serine:pyruvate aminotransferase deduced from the nucleotide sequence of the cDNA showed good agreement with the composition determined on acid hydrolysis of the purified protein. The extra 24 amino acid residues correspond to the N-terminal extension peptide (pre-sequence) that is indispensable for the specific import of the precursor protein into mitochondria. In the extension peptide there are four basic amino acids distributed among hydrophobic amino acids and, as revealed on helical wheel analysis, the putative alpha-helical structure of the peptide was amphiphilic in nature. The secondary structures of the mature serine:pyruvate aminotransferase and three other aminotransferases of rat liver were predicted from their amino acid sequences. Their secondary structures exhibited a common feature and so we propose the specific lysine residue which binds pyridoxal phosphate as the active site of serine:pyruvate aminotransferase.     

Purification of rat liver mitochondrial aspartate aminotransferase and separation of its isoforms utilizing high-performance liquid chromatography

A rapid method for purification of mitochondrial aspartate aminotransferase from rat liver employing high-performance liquid chromatography is reported. The product is purified 80-fold with a recovery greater than or equal to 50% in a single day. The amino acid composition, N-terminal amino acid sequence, specific activity, and spectral characteristics of the isolated enzyme are similar to those previously reported for this protein. The protein is homogeneous by standard electrophoretic and chromatographic criteria, but can be resolved into at least five isoforms by a carboxymethylated resin column using high-performance liquid chromatography. The principal isoform initially isolated is converted into two additional isoforms with lower specific activity upon storage at 4 degrees C.     

Purification and characterization of a mitochondrial thymine glycol endonuclease from rat liver

Mitochondrial DNA is exposed to oxygen radicals produced during oxidative phosphorylation. Accumulation of several kinds of oxidative lesions in mitochondrial DNA may lead to structural genomic alterations, mitochondrial dysfunction, and associated degenerative diseases. The pyrimidine hydrate thymine glycol, one of many oxidative lesions, can block DNA and RNA polymerases and thereby exert negative biological effects. Mitochondrial DNA repair of this lesion is important to ensure normal mitochondrial DNA metabolism. Here, we report the purification of a novel rat liver mitochondrial thymine glycol endonuclease (mtTGendo). By using a radiolabeled oligonucleotide duplex containing a single thymine glycol lesion, damage-specific incision at the modified thymine was observed upon incubation with mitochondrial protein extracts. After purification using cation exchange, hydrophobic interaction, and size exclusion chromatography, the most pure active fractions contained a single band of approximately 37 kDa on a silver-stained gel. MtTGendo is active within a broad KCl concentration range and is EDTA-resistant. Furthermore, mtTGendo has an associated apurinic/apyrimidinic-lyase activity. MtTGendo does not incise 8-oxodeoxyguanosine or uracil-containing duplexes or thymine glycol in single-stranded DNA. Based upon functional similarity, we conclude that mtTGendo may be a rat mitochondrial homolog of the Escherichia coli endonuclease III protein.     

Purification and characterization of rat liver glycosylasparaginase.

1. Rat liver glycosylasparaginase [N4-(beta-N-acetylglucosaminyl)-L-asparaginase, EC 3.5.1.26] was purified to homogeneity by using salt fractionation, CM-cellulose and DEAE-cellulose chromatography, gel filtration on Ultrogel AcA-54, concanavalin A-Sepharose affinity chromatography, heat treatment at 70 degrees C and preparative SDS/polyacrylamide-gel electrophoresis. The purified enzyme had a specific activity of 3.8 mumol of N-acetylglucosamine/min per mg with N4-(beta-N-acetylglucosaminyl)-L-asparagine as substrate. 2. The native enzyme had a molecular mass of 49 kDa and was composed of two non-identical subunits joined by strong non-covalent forces and having molecular masses of 24 and 20 kDa as determined by SDS/polyacrylamide-gel electrophoresis. 3. The 20 kDa subunit contained one high-mannose-type oligosaccharide chain, and the 24 kDa subunit had one high-mannose-type and one complex-type oligosaccharide chain. 4. N-Terminal sequence analysis of each subunit revealed a frayed N-terminus of the 24 kDa subunit and an apparent N-glycosylation of Asn-15 in the same subunit. 5. The enzyme exhibited a broad pH maximum above 7. Two major isoelectric forms were found at pH 6.4 and 6.6. 6. Glycosylasparaginase was stable at 75 degrees C and in 5% (w/v) SDS at pH 7.0.     

Purification and properties of rat liver mitochondrial glutathione peroxidase.

Glutathione peroxidase (glutathione:hydrogen peroxide oxidoreductase, EC 1.11.1.9) was purified from rat liver mitochondria. The enzyme was shown to be pure by polyacrylamide-gel electrophoresis and to contain multiple forms that differed in charge. Selenium was specifically associated with the enzyme. The enzyme was inhibited by iodoacetic acid and iodoacetamide in an unusual pattern of reduction by sulfhydryl compounds and pH dependency. The mitochondrial and cytoplasmic forms of the enzyme were compared, and an explanation of the inhibition patterns is offered.     

Species distribution and properties of hepatic phenylalanine (histidine):pyruvate aminotransferase.

Hepatic phenylalanine(histidine):pyruvate aminotransferase activity is much higher in the mouse and rat than in other animal species (human, guinea-pig, rabbit, pig, dog and chicken). The activity is elevated in the mouse and rat by the injection of glucagon but not in other species (guinea-pig, rabbit and chicken). The enzyme was purified from the mitochondrial fraction of mouse liver to homogeneity as judged by polyacrylamide disc gel electrophoresis in the presence of dodecylsulphate. With histidine as amino donor, the enzyme was active with pyruvate, oxaloacetate and hydroxypyruvate as amino acceptors but not with 2-oxoglutarate. Effective amino donors were histidine, phenylalanine and tyrosine with pyruvate, and methionine, serine and glutamine with phenylpyruvate. The apparent Km for histidine was about 6.9 mM with pyruvate and that for pyruvate was 21 mM with histidine. The enzyme is probably composed of two identical subunits with a molecular weight of approximately 40000. The pH optimum was near 9.0. Isoelectric focusing of the purified enzyme resulted in the detection of four forms with pI 6.0, 6.2, 6.5 and 6.7, respectively, all of which were responsive to glucagon. These four forms were nearly identical with the purified enzyme before the focusing with respect to physical and enzymic properties. A possible mechanism of this multiplicity is discussed.