Functional roles of valine 37 and glycine 38 in the mobile loop of porcine cytosolic aspartate aminotransferase

The functional roles of Val37 and Gly38 in porcine cytosolic aspartate aminotransferase have been studied in the site-directed mutants V37A, G38A, and G38S where the size and hydrophobic character of these residues has been altered. Previous x-ray studies have shown that Val37 and Gly38, which are part of a flexible loop, interact directly with bound substrate. From x-ray and solution experiments we find that the V37A, G38A, and G38S mutations do not cause significant perturbations to the unliganded enzyme. Replacing Val37 with a less bulky alanine residue does not affect the maximal catalytic rate (kcat), but it does increase significantly the Michaelis constants for substrates in the overall transamination reaction between aspartate and 2-oxoglutarate. On the other hand, replacing Gly38 with alanine or serine results in striking decreases in kcat to 5 and 0.6%, respectively, of the value observed for the wild-type enzyme, as well as in considerable increases in Km values. Consequently, the catalytic competence, kcat/Km, decreases by 3 orders of magnitude for G38A and by 4 orders of magnitude for G38S. Single turnover reactions of G38A and G38S with four individual substrates (aspartate, glutamate, oxalacetate, and 2-oxoglutarate) are characterized by kinetic parameters that are largely consistent with those of the overall reaction. In addition, the mutations at position 38 impair more seriously the catalytic competence of the enzyme toward C5-substrates than toward C4-substrates. We conclude that Gly38 is probably required for proper function of the enzyme because it permits a high level of flexibility for the 36-39 peptide, which in turn allows the essential substrate-induced movement of the small domain.

[Treatment of common bile duct stones by peroral cholangioscopy]

Endoscopic papillotomy (EPT) has become a popular form of treatment in managing common bile duct stones. But it may fail in difficult cases such as large stones of over 20 mm, confluence stones, and impacted stones. Over the past 5 years, our success rate in clearing the bile duct by conventional endoscopic techniques (mechanical lithotripsy) was about 90 per cent. Recently, we have performed electrohydraulic lithotripsy or laser lithotripsy with peroral cholangioscopy in these difficult cases. Complete clearance of the duct was obtained in all 23 patients who underwent peroral cholangioscopic lithotripsy. No complication occurred. In conclusion, peroral endoscopic treatment of common bile duct stones should be safe and effective.

Silent mixed ganglioneuroma/pheochromocytoma which produces a vasoactive intestinal polypeptide

An unusual pheochromocytoma was incidentally discovered in a 48-year-old woman. The patient had a 3-year history of myasthenia gravis. At the time of examination in our hospital, the right adrenal tumor was incidentally discovered by ultrasonography of the abdomen. She had no history of headache, perspiration, palpitation or hypertension. Although blood catecholamine levels were within the normal limits, urinary secretion of catecholamine was elevated. Histologically, the tumor was diagnosed to be mixed ganglioneuroma/pheochromocytoma and histochemically confirmed to produce vasoactive intestinal polypeptide. Such a tumor is quite rare.

Porcine cytosolic aspartate aminotransferase reconstituted with [4'-13C]pyridoxal phosphate. pH- and ligand-induced changes of the coenzyme observed by 13C NMR spectroscopy

Apoenzyme samples of aspartate aminotransferase (AspAT) purified from the cytosolic fraction of pig heart were reconstituted with [4'-13C]pyridoxal 5'-phosphate (pyridoxal-P). The 13C NMR spectra of AspAT samples thus generated established the chemical shift of 165.3 ppm for C4' of the coenzyme bound as an internal aldimine with lysine 258 of the enzyme at pH 5. In the absence of ligands the chemical shift of C4' was shown to be pH dependent, shifting 5 ppm upfield to a constant value of 160.2 ppm above pH 8, the resulting pKa of 6.3 in agreement with spectrophotometric titrations. The addition of the competitive inhibitor succinate to the internal aldimine raises the pKa of the imine to 7.8, consistent with the theory of charge neutralization in the active site. In the presence of saturating concentrations of 2-methylaspartic acid the C4' signal of the coenzyme was shown to be invariant with pH and located at 162.7 ppm, midway between the observed chemical shifts of the protonated and unprotonated forms of the internal aldimine. The intermediate chemical shift of the external aldimine complex is thought to reflect the observation of an equilibrium mixture composed of roughly equal populations of the protonated ketoenamine and a dipolar anion species, corresponding to their respective spectral bands at 430 and 360-370 nm. Conversion to the pyridoxamine form was accomplished via reaction of the internal aldimine with L-cysteinesulfinate or by reduction with sodium borohydride, and the resulting C4' chemical shifts were identified by difference spectroscopy. Finally, the line widths of the C4' resonance under the various conditions were measured and qualitatively compared. The results are discussed in terms of the current mechanism and molecular models of the active site of AspAT.

Structural and functional role of the amino-terminal region of porcine cytosolic aspartate aminotransferase. Catalytic and structural properties of enzyme derivatives truncated on the amino-terminal side

In porcine cytosolic aspartate aminotransferase, a dimeric enzyme, the amino-terminal region anchoring onto the neighboring subunit is linked to the adjoining floppy peptide segment (residues 12-47), an integral part of the small domain whose facile movement upon substrate binding is a striking "induced fit" feature of this enzyme. To assess the contribution by the amino-terminal region to small domain movement and protein stability, a series of enzyme derivatives truncated on the amino-terminal side (residues 1-9) was prepared by using oligonucleotide-directed in vitro mutagenesis. Deletion of residues 1-3 showed no effect on catalytic activity and heat stability. Del 1-5 mutant enzyme with an extra methionine at position 5 showed only 43% of the kappa cat value (in the overall transamination) of the wild-type enzyme. Further deletion up to residue 9 resulted in a slight decrease in kappa cat values. Del 1-9 mutant enzyme still retained a kappa cat value of 33% that of wild-type enzyme. Km values for aspartate and 2-oxoglutarate increased sharply upon deletion of residues 1-9. Accordingly, Del 1-9 mutant enzyme showed a striking decrease in the kappa cat/Km value, to only 2% of that for the wild-type enzyme. Deletion of amino-terminal residues 1-9 resulted also in a large decrease in thermostability and in an enhanced susceptibility to limited proteolysis by protease 401, which is known to cleave at Leu20 of the wild-type enzyme. These findings indicate that an increase in the conformational freedom of the floppy segment (residues 12-47) would occur upon the loss of most of the anchorage region, thereby presenting an entropic barrier to conformational changes that facilitate substrate binding with high affinity.

Import and processing of precursor to mitochondrial aspartate aminotransferase. Structure-function relationships of the presequence

The precursor protein of pig mitochondrial aspartate aminotransferase (pre-mAspAT) contains a 29-residue presequence (Joh, T., Nomiyama, H., Maeda, S., Shimada, K., and Morino, Y. (1985) Proc. Natl. Acad. Sci. U. S. A. 82, 1-5). Pre-mAspAT produced in an in vitro transcription and translation system was avidly imported into pig and rat liver mitochondria to be processed to the mature form of the enzyme. The pre-mAspAT was also processed to the mature form upon incubation with mitochondrial extracts. We synthesized precursor proteins with alterations within the presequence and compared quantitatively the effects of these mutations on the rates of both import and processing. Single and multiple substitutions of four basic residues with neutral amino acids at positions 5, 8, 18, and 28 showed that each residue contributes differentially to import and processing. Substitutions of His5 and Arg8 with glycines abolished the import activity but did not appreciably affect the rate of processing. Substitution of Arg28 with leucine at the position adjacent to the cleavage site seriously impaired the processing without appreciably affecting the rate of import. Analysis of deletions revealed that the amino-terminal region from position 2 to 8 was essential for both the import and processing. Thus the positive charges in the amino-terminal region are critical for import while the amino-terminal peptide segment and the cleavage site region appear to be requisite for recognition by a processing protease.

cDNA cloning and expression of pig cytosolic aspartate aminotransferase in Escherichia coli: amino-terminal heterogeneity of expressed products and lack of its correlation with enzyme function

A full-length cDNA encoding the pig cytosolic aspartate aminotransferase (EC 2.6.1.1) (cAspAT) was constructed from two overlapping cDNA clones. One clone (Lm pcAAT-8) isolated from a lambda gt10 pig heart cDNA library contained a 3' untranslated sequence, a poly(A) segment, and a part of the coding region for amino acid positions 127-412. Another clone (Lm pcAAT-107) isolated from a lambda gt10 primer extension library contained the coding region for amino acid positions 1-148 and a 5' untranslated sequence. Rejoining of the cDNA inserts of the two clones and recloning into pUC18 gave rise to a cDNA covering an entire coding sequence for pig cAspAT mRNA. Insertion into pKK223-3 yielded an expression plasmid, ppcAAT200. Escherichia coli JM105 cells transfected with ppcAAT200 overproduced pig cAspAT to an extent of about 3% of the total cellular soluble proteins. The expressed product was indistinguishable from the alpha subform of cAspAT isolated from pig heart in terms of specific activity, absorption spectra, molecular size, crystalline form, and immunological reactivity with anti pig cAspAT antibody. Compared with the amino-terminal sequence (Ala-Pro-Pro-) reported for pig heart cAspAT, the recombinant pig cAspAT showed heterogeneity in the amino-terminal sequence: Ala 1 (26%), Pro2 (54%), and Pro3 (19%). Construction of a mutant cAspAT with deletion of residues 1-3 and its comparison with the wild-type enzyme revealed that loss of the three amino-terminal residues does not affect the catalytic activity and structural integrity of the enzyme.

Rat cytosolic aspartate aminotransferase: molecular cloning of cDNA and expression in Escherichia coli

cDNA clones for rat cytosolic aspartate aminotransferase (cAspAT, L-aspartate:2-oxoglutarate aminotransferase) [EC 2.6.1.1] were isolated from a rat cDNA library, and the primary structure of the gene for cAspAT was deduced from its cDNA sequence. Rat cAspAT consists of 412 amino acids and its molecular weight is 46,295. The deduced amino acid sequence of rat cAspAT was compared with the sequences of AspATs from other species. The degree of sequence identities of rat/mouse cAspAT, rat/pig cAspAT, rat/chicken cAspAT, rat/pig mAspAT, and rat/Escherichia coli AspAT were 97.1, 89.6, 81.7, 48.1, and 41.2%, respectively. A coding region of rat cAspAT cDNA was inserted into E. coli expression vector pUC9, and enzymatically active cAspAT was expressed as a beta-galactosidase-cAspAT hybrid protein. This hybrid protein represented about 18% of the soluble proteins in E. coli and its kinetic properties were comparable with those of cAspAT preparations purified from rat liver.

Kinetic studies on the binding of gostatin, a suicide substrate for aspartate aminotransferase, with the isoenzymes from porcine heart mitochondria and cytosol

The reaction of pig heart mitochondrial and cytosolic aspartate aminotransferases (abbreviated to mAspAT and cAspAT, respectively) with an enzyme-suicide substrate (mechanism-based inhibitor), gostatin (5-amino-2-carboxyl-4-oxo-1,4,5,6-tetrahydropyridine-3-acetic acid) was studied kinetically, by following the spectral change with a micro-stopped-flow apparatus, as well as the inactivation of the enzyme activity. No significant difference in kinetic behavior was observed between mAspAT and cAspAT. From the analysis of time-dependent spectral change, no positive evidence for the existence of spectrophotometrically distinguishable intermediates was obtained. Both the spectral change and the inactivation followed, at least in appearance, simple bimolecular association kinetics, under the conditions studied. However, the second-order rate constant of the spectral change was found to be 1.5 to 2 times as large as that of the inactivation. The effects of pH and temperature on k(on) (the second-order rate constant of the spectral change) were also studied.

Cloning and sequence analysis of mRNA for mouse aspartate aminotransferase isoenzymes

The nucleotide sequences of mRNAs for the mouse mitochondrial and cytosolic aspartate aminotransferase isoenzymes (mAspAT and cAspAT) (EC 2.6.1.1) were determined from complementary DNAs. The mAspAT mRNA comprises minimally 2460 nucleotides and codes for a polypeptide of 430 amino acid residues corresponding to the precursor form of the mAspAT (pre-mAspAT). The cAspAT mRNA comprises minimally 2086 nucleotides and codes for a polypeptide of 413 amino acid residues. The region coding for the mature mAspAT and that for the cAspAT show about 53% overall homology. The former shares 49% and the latter 48% of homology, respectively, with that of the Escherichia coli aspC gene, which has been shown to code for the E. coli AspAT (Kuramitsu, S., Okuno, S., Ogawa, T., Ogawa, H., and Kagamiyama, H. (1985) J. Biochem. (Tokyo) 97, 1259-1262). When the deduced amino acid sequence of the mouse pre-mAspAT was compared with that of the pig pre-mAspAT polypeptide, we found that they share a 94% homology and that the mouse pre-mAspAT yields a presequence consisting of 29 amino acid residues and a mature mAspAT, consisting of 401 amino acid residues. These numbers and the amino acid residues present at the putative cleavage site are all in complete agreement in these two species. The deduced amino acid sequence of the mouse cAspAT shares 91% homology with that of the pig cAspAT. Comparisons of the nucleotide and deduced amino acid sequences between the mouse and E. coli AspATs suggest that the mammalian mAspAT gene is more closely related to the E. coli aspC gene than is the mammalian cAspAT gene.

Identification of coenzyme aldimine proton in 1H NMR spectra of pyridoxal 5'-phosphate dependent enzymes: aspartate aminotransferase isoenzymes

The pyridoxal form of the alpha subform of cytosolic aspartate aminotransferase (EC 2.6.1.1) is fully active and binds pyridoxal 5'-phosphate via an aldimine formation with Lys-258 whereas the gamma subform is virtually inactive and lacks the aldimine linkage. Comparison of 1H NMR spectra between the alpha and gamma subforms suggested that peak 1 of the alpha subform at 8.89 ppm contains a resonance assignable to the internal aldimine 4'-H. Reaction with a reagent that cleaves or modifies the internal aldimine bond [(amino-oxy)acetate, L-cysteinesulfinate, NH2OH, NaBH4, or NaCNBH3] caused the disappearance of a resonance line at 8.89 ppm that possessed a broad line width and corresponded in intensity to a single proton. These reagents were also used successfully for the identification of the aldimine 4'-H resonance in the mitochondrial isoenzyme. In contrast to the cytosolic isoenzyme whose resonance for the 4'-H did not show any detectable change in chemical shift with pH, the corresponding resonance in the mitochondrial isoenzyme exhibited pH-dependent chemical shift change (8.84 ppm at pH 5 and 8.67 ppm at pH 8) with a pK value of 6.3, reflecting the interisozymic difference in the microenvironment provided for the internal aldimine. Validity of the signal assignment was further shown by the two findings: the resonance assigned to the 4'-H emerged upon conversion of the pyridoxamine into the pyridoxal form, and the resonance appeared upon reconstitution of the apoenzyme with [4'-1H]pyridoxal phosphate but not with [4'-2H]pyridoxal phosphate.

Selective proteolysis of cytosolic aspartate aminotransferase by a new microbial protease

A protease from Streptomyces violaceochromogenes (Murao, S., Nishino, Y., & Maeda, Y. (1984) Agric. Biol. Chem. 48, 2163-2166) is known to inactivate pig heart aspartate aminotransferase [EC 2.6.1.1]. Chemical analysis of the core proteins and peptide fragments produced upon proteolysis of the aminotransferase revealed that peptide bond cleavage occurred specifically at Leu 20 with concomitant inactivation. Neither inactivation nor peptide bond cleavage was observed with the mitochondrial isoenzyme. The proteolytically produced derivative 21-412 of the cytosolic isoenzyme retained approximately 0.1% enzymic activity for transamination with natural dicarboxylic substrates. The pyridoxal form of the derivative 21-412 was fully converted by cysteinesulfinate or alanine to the pyridoxamine form and conversely the pyridoxamine form of the derivative was also fully converted by 2-oxoglutarate or pyruvate into the pyridoxal form, indicating that the derivative was still catalytically competent. However, the rates of reaction with dicarboxylic substrates were much reduced whereas the rates with monocarboxylic substrates remained at an order of magnitude similar to that observed with the native enzyme. Thus the NH2-terminal segment appears to be an import structural component which determines the substrate specificity of aspartate aminotransferase for dicarboxylic keto and amino acids. A substantial alteration in the molecular structure accompanying the loss of the NH2-terminal 20 residues was also reflected by the decrease in heat stability and in the lowering of the pKa value for His 68, which is involved in the intersubunit interaction of this dimeric enzyme.

Inactivation of cytosolic aspartate aminotransferase accompanying modification of Trp 48 by N-bromosuccinimide

Reaction of N-bromosuccinimide with pig heart cytosolic aspartate aminotransferase led to loss of the enzymatic activity. Chemical analysis indicated the modification of two tryptophan residues. At a low ratio of N-bromosuccinimide to enzyme, oxidation of Trp 122 occurred without affecting the enzymatic activity. Increase in the ratio resulted in the oxidation of Trp 48 with a concomitant decrease in enzyme activity. The modified enzyme did not react with substrates and their analogs. Trp 48 is not within the active site but in the hinge region linking the large domain of the enzyme to the small domain that shows dynamic movement upon binding substrates. The present result suggests that oxidation of Trp 48 may impair the structural integrity of the interdomain interface.

1H NMR studies of aspartate aminotransferase. Histidyl residues of cytosolic and mitochondrial isoenzymes

200 MHz proton nuclear magnetic resonance spectra were compared between the cytosolic (cAAT) and mitochondrial (mAAT) isoenzymes of aspartate aminotransferase (EC 2.6.1.1) from pig heart. The pattern of signal distribution in the whole spectral region differed considerably between the two isoenzymes, reflecting the difference in their amino acid sequences. A group of distinct signals were resolved at elevated temperatures (50 to 70 degrees C) in the low field region (9.0 to 7.5 ppm) of the spectra of both isoenzymes in the pyridoxal form. Most of these signals were also observable at 28 degrees C although some showed considerable line broadening. Among resonance lines in this spectral region, cAAT in the pyridoxal form showed four pH-titratable resonances with pKa of 9.54, 6.72, 5.69, and 4.87 at 28 degrees C. Variation in pK and line width of these signals indicated differences in the microenvironment of histidyl residues. On the other hand, mAAT showed six pH-titratable resonances with pKa of 6.73 (peak 2), 6.77 (peak 3), 6.07 (peak 4), 4.71 (peak 5), 4.54 (peak 6), and 4.33 (peak 7). Peaks 2, 3, and 4 were narrow and others were considerably broad. Thus, only part of the histidyl residues present in each isoenzyme (8 and 10 His/monomeric unit of cAAT and mAAT, respectively) appeared on the spectra as pH-titratable resonances. With both isoenzymes, chemical shift and pKa values of these signals obtained for the pyridoxal form were indistinguishable from those for the pyridoxamine form and the borohydride-reduced form. None of the observable signals were affected upon the interaction of cAAT with glutarate. By contrast, peaks 2 and 4 in mAAT showed subtle but distinct chemical shift changes upon complex formation with succinate, suggesting that these two resonances are due to histidyl residues located at the part of the enzyme molecule which undergoes a conformational change upon the interaction with the dicarboxylate.

Affinity chromatography of hepatic glutathione S-transferases on omega-aminoalkyl sepharose derivatives of glutathione

Rat liver glutathione S-transferases (RX: glutathione R-transferase, EC 2.5.1.18) were found to adsorb S-carbamidomethyl glutathione linked to Sepharose CL-4B via lysyl or aliphatic diamine spacers of various carbon chain lengths (-NH-(CH2)n-NH-, n = 2, 4, 5, 6, 8 and 10). Proteins were eluted specifically by reduced glutathione. The affinity of the enzymes for the adsorbent increased with increase in the carbon chain length of aliphatic diamine spacers used. Adsorbent having a free carboxyl group within the spacer moiety had high capacity and was specific for glutathione S-transferases. The transferases were specifically eluted from the column in high yield by low concentrations of glutathione. Enzymes purified by the lysyl spacer adsorbent were homogeneous in sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and contained most of the hepatic glutathione S-transferase isozymes in isoelectric focusing. Oxidized glutathione and S-methyl glutathione were equally effective as reduced glutathione in eluting glutathione S-transferases from the adsorbent, but gamma-glutamylcysteinylglycineamide or gamma-glutamylcysteinylglycine-1-methyl ester were not effective. These data suggested that the free carboxyl group of glycyl moiety of glutathione might also be important for the specific binding of the transferases to this adsorbent.