Characterization of glutamate-1-semialdehyde aminotransferase of Synechococcus. Steady-state kinetic analysis

Synechococcus glutamate-1-semialdehyde aminotransferase was expressed in large amounts in transformed cells of Escherichia coli. The resulting purified enzyme has an absorption spectrum characteristic of B6-containing enzymes and could be converted to the pyridoxal-phosphate form with excess dioxovalerate (O2Val), and back to the pyridoxamine-phosphate form with diaminovalerate (A2Val). Both enzyme forms are similarly active in the conversion of glutamate 1-semialdehyde (GSA) to 5-aminolevulinate (ALev), suggesting that A2Val and O2Val are intermediates. Initial rates of ALev synthesis at various fixed concentrations of GSA followed typical Michaelis-Menten kinetics (Km of GSA for the pyridoxamine-phosphate form of GSA aminotransferase = 12 microM, kcat = 0.23 s-1). In submicromolar amounts A2Val stimulates ALev synthesis, and in a series of concentrations with various fixed concentrations of GSA, gives a family of parallel lines in Lineweaver-Burk plots (Km for A2Val = 1.0 microM). On the other hand, O2Val gives competitive inhibition of the pyridoxamine-phosphate form of GSA-aminotransferase and mixed-type inhibition of the pyridoxal-phosphate form (Ki for O2Val = 1.4 mM). In general the kinetics were typical of ping-pong bi-bi mechanisms in which A2Val is the second substrate (intermediate) and O2Val is an alternative first substrate. There is no compelling evidence that O2Val accepts an amino group at its C5 position resulting in the direct formation of ALev, or the reverse involving the apparent formation of O2Val from ALev. These results are consistent with the hypothesis that the mechanism of GSA aminotransferase mimics that of other aminotransferases and that A2Val is the intermediate.

Determination of pyridoxamine 5'-phosphate in human blood plasma

Pyridoxamine 5'-phosphate in 18 microliters of human capillary blood plasma is determined by catalytic amplification using the apoenzyme of aspartate aminotransferase. Prior isolation from interfering substances is accomplished by employment of a cation exchange resin in batch operation. The procedure consists of the following stages. Stage I, denaturation of proteins. Trichloroacetic acid is used to precipitate plasma proteins and liberate any bound coenzyme. Dilute NaCl is added to expand the volume thus minimizing coenzyme entrapment in the precipitate. Stage II, isolation of the coenzyme. A sulfonated polystyrene ion exchange resin is used inside a centrifugal filter. Pyridoxamine 5'-phosphate in the supernatant from Stage I adsorbs to the resin. Pyridoxal 5'-phosphate, other organic phosphates, and Pi are removed by centrifugation. Rinsing with dilute NaBH4 destroys traces of pyridoxal 5'-phosphate and washes off residual inhibitors. Pyridoxamine 5'-phosphate is then desorbed with NaOH and Tris buffer and recovered by centrifugation. Stage III, reconstitution and assay. The desorbate from Stage II is incubated with excess apoenzyme. Specific activity of the reconstituted enzyme is measured. Interpolation from a standard curve relating enzyme specific activity and pyridoxamine 5'-phosphate concentration yields the plasma level of the cofactor. Approximately 3 h are required to carry out the procedure. Much of the coenzyme was found not be assayable if plasma was refrigerated overnight or if whole blood was left standing at room temperature for a few hours. The degradation was arrested with freezing at -80 degrees C. In a 13-day experiment involving a healthy subject, sharp rises of plasma pyridoxamine 5'-phosphate were found to occur in response to small doses of oral vitamin B6.(ABSTRACT TRUNCATED AT 250 WORDS)

Affinity labeling of pig kidney 3,4-dihydroxyphenylalanine (Dopa) decarboxylase with N-(bromoacetyl)pyridoxamine 5'-phosphate. Modification of an active-site cysteine

Pig kidney 3,4-dihydroxyphenylalanine (Dopa) decarboxylase is inactivated by N-(bromoacetyl)pyridoxamine 5'-phosphate (BAPMP) in a reaction which follows first-order kinetics at pH 7.5 and 25 degrees C. The concentration dependence of inactivation reveals saturation kinetics with an apparent Ki of 0.16 mM and kinact of 0.086 min-1 at saturating inhibitor concentration. Enzyme can be protected from inactivation by pyridoxal 5'-phosphate. Inactivation of enzyme by [14C]BAPMP proceeds with the incorporation of a stoichiometric amount of labeled inhibitor. Proteolytic digestions of the radioactively labeled enzyme followed by high-performance liquid chromatography allow the isolation of the modified peptide corresponding to the sequence Ala-Ala-Ser-Pro-Ala-Cys-Thr-Glu-Leu in which cysteine (Cys111) is the modified residue. The conservation of this residue and also of an extended region around it in all Dopa decarboxylases so far sequenced is underlined. The overall conclusion of these findings is that Cys111 may be at, or near, the pyridoxal-5'-phosphate binding site of pig kidney Dopa decarboxylase and plays a critical role in the catalytic function of the enzyme. Furthermore, fluorescence studies of BAPMP-modified apoenzyme provide useful information on the microenvironment of the affinity label at its binding site.

Separation and partial characterization of enzymes catalyzing delta-aminolevulinic acid formation in Synechocystis sp. PCC 6803

Formation of the universal tetrapyrrole precursor, delta-aminolevulinic acid (ALA), from glutamate via the five-carbon pathway requires three enzymes: glutamyl-tRNA synthetase, glutamyl-tRNA reductase, and glutamate-1-semialdehyde (GSA) aminotransferase. All three enzymes were separated from extracts of the unicellular cyanobacterium Synechocystis sp. PCC 6803, and two of them, glutamyl-tRNA synthetase and GSA aminotransferase, were partially characterized. After an initial high speed centrifugation and differentiatial ammonium sulfate fractionation of cell extract, the enzymes were separated by successive affinity chromatography on Reactive Blue 2-Sepharose and 2',5'-ADP-agarose. All three enzyme fractions were required to reconstitute ALA formation from glutamate. The apparent native molecular masses of glutamyl-tRNA synthetase and GSA aminotransferase were determined by gel filtration chromatography to be 63 and 98 kDa, respectively. Neither glutamyl-tRNA synthetase nor GSA aminotransferase activity was affected by hemin concentrations up to 10 and 30 microM, respectively, and neither activity was affected by protochlorophyllide concentrations up to 2 microM. GSA aminotransferase was inhibited 50% by 0.5 microM gabaculine. The gabaculine inhibition was reversible for up to 1 h after its addition, if the gabaculine was removed by gel filtration before the enzyme was incubated with substrate. However, irreversible inactivation was obtained by preincubating the enzyme at 30 degrees C either for several hours with gabaculine alone or for a few minutes with both gabaculine and GSA. Neither pyridoxal phosphate nor pyridoxamine phosphate significantly affected the activity of GSA aminotransferase at physiologically relevant concentrations, and neither of these compounds reactivated the gabaculine-inactivated enzyme. It was noted that the presence of pyridoxamine phosphate in the ALA assay mixture produced a false positive color reaction even in the absence of enzyme.

Kinetics and equilibria for the reactions of coenzymes with wild type and the Y70F mutant of Escherichia coli aspartate aminotransferase

The Y70F mutant of aspartate aminotransferase has reduced affinity for coenzymes compared to the wild type. The equilibrium dissociation constants for pyridoxamine phosphate (PMP) holoenzymes, KPMPdiss, were determined from the association and dissociation rate constants to be 1.3 nM and 30 nM for the wild type and mutant, respectively. This increase in KPMPdiss for Y70F is due to a 27-fold increase in the dissociation rate constant. Pyridoxal phosphate (PLP) association kinetics are complex, with three kinetic processes detectable for wild type and two for Y70F. A directly determined, accurate value of KPLPdiss for wild type enzyme has been difficult to obtain because of the low value of this constant. The values of KPLPdiss for the holoenzymes were determined indirectly through the measured values for KPMPdiss, glutamate-alpha-ketoglutarate half-reaction equilibrium constants, and the equilibrium constant for the transamination of PLP by glutamate catalyzed by Y70F. The values of KPLPdiss obtained by this procedure are 0.4 pM for wild type and 40 pM for Y70F. The increases in KPMPdiss and KPLPdiss for Y70F correspond to delta delta G values of 1.9 and 2.7 kcal/mol, respectively, and are directly attributed to the loss of the hydrogen bond from the phenolic hydroxyl group of Tyr70 to the coenzyme phosphate. The delta G for association of PLP with wild type enzyme is 4.7 kcal/mol more favorable than that for PMP.

The Escherichia coli hemL gene encodes glutamate 1-semialdehyde aminotransferase

delta-Aminolevulinic acid (ALA), the first committed precursor of porphyrin biosynthesis, is formed in Escherichia coli by the C5 pathway in a three-step, tRNA-dependent transformation from glutamate. The first two enzymes of this pathway, glutamyl-tRNA synthetase and Glu-tRNA reductase, are known in E. coli (J. Lapointe and D. Söll, J. Biol. Chem. 247:4966-4974, 1972; D. Jahn, U. Michelsen, and D. Söll, J. Biol. Chem. 266:2542-2548, 1991). Here we present the mapping and cloning of the gene for the third enzyme, glutamate 1-semialdehyde (GSA) aminotransferase, and an initial characterization of the purified enzyme. Ethylmethane sulfonate-induced mutants of E. coli AB354 which required ALA for growth were isolated by selection for respiration-defective strains resistant to the aminoglycoside antibiotic kanamycin. Two mutations were mapped to min 4 at a locus named hemL. Map positions and resulting phenotypes suggest that hemL may be identical with the earlier described porphyrin biosynthesis mutation popC. Complementation of the auxotrophic phenotype by wild-type DNA from the corresponding clone pLC4-43 of the Clarke-Carbon bank (L. Clarke and J. Carbon, Cell 9:91-99, 1976) allowed the isolation of the gene. Physical mapping showed that hemL mapped clockwise next to fhuB. The hemL gene product was overexpressed and purified to apparent homogeneity. The pure protein efficiently converted GSA to ALA. The reaction was stimulated by the addition of pyridoxal 5' -phosphate or pyridoxamine 5' -phosphate and inhibited by gabaculine or aminooxyacetic acid. The molecular mass of the purified GSA aminotransferase under denaturing conditions was 40,000 Da, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The enzyme has apparent native molecular mass of approximately 80,000 Da, as determined by rate zonal sedimentation on glycerol gradients and molecular sieving through Superose 12, which indicates a homodimeric alpha2, structure of the protein.

The role of His143 in the catalytic mechanism of Escherichia coli aspartate aminotransferase

In aspartate aminotransferase (AspAT), His143 is located within a hydrogen-bonding distance to Asp222 that forms a strong ion pair with the ring nitrogen of the coenzyme, pyridoxal 5'-phosphate (PLP) or pyridoxamine 5'-phosphate (PMP). His143 of Escherichia coli AspAT was replaced by Ala or Asn. The mutant enzyme H143A showed a slight increase in the maximum velocity of the overall transamination reaction between aspartate and 2-oxoglutarate, while H143N AspAT showed a decrease to 60% in the maximum rate of the overall reactions in both directions. In all of the half-transamination reactions with four substrates, aspartate, glutamate, oxalacetate, and 2-oxoglutarate, the catalytic competence as defined by kmax/Kd decreased by 3-18-fold upon replacing His143 by either Ala or Asn. The extent of the decrease varied from one substrate to another; it was largely contributed to by the decrease in affinities for all substrates. The equilibrium constants, [PMP-form] [keto acid]/[( PLP-form] [amino acid]), decreased by over 10-fold upon the mutations at position 143. Both H143A and H143N AspATs exhibited a considerably decreased affinity for 2-methylaspartate, an external-aldimine-forming substrate analogue, yet without appreciable alteration in the affinity for succinate and glutarate, which are non-aldimine-forming analogues. All these findings suggest that, although His143 is not essential for catalysis, it might assist the formation of enzyme-substrate complex.

The ionization states of the 5'-phosphate group in the various coenzyme forms bound to mitochondrial aspartate aminotransferase

We have carried out a Fourier transform infrared spectroscopic study of mitochondrial aspartate aminotransferase in the spectral region where phosphate monoesters give rise to absorption. Infrared spectra in the above-mentioned region are dominated by protein absorption. Yet, below 1020 cm-1 protein interferences are minor, permitting the detection of the band arising from the symmetric stretching of dianionic phosphate monoesters [T. Shimanouchi, M. Tsuboi, and Y. Kyogoku (1964) Adv. Chem. Phys. 8, 435-498]. The integrated intensity of this band in several enzyme forms (pyridoxal phosphate, pyridoxamine phosphate, and sodium borohydride-reduced, pyridoxyl phosphate form) does not change with pH in the range 5-9. This behavior contrasts that of free pyridoxal phosphate (PLP) and pyridoxamine phosphate (PMP) in solution, where the dependence of the same infrared band intensity with pH can be correlated to the known pK values for the 5'-phosphate ester in solution. The integrated intensity value of this infrared band for the PLP enzyme form before and after reduction with sodium borohydride is close to that given by free PLP at pH 8-9. These results are taken as evidence that in the active site of mitochondrial aspartate aminotransferase the 5'-phosphate group of PLP remains mostly dianionic even at a pH near 5. Thus, it is suggested that the chemical shift changes associated with pH titrations of various PLP forms reported in a previous 31P NMR study of this enzyme [M. E. Mattingly, J. R. Mattingly, and M. Martinez-Carrion (1982) J. Biol. Chem. 257, 8872] are due to the fact that the phosphorus chemical shift senses the O-P-O bond distortions induced by the ionization of a nearby residue. Since no chemical shift changes were observed in pH titrations of the PMP forms (lacking an ionizable internal aldimine) of this isozyme, the Schiff base between PLP and Lys-258 at the active site is the most likely candidate for the ionizing group influencing the phosphorus chemical shift in this enzyme.

Relationship between blood, liver and brain pyridoxal phosphate and pyridoxamine phosphate concentrations in mice

Plasma pyridoxal 5'-phosphate (PLP) concentrations are considered to be the most reliable single indicator of vitamin B-6 nutritional status and are thought to reflect tissue PLP and pyridoxamine 5'-phosphate (PMP) levels. We investigated the relationship between dietary level of pyridoxine hydrochloride (PN-HCl) and concentrations of PLP in blood and PLP and PMP in liver and brain of mice. Female heterogeneous stock mice, 60 to 90 d old, were fed purified diets containing 0.5, 1.0, 2.0, 3.0, 5.0, or 7.0 mg PN-HCl/kg diet for 5 wk. PLP and PMP concentrations were determined by a spectrophotometric apotryptophanase assay. PLP content of plasma, erythrocytes, whole blood, liver and brain and PMP levels in liver and brain were highly correlated with dietary level of PN-HCl (r values ranged from 0.81 to 0.94, n per correlation = 32 to 43). By using the entire range of dietary levels of PN-HCl, both plasma and erythrocyte PLP were found to be significantly correlated with tissue PLP and PMP concentrations. For any one dietary level, however, correlations between plasma or erythrocyte PLP and tissue PLP and PMP concentrations were low and nonsignificant. These results suggest that plasma PLP levels may be suitable to determine vitamin B-6 status of populations, but not to reliably predict tissue concentrations of PLP or PMP in individuals.

Binding of a photoaffinity analogue of pyridoxal to pyridoxal kinase

The binding of pyridoxal analogues to the structural domains of pyridoxal kinase was studied by fluorescence spectroscopy and chromatographic techniques. Two fragments of 24 and 16 kDa, arising from limited proteolysis of the native enzyme, were separated by ion-exchange chromatography and used for binding studies with pyridoxal oxime. Fluorometric titrations yielded dissociation constants of 6 and 12.4 MicroM for pyridoxal oxime bound to the native enzyme and 24-kDa fragment, respectively. 4-(4-Azido-2-nitrophenyl)-pyridoxamine, a new photolabeling reagent, binds irreversibly to the kinase with concomitant loss of catalytic activity. The modified kinase (2.1 mol label/mol dimer) yields two fragments upon limited proteolysis with chymotrypsin. The two fragments were separated by reverse-phase HPLC and SDS/polyacrylamide gel electrophoresis. Radiolabeled ligand was detected only in the 24-kDa fragment. It is postulated that the pyridoxal binding site is located in the 24-kDa structural domain.

Distribution of B-6 vitamers in human milk during a 24-h period after oral supplementation with different amounts of pyridoxine

The relative distribution of B-6 vitamers, separated by reverse-phase liquid chromatography, was examined in human milk during a 24-h period after supplementation with 2.5 or 15 mg pyridoxine hydrochloride. Consistently, pyridoxal (PL) was the predominate vitamer and the most responsive to vitamin B-6 intake. During 3-8 h after supplement ingestion, PL, pyridoxal phosphate, and pyridoxamine concentrations were significantly higher than at other times examined. In the first two periods after supplementation, PL as a percentage of total vitamin B-6 was slightly but significantly higher in milk from the group supplemented with 15 mg than from the group supplemented with 2.5 mg. With the exception of PL, the distribution of B-6 vitamers, expressed as percent of total vitamin B-6, was similar for the two supplemented groups at all times examined. Percentage PL of total vitamin B-6 in milk was approximately 25% lower in unsupplemented than in supplemented women.

Pyridoxal 5'-phosphate and pyridoxamine 5'-phosphate concentrations in blood and tissues of mice fed ethanol-containing liquid diets

The effects of chronic ethanol administration on vitamin B-6 metabolism were studied in female Long-Sleep (LS) and Short-Sleep (SS) mice. Animals were fed an ethanol containing liquid diet (AIN-76) for four weeks. Concentration of ethanol in the diet increased from 10 to 25% ethanol-derived calories (EDC) during weeks 1-3 and was maintained at 30% EDC for 1 additional week. We measured concentrations of pyridoxal 5'-phosphate (PLP) in plasma, erythrocytes and whole blood, and liver and brain PLP and pyridoxamine 5'-phosphate (PMP) in ethanol-fed and pair-fed control mice. Chronic ethanol administration significantly increased PMP and total (PLP + PMP) levels in the liver of SS mice. In LS mice ethanol feeding significantly decreased PMP and total (PLP + PMP) levels in the brain, but these values were still within normal limits. These results suggest that both control and ethanol-containing liquid diets are nutritionally adequate with respect to vitamin B-6, and that chronic ethanol administration does not adversely affect vitamin B-6 metabolism in adult mice.

Reaction of pyridoxamine-5-P with pyrroloquinoline quinone (coenzyme PQQ)

PQQ catalyzes the oxidation of pyridoxamine (PM) and pyridoxamine-5-P (PMP) to pyridoxal and pyridoxal-5-P (PLP) at 37 degrees C in the absence of micelles and proteins. The time course of conversion of PMP into PLP was monitored by absorption spectroscopy; a rate of 10 nmol PLP/min was determined. The product of the reaction was identified by TLC, HPLC and its ability to restore the catalytic activity of apoaspartate aminotransferase. The conversion of PMP into PLP by free PQQ is more efficient than reactions catalyzed by the enzymes plasma amine oxidase and pyridoxamine-5-P oxidase at optimal pH values.

Complexes of vitamin B6XX: equilibrium and mechanistic studies of the reaction of pyridoxal-5'-phosphate with pyridoxamine-5'-phosphate in the presence of copper(II)

The interaction of Cu(II) with pyridoxamine-5'-phosphate (PMP) and pyridoxal-5'-phosphate (PLP) was studied potentiometrically. The titration data were assessed by MINIQUAD program. Several protonated and nonprotonated complexes have been found to exist in solution. The reaction of PLP with Cu(II)-PMP has been studied kinetically, using the stopped-flow technique. Two rate steps have been observed. The first step has been attributed to the formation of a Schiff's base metal complex. The second step may be due to the formation of a ternary complex formation. A mechanism was suggested.

Identification of amino acid residues modified by pyridoxal 5'-phosphate in Escherichia coli glutamine synthetase

Chemical modification studies with pyridoxal 5'-phosphate have indicated that lysine(s) appear to be at or near the active site of Escherichia coli glutamine synthetase (Colanduoni, J., and Villafranca, J. J. (1985) J. Biol. Chem. 260, 15042-15050; Whitley, E. J., Jr., and Ginsburg, A. (1978) J. Biol. Chem. 253, 7017-7025). Enzyme samples were prepared that contained approximately 1, approximately 2, and approximately 3 pyridoxamine 5'-phosphate residues/50,000-Da monomer; the activity of each sample was 100, 25, and 14% of the activity of unmodified enzyme, respectively. Cyanogen bromide cleavage of each enzyme sample was performed, the peptides were separated by high performance liquid chromatography, and the peptides containing pyridoxamine 5'-phosphate were identified by their absorbance at 320 nm. These isolated peptides were analyzed for amino acid composition and sequenced. The N terminus of the protein (a serine residue) was modified by pyridoxal 5'-phosphate at a stoichiometry of approximately 1/50,000 Da and this modified enzyme had full catalytic activity. Beyond a stoichiometry of approximately 1, lysines 383 and 352 reacted with pyridoxal 5'-phosphate and each modification results in a partial loss of activity. When various combinations of substrates and substrate analogs (ADP/Pi or L-methionine-SR-sulfoximine phosphate/ADP) were used to protect the enzyme from modification, Lys-352 was protected from modification indicating that this residue is at the active site. Under all experimental conditions employed, Lys-47, which reacts with the ATP analog 5'-p-fluorosulfonylbenzoyl-adenosine does not react with pyridoxal 5'-phosphate.

Synthesis of adenosine-N6-methyl, propylthioether-N-pyridoxamine: an analog of a novel vitamin B6 tumor product

Studies on the time-course utilization of radiolabeled pyridoxine in rats with hepatomas led to the discovery of a novel vitamin B6 product. It is found in a spectrum of tumor lines but it is absent or occurs minimally in normal tissues. Hepatomas incorporate up to 20-30% of labeled pyridoxine into the novel product. Its structure was tentatively identified as adenosine-N6-methyl, propylthioether-N-pyridoximine-5'-PO4. However, results of tests on the incorporation of labeled precursors into the novel product by 3B3 hybridoma or HL-60 cells support an N6-diethylthioether bridge linking the adenosyl and pyridoxyl moieties. The synthesis of adenosine-N6-methyl, propylthioether-N-pyridoxamine is reported in this paper. The mass spectrum of this analog is similar to that of the tumor product as seen by its fragmentation in further support of the structure of the tumor product. Whether the latter may be part of tumor RNA is questionable. RNA was isolated for 3B3 or HL-60 cells after incubation with tritiated or 14C-pyridoxine using SDS-phenol repeated extractions in the presence of RNase inhibitors. Centrifugation of cRNA on 5-20% linear sucrose density gradients showed practically all the label at the top of the gradient. RNase treatment resulted in a labeled product which coeluted with the tumor product on reverse phase paired-ion HPLC and chromatographed as dinucleotide on paper. These results suggest that the novel tumor product may occur as a short oligonucleotide.

Changes in pyridoxal phosphate and pyridoxamine phosphate in blood, liver and brain in the pregnant mouse

A decrease in plasma pyridoxal-5'-phosphate (PLP) occurs during pregnancy in humans and experimental animals for reasons that are not known. To determine if mice also develop decreased plasma PLP concentrations during pregnancy, and if plasma PLP levels in pregnancy reflect tissue levels of PLP and pyridoxamine-5'-phosphate (PMP), we measured PLP concentrations in plasma, erythrocytes and whole blood, and liver and brain PLP and PMP in control and pregnant mice. Mice were fed a nonpurified diet containing 8.13 mg pyridoxine-HCl/kg. The PLP analyses were performed in our newly developed apotryptophanase method in which the substrate S-benzyl-L-cysteine is hydrolyzed to benzyl mercaptan, reacted with Ellman's reagent and measured spectrophotometrically. During pregnancy, plasma PLP levels decreased 50% below control levels, but erythrocyte and whole blood PLP levels increased 2.9- and 1.6-fold, respectively. Liver PLP and PMP decreased 25%, in parallel with plasma PLP, but brain PLP and PMP concentrations were unchanged during pregnancy. These results suggest that metabolism or utilization of vitamin B-6 is altered in pregnancy, and that plasma PLP concentrations alone may not be a good indicator of nutritional status in pregnancy.

L-Histidinol phosphate aminotransferase from Salmonella typhimurium. Kinetic behavior and sequence at the pyridoxal-P binding site

A coupled assay with alpha-hydroxyglutarate dehydrogenase was used to analyze the kinetic behavior of histidinol phosphate aminotransferase from Salmonella typhymurium. Data obtained from studies of initial velocity, inhibition by products or substrate analogues, isotope exchange rates, and the determination of the equilibrium constant were consistent only with a Ping-Pong Bi Bi mechanism. Variations in inhibition patterns by different substrate analogues indicate that the microenvironment about the pyridoxal phosphate and the pyridoxamine phosphate forms of histidinol phosphate amino-transferase are different, and favor the presence of one active site with partially overlapping substrate-binding subsites for these 2 forms of the enzyme. Histidinol phosphate aminotransferase also catalyzes decomposition of beta-chloro-L-alanine to pyruvate, NH3 and Cl-; no transamination of this substrate occurs and inactivation of the enzyme accompanies this reaction. After reduction of histidinol-P aminotransferase with [3H]NaBH4, carboxymethylation, and tryptic digestion, one major radioactive peptide absorbing at 325 nm was isolated. Its primary structure was determined to be TLSK*AFALAGLR, where K* is the P-pyridoxyllysine residue. Although this peptide is only 30-40% homologous with the corresponding segment reported for other transaminases, all of these peptides are similar in placement of an hydroxyamino acid residue three residues upstream from the lysine residue, and in the cluster of hydrophobic amino acid residues immediately following the lysine residue.

[Aspartate aminotransferase and glutamate dehydrogenase activity in the rat brain during infrared laser exposure]

In experiments in vivo it was shown that upon low-intensity infrared irradiation changes in the activity of main enzymes of glutamic acid metabolism are a function of time of exposure and flux density.

[The content of pyridoxal coenzymes in the brain and liver of rats exposed to a single irradiation with x-rays]

A study was made of the effect of a single exposure of rats to 0.4 Gy X-radiation on the content of pyridoxal phosphate and pyridoxamine phosphate in gray and white brain substances and liver. At the same time changes were noted in the activity of pyridoxal kinase in the tissues under study.

Fluorescence resonance energy transfer studies on the proximity between lysine-107 and cysteine-239 in rabbit muscle aldolase

Spatial relationships between Lys-107, which binds the C-6 phosphate group of the substrate, and fast-reacting Cys-239, located outside the active site of rabbit muscle aldolase, were studied by means of resonance energy transfer. The Lys-107 residue was covalently linked to pyridoxal phosphate (fluorescence donor) and the Cys-239 residue was modified by 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (fluorescence acceptor). The energy transfer between donor and acceptor has been demonstrated. The steady-state and the lifetime measurements indicate that in solution the distance between Lys-107 and Cys-239 in the aldolase molecule is 12.4 A assuming chi 2 = 2/3.

Correlation of polarized absorption spectroscopic and X-ray diffraction studies of crystalline cytosolic aspartate aminotransferase of pig hearts

Absorption spectra of large, well-formed crystals of cytosolic aspartate aminotransferase have been recorded using plane polarized light. Making use of measurements of crystal thickness we have calculated extinction coefficients with the electric vector of the light parallel to both the a and c axes of the crystals of the enzyme in space group P2(1)2(1)2(1). The spectra have been resolved into components with lognormal distribution curves and the resulting integrated intensities have been used to calculate the c/a polarization ratios for the absorption bands of the bound co-enzyme pyridoxal 5'-phosphate. From the polarization ratio and the co-ordinates of the co-enzyme ring atoms, provided by X-ray crystallography, we have assigned principal molecular directions of the transition dipole moment within the plane of the co-enzyme ring. Of two possible orientations, only one predicts the correct crystal extinction coefficients for the 436 nm band. In this orientation, when viewed from the B face of the ring (i.e. looking into the active site of the enzyme), the transition moment is related to the N-1-C-4 axis of the ring by counterclockwise rotation by 27 degrees. A tentative assignment of the principal molecular directions of the transition moment has also been made for the 368 nm band of the high pH form of the enzyme. In each case, the plane of the co-enzyme ring was located from the atomic co-ordinates of the ring atoms and of those atoms attached directly to the ring. The projection of the N-1 to C-4 axis on to this plane was used to evaluate the orientation of the transition moment, which was presumed to lie precisely within the plane of the ring. We have tilted this plane systematically to evaluate the error in transition moment direction resulting from uncertainties in the atomic co-ordinates. When 2-methylaspartate is diffused into the crystals if forms a Schiff base with the co-enzyme in which the ring has tilted about 32 degrees from its original position and the polarization ratio of the 436 nm band drops from 1.6 in the free enzyme to about 0.38. On the assumption that the orientation of the transition moment within the co-enzyme does not change during this rotation, this value of the polarization ratio is within experimental error of that predicted from X-ray structures on the two forms. The 2-methylaspartate binds only to subunit 1.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and characterization of aspartate aminotransferase from the thermoacidophilic archaebacterium Sulfolobus solfataricus

Aspartate aminotransferase from the archaebacterium Sulfolobus solfataricus, a thermoacidophilic organism isolated from an acidic hot spring (optimal growth conditions: 87 degrees C, pH 3.5) was purified to homogeneity. The enzyme is a dimer (Mr subunit = 53,000) showing microheterogeneity when submitted to chromatofocusing and/or isoelectric focusing analysis (two main bands having pI = 6.8 and 6.3 were observed). The N-terminal sequence (22 residues) does not show any homology with any stretch of known sequence of aspartate aminotransferases from animal and bacterial sources. The apoenzyme can be reconstituted with pyridoxamine 5'-phosphate and/or pyridoxal 5'-phosphate, each subunit binding 1 mol of coenzyme. The absorption maxima of the pyridoxamine and pyridoxal form are centered at 325 and 335 nm, respectively; the shape of the pyridoxal form band does not change with pH. The enzyme has an optimum temperature higher than 95 degrees C, and at 100 degrees C shows a half-inactivation time of 2 h. The above properties seem to be unique even for enzymes from extreme thermophiles (Daniel, R. M. (1986) in Protein Structure, Folding, and Design (Oxender, D. L., ed) pp. 291-296, Alan R. Liss, Inc., New York) and lead to the conclusion that aspartate aminotransferase from S. solfataricus is one of the most thermophilic and thermostable enzymes so far known.