Formation of delta-Aminolevulinic Acid from Glutamic Acid in Algal Extracts : Separation into an RNA and Three Required Enzyme Components by Serial Affinity Chromatography

Extracts from plant chloroplasts and algae catalyze the conversion of glutamate to δ-aminolevulinic acid (ALA) in the first committed step of the tetrapyrrole biosynthetic pathway leading to chlorophylls, hemes, and bilins. The conversion requires ATP, Mg²⁺, and NADPH as cofactors. Soluble extracts from Chlorella vulgaris have now been resolved into four macromolecular fractions, all of which are required to reconstitute activity. One fraction contains a low molecular weight RNA which can be separated from the protein components in an active high-speed supernatant by treatment with 1 molar NaCl followed by precipitation of the proteins with (NH₄)₂SO₄ at 70% saturation. The proteins recovered from the (NH₄)₂SO₄ precipitate are reactivated by addition of a fraction containing tRNAs isolated from Chlorella by phenol-chloroform extraction and DEAE cellulose chromatography. Three required protein fractions were resolved from the RNA-depleted (NH₄)₂SO₄ precipitate by serial affinity chromatography on Reactive Blue 2-Sepharose and 2′,5′-ADP-agarose. Glycerol was found to stabilize the enzyme activity during the separation process. The majority of the glutamate:tRNA ligase activity was associated with the fraction which was retained by Blue-Sepharose and not retained by ADP-agarose, in agreement with the reported properties of the affinity ligands. The active material in the fraction not retained by Blue-Sepharose eluted as a single component on gel filtration chromatography, with an apparent molecular weight of 67,000. The active component in the RNA fraction also eluted as a single component on gel filtration chromatography.     

Biosynthesis of the Tetrapyrrole Pigment Precursor, delta-Aminolevulinic Acid, from Glutamate

δ-Aminolevulinic acid (ALA), the common biosynthetic precursor of hemes, chlorophylls, and bilins, is synthesized by two distinct routes. Among phototrophic species, purple nonsulfur bacteria form ALA by condensation of glycine with succinyl-CoA, catalyzed by ALA synthase, in a reaction identical to that occurring in the mitochondria of animals, yeast, and fungi. Most or all other phototrophic species form ALA exclusively from the intact carbon skeleton of glutamic acid in a reaction sequence that begins with activation of the α-carboxyl group of glutamate by an ATP-dependent ligation to tRNA^Glu, catalyzed by glutamyl-tRNA synthetase. Glutamyl-tRNA is the substrate for a pyridine nucleotide-dependent dehydrogenase reaction whose product is glutamate-1-semialdehyde or a similar reduced compound. Glutamate-1-semialdehyde is then transaminated to form ALA. Regulation of ALA formation from glutamate is exerted at the dehydrogenase step through end product feedback inhibition and induction/repression. In some species, end product inhibition of the glutamyl-tRNA synthetase step and developmental regulation of tRNA^Glu level may also occur.     

The tRNA Required for in Vitro delta-Aminolevulinic Acid Formation from Glutamate in Synechocystis Extracts : Determination of Activity in a Synechocystis in Vitro Protein Synthesizing System

RNA is an essential component for the enzymic conversion of glutamate to δ-aminolevulinic acid (ALA), the universal heme and chlorophyll precursor, as carried out in plants, algae, and some bacteria. The RNA required in this process was reported to bear a close structural resemblance to tRNA^Glu(UUC), and it can be isolated by affinity chromatography directed against the UUC anticodon. Affinity-purified tRNA^Glu(UUC) from the cyanobacterium Synechocystis sp. PCC 6803 was resolved into two major subfractions by reverse-phase HPLC. Only one of these was effectively charged with glutamate in enzyme extract from Synechocystis, but both were charged in Chlorella vulgaris enzyme extract. When charged with glutamate, the two glutamyl-tRNA^Glu(UUC) species produced were equally effective in supporting both ALA formation and protein synthesis in vitro, as measured by label transfer from [³H]glutamyl-tRNA to ALA and protein. These results indicate that one of the two tRNA^Glu(UUC) species is used by Synechocystis for both protein biosynthesis and ALA formation. Both of the tRNA^Glu(UUC) subfractions from Synechocystis supported ALA formation in Chlorella enzyme extract. Escherichia coli tRNA^Glu(UUC) was charged with glutamate, but did not support ALA formation in Synechocystis enzyme extract. Unfractionated tRNA from Chlorella, pea, and E. coli, having been charged with [³H] glutamate by Chlorella enzyme extract and then re-isolated, were all able to transfer label to proteins in the Synechocystis enzyme extract.     

Purification, Characterization, and Fractionation of the delta-Aminolevulinic Acid Synthesizing Enzymes from Light-Grown Chlamydomonas reinhardtii Cells

The synthesis of δ-aminolevulinate from glutamate by Chlamydomonas reinhardtii membrane-free cell homogenates requires Mg²⁺, ATP, and NADPH as cofactors. The pH optimum is about 8.3. When analyzed by a Fractogel TSK gel filtration column the δ-aminolevulinate synthesizing enzymes, including glutamate-1-semialdehyde aminotransferase, elute with an apparent molecular weight of about 45,000. The enzymes obtained from the gel filtration column were separated into three fractions by affinity column chromatography. One fraction binds to heme-Sepharose, one to Blue Sepharose, while the enzyme converting the putative glutamate-1-semialdehyde to δ-aminolevulinic acid is retained by neither column. All three fractions are necessary for the conversion of glutamate to δ-aminolevulinate. The δ-aminolevulinate synthesizing enzymes from Chlamydomonas are sensitive to inhibition by heme but not sensitive to inhibition by protoporphyrin.     

Phage-Displayed Libraries for the Selection of Optimal Affinity Peptides for Protein Purification Using Ni-Nitrilotriacetic Acid Chromatography

A random hexapeptide library, cloned in bacteriophage, was used to select affinity peptides using nickel-nitrilotriacetic acid (Ni-NTA) columns. The screening protocol was successful by isolating peptides sharing common features and, in most cases, common amino acid sequences were isolated (e.g. WHHHPH, AQHHHH). Ni-NTA chromatography of the fusion phage of the selected peptides exhibited a more homogeneous elution behavior (i.e. elution in one peak) than the most commonly used His₆peptide (elution in multiple peaks).     

The Affinity Purification and Characterization of a Dehydrogenase from Aspergillus parasiticus Involved in Aflatoxin B1, Biosynthesis

Abstract

A two step scheme has been developed for the purification of a dehydrogenase from mycelia of 84 hours old Aspergillus parasiticus (1-11-105 Wh 1), which catalyzes the conversion of norsolorinic acid (NA) to averantin (AVN). The dehydrogenase was purified from cell-free extracts using reactive green 19-agarose and norsolorinic acid-agarose affinity chromatography. The latter affinity matrix was synthesised by attaching norsolorinic acid to ω-aminohexylagarose. The purified protein was shown to be homogenous on non-denaturing polyacrylamide gel electrophoresis. A final purification of 215-fold was achieved. Results of gel filtration chromatography indicated the approximate molecular mass of the native protein to be 140 000 daltons. The isoelectric point of the protein was about 5.5 as determined by chromatofocusing. The reaction catalyzed by the dehydrogenase was optimum at pH 8.5 and between 25[ddot] to 35[ddot]C. The Km of the enzyme for NA and NADPH was determined to be 3.45 μM and 103 μM respectively.     

Purification of the Major Soybean Leaf Acid Phosphatase That Is Increased by Seed-Pod Removal

Fruit removal for 5 weeks after flowering increased acid phosphatase activity 10-fold in soybean (Glycine max L. Merr. Var Hobbit) leaves compared with normal seed-pod-bearing plants. The major acid phosphatase activity in leaves was purified over 2700-fold, yielding a single polypeptide of 51 kD with a specific activity of 1353 units/mg protein using p-nitrophenylphosphate as the substrate. Isoelectric focusing demonstrated that the purified protein co-migrated with a majority of the activity that increased in leaves following seed-pod removal. Immunoblot analysis demonstrated that at least part of the increased activity was due to an increased abundance of the phosphatase protein. In situ enzyme activity staining localized most of the total phosphatase activity to vascular tissues, the leaf paraveinal mesophyll cell layer, and the lower epidermis. This distribution and the response to seed-pod removal paralleled previous results for soybean vegetative storage protein (VSP) [alpha] and [beta]. However, in a native polyacrylamide gel the VSP detected by immunological staining of electrophoretically transferred protein did not migrate with the majority of the phosphatase activity. Fractionation of crude leaf protein on concanavalin A-Sepharose yielded a fraction containing 97% of the total VSP but only 0.1% of the total acid phosphatase activity.     

Biosynthesis of Tetrapyrrole Pigment Precursors : Formation and Utilization of Glutamyl-tRNA for delta-Aminolevulinic Acid Synthesis by Isolated Enzyme Fractions from Chlorella Vulgaris

The universal tetrapyrrole precursor δ-aminolevulinic acid (ALA) is formed from glutamate (Glu) in algae and higher plants. In the postulated reaction sequence, Glu-tRNA is produced by a Glu-tRNA synthetase, and the product serves as a substrate for a reduction step catalyzed by a pyridine nucleotide-requiring Glu-tRNA dehydrogenase. The reduced intermediate is then converted into ALA by a transaminase. An RNA and three enzyme fractions required for ALA formation from Glu have been isolated from soluble Chlorella extracts. The recombined fractions catalyzed ALA production from Glu or Glu-tRNA. The fraction containing the synthetase produced Glu-tRNA from Glu and tRNA in the presence of ATP and Mg²⁺. The isolated product of this reaction served as substrate for ALA production by the partially reconstituted enzyme system lacking the synthetase fraction and incapable of producing ALA from Glu. The production of ALA from Glu-tRNA by this partially reconstituted system did not require free Glu or ATP, and was not affected by added ATP. These results show that (a) free Glu-tRNA is an intermediate in the formation of ALA from Glu, (b) ATP is required only in the first step of the reaction sequence, and NADPH only in a later step, (c) Glu-tRNA production is the essential reaction catalyzed by one of the enzyme fractions, (d) this enzyme fraction is active in the absence of the other enzymes and is not required for activity of the others. The specific Glu-tRNA synthetase required for ALA formation has an approximate molecular weight of 73,000 ± 5,000 as determined by Sephadex G-100 gel filtration and native polyacrylamide gel electrophoresis. Other Glu-tRNA synthetases were present in the cell extracts but were ineffective in the the ALA-forming process.     

Purification of the Fusicoccin-Binding Protein from Oat Root Plasma Membrane by Affinity Chromatography with Biotinylated Fusicoccin

Fusicoccin (FC), a fungal phytotoxin, evokes a number of physiological responses after binding to the FC-binding protein (FCBP). For characterization of this plasma membrane protein and elucidation of the signal transduction pathway, we purified active FCBP from oat (Avena sativa L. cv Valiant) root plasma membranes using avidin-biotin affinity chromatography. For the binding of FCBP to immobilized avidin, a bifunctional FC derivative (FC-biotin, FCBio) was synthesized. FCBio retained high binding affinity for the FCBP (KD = 70 nM), it elicited a biological response comparable to FC, and it was bound by avidin. The purification of the FCBP involved three important steps. First, FCBio was bound to the FCBP in purified plasma membrane vesicles. Next, plasma membrane proteins were solubilized in detergent, and part of the solubilized proteins was precipitated by decreasing the detergent concentration below the critical micelle concentration. The FCBP remained in the soluble fraction, and this fraction was loaded on a "low-affinity" avidin column. Proteins, bound through a biotin moiety to the column, were specifically eluted with excess biotin. This resulted in fractions active in [3H]FC binding and two bands on sodium dodecyl sulfate-polyacrylamide gel electrophoresis of 31 and 30 kD. The nonhydrophobic behavior of the FCBP was confirmed by means of phase separation with Triton X-114, wherein the FCBP migrated to the hydrophilic phase. Purification of the FCBP in active form using this novel affinity technique opens the possibility to study other features of the FCBP necessary for inducing physiological responses in plant cells.     

Sph3 Is a Glycoside Hydrolase Required for the Biosynthesis of Galactosaminogalactan in Aspergillus fumigatus

Aspergillus fumigatus is the most virulent species within the Aspergillus genus and causes invasive infections with high mortality rates. The exopolysaccharide galactosaminogalactan (GAG) contributes to the virulence of A. fumigatus. A co-regulated five-gene cluster has been identified and proposed to encode the proteins required for GAG biosynthesis. One of these genes, sph3, is predicted to encode a protein belonging to the spherulin 4 family, a protein family with no known function. Construction of an sph3-deficient mutant demonstrated that the gene is necessary for GAG production. To determine the role of Sph3 in GAG biosynthesis, we determined the structure of Aspergillus clavatus Sph3 to 1.25 Å. The structure revealed a (β/α)₈ fold, with similarities to glycoside hydrolase families 18, 27, and 84. Recombinant Sph3 displayed hydrolytic activity against both purified and cell wall-associated GAG. Structural and sequence alignments identified three conserved acidic residues, Asp-166, Glu-167, and Glu-222, that are located within the putative active site groove. In vitro and in vivo mutagenesis analysis demonstrated that all three residues are important for activity. Variants of Asp-166 yielded the greatest decrease in activity suggesting a role in catalysis. This work shows that Sph3 is a glycoside hydrolase essential for GAG production and defines a new glycoside hydrolase family, GH135.     

Purification of a Lipolytic Acyl-hydrolase from Phaseolus vulgaris Leaves by Affinity Chromatography on Palmitoylated Gauze and Its Properties

A lipolytic acyl-hydrolase was purified about 220-fold from the homogenate of the leaves of Phaseolus vulgaris L. cv. Kurodane-kinugasa by acetone precipitation, affinity chromatography on a palmitoylated gauze column and isoelectric focusing. The purified enzyme showed a single protein band by polyacrylamide gel disc electrophoresis. The enzyme had an isoelectric point of 4.4 and a molecular weight of about 90,000. It had pH optima of 5.5 and 6.5, and Km values of 0.24 and 0.53 mm for monogalactosyldiacylglycerol and phosphatidylcholine, respectively. The pH dependences were changed by Triton X–100. No separation of these two hydrolyzing activities were achieved, and the ratio of the specific activity of galactolipase to that of phospholipase (about 3/1) remained constant throughout the purification procedures. Both the activities changed in parallel with each other by the addition of reagents and by heat treatment. The enzyme clearly catalyzed the deacylation of the several classes of glyco- and phospholipids. These results suggest that a single enzyme is responsible for both the activities.     

Purification of Phosphatidylinositol Synthetase from Rat Brain by CDP-Diacylglycerol Affinity Chromatography and Properties of the Purified Enzyme

A purification procedure for rat brain phosphatidylinositol synthetase (PI synthetase; CDP-1,2-diacyl-sn-glycerol:myo-inositol 3-phosphatidyltransferase; EC 2.7.8.11) is described. The enzyme was purified 200-250-fold from the homogenate by solubilization with Triton X-100 from microsomal membranes and affinity chromatography on CDP-diacylglycerol-Sepharose. Elution of enzyme activity required the presence of Triton X-100, CDP-diacylglycerol, and either phosphatidylcholine or asolectin. The product that was obtained in 5-10% yield from whole brain and in 70% yield from the microsomal fraction contained three protein bands as determined by sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The final preparation contained levels of CDP-diacylglycerol hydrolase and CDP-diacylglycerol: sn-glycero-3-phosphate 3-phosphatidyltransferase activities that were less than 1% of PI synthetase activity. The purified enzyme displayed a pH optimum of 8.5-9.0, required either Mg2+ or Mn2+ and exhibited a Km of 4.6 mM for myo-inositol.     

Complementable Fraction and Complemented Enzyme of Mutant Ml5 from Escherichia Coli: Partial Purification by Affinity Chromatography

The technique of affinity chromatography has been used in the partial purification of complementable fractions and complemented enzyme of β-galactosidase from Escherichia coli mutant M15. The crude extract of mutant ML5 was incubated with fragment CM-B. The complemented enzyme and complementable fractions were passed through a small column of p-amino-phenyl-β-D-thiogalactoside to which inhibitors had been covalently attached. A high percentage of the nonspecific protein passed directly through the affinity column while the specific enzymatic protein remained bound to the gel. Phosphate buffer with NaCl was used to elute the complementable fractions from the column. Sodium borate buffer was used to elute the bound complemented enzyme from the affinity support. The results of this study show that 100% of the complemented enzyme was bound to the column. The partially purified enzyme had the same position in disc gel electrophoresis as β-galactosidase from E. coli.     

Activation of Protein Kinase C by Phorbol Esters and Arachidonic Acid Required for the Optimal Potentiation of Glutamate Exocytosis

Abstract: The effects of arachidonic acid and phorbol esters in the Ca²⁺-dependent release of glutamate evoked by 4-aminopyridine (4-AP) in rat cerebrocortical synaptosomes were studied. In the absence of arachidonic acid, high concentrations (500 n M ) of 4β-phorbol dibutyrate (4β-PDBu) were required to enhance the release of glutamate. However, in the presence of arachidonic acid, low concentrations of 4β-PDBu (1–50 n M ) were effective in potentiating glutamate exocytosis. This potentiation of glutamate release by phorbol esters was not observed with the methyl ester of arachidonic acid, which does not activate protein kinase C. Moreover, pretreatment of synaptosomes with the protein kinase inhibitor staurosporine also prevented the stimulatory effect by arachidonic acid and phorbol esters. These results suggest that the activation of protein kinase C by both arachidonic acid and phorbol esters may play a role in the potentiation of glutamate exocytosis.