Purification of a prenyltransferase that elongates cis-polyisoprene rubber from the latex of Hevea brasiliensis

We have purified "rubber transferase" from latex of the commercial rubber tree Hevea brasiliensis and find that it is a dimer with a monomeric molecular mass of 38,000 Da, requires Mg2+, and is stabilized by thiols in agreement with studies of a partially purified preparation previously described (Archer, B. L., and Cockbain, E. G. (1969) Methods Enzymol. 15, 476-480). Greater than 90% of the [1-14C]isopentenyl pyrophosphate which is incorporated into deproteinated rubber particles by the purified prenyltransferase is added to high molecular mass polyisoprene (greater than 20,000 Da). Purified prenyltransferase and deproteinated rubber particles reconstitute 40-60% of the biosynthetic activity of whole latex in samples matched for rubber content. Incorporation is linear with added rubber particles up to at least 10 mg/ml rubber or 20 microM rubber molecules (based on a number average molecular mass of 500,000 Da). Prenyltransferase concentrations estimated in whole latex (0.37% or 160 nM) are sufficient to saturate all elongation sites in whole latex, and addition of purified prenyltransferase does not increase [1-14C]isopentenyl pyrophosphate incorporation. Deproteinated rubber particles can be titrated with the pure enzyme (Kd = 9 nM) demonstrating that the fraction of rubber molecules available for addition is low (approximately 0.01%). An estimated 7,000 isoprene units are added per complex at a rate of 1/s in a typical assay. Hevea prenyltransferase catalyzes the formation of cis-isoprene in the presence of rubber particles. However, in the absence of rubber particles and in the presence of dimethylallyl pyrophosphate, the purified prenyltransferase catalyzes the formation of geranyl pyrophosphate and all trans-farnesyl pyrophosphate as demonstrated by thin layer chromatography, gas chromatography, and molecular exclusion chromatography.

Direct observation of the enzyme-intermediate complex of 5-enolpyruvylshikimate-3-phosphate synthase by 13C NMR spectroscopy

The interaction of 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase, 4,5-dideoxyshikimate 3-phosphate (ddS3P), and [2-13C]-and [3-13C]phosphoenolpyruvate (PEP) has been examined by 13C NMR spectroscopy. Although no resonances due to a dead-end intermediate complex could be detected, an enzyme active site specific formation of pyruvate was observed. The interaction of EPSP synthase with shikimate 3-phosphate (S3P) and [2-13C]- or [3-13C]PEP has been examined by 13C NMR spectroscopy. With [2-13C]PEP, in addition to the resonances due to [2-13C]PEP and [8-13C]EPSP, new resonances appeared at 164.8, 110.9, and 107.2 ppm. The resonance at 164.8 ppm has been assigned to enzyme-bound EPSP. The resonance at 110.9 ppm has been assigned to C-8 of an enzyme-free tetrahedral intermediate of the sort originally proposed by Levin and Sprinson [Levin, J. G., & Sprinson, D. B. (1964) J. Biol. Chem. 239, 1142-1150] and recently independently observed by Anderson et al. [Anderson, K. S., Sikorski, J. A., Benesi, A. J., & Johnson, K. A. (1988) J. Am. Chem. Soc. 110, 6577-6579]. The resonance at 107.2 ppm has been assigned to an enzyme-bound intermediate whose structure is closely related to that of the tetrahedral intermediate. With [3-13C]PEP, new resonances appeared at 88.9, 26.2, 25.5, and 24.5 ppm. The resonance at 88.9 ppm has been assigned to enzyme-bound EPSP. The resonance at 26.2 ppm, which was found to correlate with 1.48 ppm by isotope-edited multiple quantum coherence 1H NMR spectroscopy, has been assigned to the methyl group 4-hydroxy-4-methylketoglutarate.(ABSTRACT TRUNCATED AT 250 WORDS)

Essential role of T4 phage deoxycytidylate hydroxymethylase in a multienzyme complex for deoxyribonucleotide synthesis

Several enzymes of deoxyribonucleoside triphosphate (dNTP) biosynthesis interact in T4 phage-infected Escherichia coli to form a multienzyme aggregate, the T4 dNTP synthetase complex. To test the specificity of enzyme interactions seen in vitro with this complex, we analyzed bacteria infected with four T4 gene 42 amber mutants, which specify truncated forms of dCMP hydroxymethylase, one of the constituent enzymes in the complex. Mutants that specify a nearly full length gene 42 product can form an intact complex, as revealed by two criteria: kinetic coupling among constituent enzymes in crude extracts of infected bacteria, and co-elution of enzyme activities from a gel filtration column. By these criteria, mutations that specify truncated proteins less than half the size of the full length protein cause disruption of the complex. These findings suggest that an enzymatically inactive form of dCMP hydroxymethylase can contribute toward assembly of an intact complex, so long as the incomplete protein is of sufficient size to fold normally, allowing interaction with other proteins in the complex.

Partial purification and properties of geranyl pyrophosphate synthase from Lithospermum erythrorhizon cell cultures

A prenyltransferase (EC 2.5.1.1) was isolated from cell cultures of Lithospermum erythrorhizon. The enzyme was purified 92-fold by subsequent chromatography on DEAE-Sephacel, phenyl-Sepharose, and Sephadex G-150. Geranyl pyrophosphate (GPP) was the sole product of the enzymatic reaction with dimethylallyl pyrophosphate and isopentenyl pyrophosphate as the substrates. The enzyme showed a molecular weight of 73,000, estimated by gel chromatography on Sephadex G-150, and an isoelectric point at pH 4.95, determined by analytical isoelectric focusing. It had an absolute requirement for a divalent cation with Mg2+ and Mn2+ being most effective. The enzyme was soluble rather than membrane-bound. The physiological role of this prenyltransferase probably is to supply GPP for the biosynthesis of shikonin. It is the first chain-length specific geranyl pyrophosphate synthase reported from eukaryotic cells.

Some evidence of the enzymatic conversion of bovine suppressor phosphoseryl-tRNA to selenocysteyl-tRNA

In order to clarify the mechanisms of selenocysteine incorporation into glutathione peroxidase, some evidence to show the in vitro conversion of phosphoseryl-tRNA to selenocysteyl-tRNA is reported. [3H]Phosphoseryl-tRNA was incubated in a reaction mixture composed of SeO2, glutathione and NADPH in the presence of selenium-transferase partially purified. Analyses of amino acids on the product tRNA showed that a part (4%) of [3H]phosphoseryl-tRNA was changed to [3H]selenocysteyl-tRNA. The conversion from seryl-tRNAsu or major seryl-tRNAIGA was not found. Selenium-transferase was essential for the conversion. [3H]Selenocysteine, liberated from the tRNA, was modified with iodoacetic acid. The product was confirmed to be carboxymethyl-selenocysteine by two-dimensional TLC. Selenocysteyl-tRNAsu should be used to synthesize glutathione peroxidase by co-translational mechanisms.

N-acetylglucosaminyltransferase III, IV and V activities in Novikoff ascites tumour cells, mouse lymphoma cells and hen oviduct. Application of a sensitive and specific assay by use of high-performance liquid chromatography

A specific and fast method for the determination of N-acetylglucosaminyltransferase III, IV and V activity in one assay is described. The method is based on the separation by HPLC of the three transferase products formed from the common acceptor oligosaccharide substrate GlcNAc beta 1----2Man alpha 1----3(GlcNAc beta 1----2Man alpha 1---- 6)Man beta 1----4GlcNAc. Assays are not interfered with by substances that result from enzymatic or chemical breakdown of the donor substrate UDP-[14C]GlcNAc. Using this assay system N-acetylglucosaminyltransferase III, IV and V activities were estimated in Novikoff ascites tumour cells, mouse lymphoma BW 5147 cells and hen oviduct.

Purification and partial characterization of human polyamine synthases

Spermidine synthase was purified to apparent homogeneity from human spleens (8700-fold) by affinity chromatography. The native enzyme was composed of two subunits of identical Mr (35,000) and showed an apparent Mr of 62,000 in pore-gradient gel electrophoresis. Its pI was 5.1, Spermine synthase was purified to apparent homogeneity from placenta (5300-fold) and from kidney (4600-fold). The native enzyme was composed of two subunits of identical Mr (45,000) and showed an apparent Mr of 78,000 in pore-gradient gel electrophoresis. In isoelectric focusing it revealed two bands, with pI values of 4.9 and 5.0. Both synthases were present in all human tissues studied, but revealed a clear tissue-specific pattern. Mouse antisera against spermidine synthase revealed only one band, of Mr 35,000, in all purified enzyme preparations and in crude human tissue extracts in immunoblotting. Antisera against spermine synthase showed an immunoreactive band corresponding to the Mr of the subunit of spermine synthase. These antisera did not indicate any cross-reactivity in immunoblotting. Thus spermine synthase and spermidine synthase do not share homologous antigenic sites and are totally different proteins.

Purification and partial characterization of the glycine decarboxylase multienzyme complex from Eubacterium acidaminophilum

The proteins P1, P2, and P4 of the glycine cleavage system have been purified from the anaerobic, glycine-utilizing bacterium Eubacterium acidaminophilum. By gel filtration, these proteins were determined to have Mrs of 225,000, 15,500, and 49,000, respectively. By sodium dodecyl sulfate-polyacrylamide gel electrophoresis, protein P1 was determined to have two subunits with Mrs of 59,500 and 54,100, indicating an alpha 2 beta 2 tetramer, whereas the proteins P2 and P4 showed only single bands with estimated Mrs of 15,500 and 42,000, respectively. In reconstitution assays, proteins P1, P2, P4 and the previously reported lipoamide dehydrogenase (P3) had to be present to achieve glycine decarboxylase or synthase activity. All four glycine decarboxylase proteins exhibited highest activities when NADP+ was used as the electron acceptor or when NADPH was used as the electron donor in the glycine synthase reaction. The oxidation of glycine depended on the presence of tetrahydrofolate, dithioerythreitol, NAD(P)+, and pyridoxal phosphate. The latter was loosely bound to the purified protein P1, which was able to catalyze the glycine-bicarbonate exchange reaction only in combination with protein P2. Protein P2 could not be replaced by lipoic acid or lipoamide, although lipoic acid was determined to be a constituent (0.66 mol/mol of protein) of protein P2. Glycine synthase activity of the four isolated proteins and in crude extracts was low and reached only 12% of glycine decarboxylase activity. Antibodies raised against P1 and P2 showed cross-reactivity with crude extracts of Clostridium cylindrosporum.

Serine hydroxymethyltransferase from pyrimethamine-sensitive and -resistant strains of Plasmodium chabaudi

Serine hydroxymethyltransferase (EC 2.1.2.1) was partially purified from a pyrimethamine sensitive strain of Plasmodium chabaudi. Km values of 2.91 and 1.08 mM were determined for tetrahydrofolate and serine, respectively. The effects of pH, of temperature and of some potential inhibitors were determined. The enzyme was also partially purified from a pyrimethamine-resistant strain of P. chabaudi and subjected to the same regime. No differences between the enzymes from the two sources could be detected. It would appear that the changes in properties in the enzymes dihydrofolate reductase and thymidylate synthetase associated with the development of drug resistance in P. chabaudi were not reflected in any obvious alterations in serine hydroxymethyltransferase.

Partial purification and properties of the isozymes of S-adenosylmethionine synthetase from sheep liver

Three forms of AdoMet synthetase were separated from sheep liver. The apparent molecular weights of the native isozymes were 122,000, 62,400 and 70,800 for the alpha-, beta 1- and beta 2-form, respectively and beta 1 was the predominant form. The alpha-form exhibited negative cooperativity with [S] 0.5 values of 31 microM for methionine and 62 microM for ATP; while the two beta-forms exhibited positive cooperativity with [S]0.5 values for methionine of 82 microM and 70 microM and those for ATP of 572 microM and 505 microM for the beta 1- and beta 2-form, respectively. Dimethylsulfoxide markedly stimulated the activities of the two beta-forms at low methionine concentrations. However, at high methionine levels, it inhibited the activity of the beta 2-form but not that of the beta 1-form. The effect of dimethylsulfoxide on the alpha-form was not significant. AdoMet was inhibitory at high concentrations. However, it had a slight stimulatory effect on the two beta-forms at low concentrations when methionine level was also low. These results suggest that AdoMet synthetase is a regulatory enzyme and the reaction rate in vivo can be directly influenced by substrate and product concentrations.

Purification of geranylgeranyl diphosphate synthase from Phycomyces blakesleanus

Geranylgeranyl diphosphate synthase has been purified to homogeneity from the carotene-overproducing strain M1 of Phycomyces blakesleanus. Usually two activity peaks with molecular weights of 60,000 and 30,000 eluted on gel exclusion chromatography, suggesting that the enzyme consists of two subunits, with a tendency to dissociate. With homogeneous protein, a single-staining band with molecular weight of 30,000 appeared on sodium dodecyl sulfate gel electrophoresis, confirming a subunit molecular weight of 30,000. Only isopentenyl diphosphate and farnesyl diphosphate were accepted by this enzyme for geranylgeranyl diphosphate formation. The smaller allylic compounds, dimethylallyl and geranyl diphosphate, were utilized at less than 1/20th the rate of farnesyl diphosphate. Michaelis constants of 9 microM for isopentenyl diphosphate and 60 microM for farnesyl diphosphate were found. The isoelectric point is 4.8.

Complete purification and immunochemical analysis of S-adenosylmethionine synthetase from bovine brain

We have purified S-adenosylmethionine (AdoMet) synthetase about 3000-fold from bovine brain extract. The Km values of the enzyme for L-methionine and ATP were 10 and 50 microM, respectively. An apparent molecular mass of the enzyme was estimated to be 160 kDa by gel filtration on a Sephacryl S-200 column. Sucrose density gradient centrifugation gave a sedimentation coefficient of 8 S. Polyacrylamide gel electrophoresis of the purified enzyme in native system revealed a single protein band, whereas two polypeptide bands with molecular masses of 48 kDa (p48) and 38 kDa (p38) were observed in sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the purified enzyme. Antibody against bovine brain AdoMet synthetase was prepared by injecting the purified enzyme into a rabbit. Immunoblot analysis revealed that the antibody recognized both p48 and p38 in the impure enzyme preparations from bovine brain as well as in the purified enzyme. Specific antibodies against p48 and p38 were separated from the immunoglobulin fraction by an affinity purification, both of which inhibited the enzyme activity. These results indicate that AdoMet synthetase from bovine brain consists of two different polypeptides, p48 and p38.

Undecaprenyl diphosphate synthase from Micrococcus luteus B-P 26: essential factors for the enzymatic activity

An undecaprenyl diphosphate synthase fraction, which was free of other prenyltransferases and was active without the addition of detergent or phospholipid, was obtained by Sephadex G-100 chromatography of cell-free extracts of Micrococcus luteus B-P 26 cells. The addition of small amounts of Triton X-100 to this fraction caused a marked loss of the enzyme activity, but the activity was gradually restored as further detergent was added. When the enzyme fraction was chromatographed on DEAE-cellulose, the synthase was partially purified, but the activity was not detected unless assayed with addition of the detergent or a lipid fraction of this bacterium. Among the three phospholipids isolated from this bacterium, cardiolipin and phosphatidylglycerol had a marked effect in activating lipid-depleted undecaprenyl diphosphate synthase, but O-lysylphosphatidylglycerol, which occurs prominently in this bacterium, had little effect.

Partial purification and characterization of two sesquiterpene cyclases from sage (Salvia officinalis) which catalyze the respective conversion of farnesyl pyrophosphate to humulene and caryophyllene

Humulene cyclase and caryophyllene cyclase, two enzymes which catalyze the cyclization of farnesyl pyrophosphate to the respective sesquiterpene olefins, have been partially purified from the supernatant fraction of a sage (Salvia officinalis) leaf epidermis extract and separated from each other by a combination of hydrophobic interaction, gel filtration, and ion-exchange chromatography. The molecular weight of both cyclases was estimated by gel filtration to be 57,000 and both cyclases exhibited a pH optimum of 6.5 and preferred Mg2+ (Km approximately 1.5 mM) as the required divalent metal cation. Both enzymes possessed a Km of about 1.7 microM for farnesyl pyrophosphate, were strongly inhibited by p-hydroxymercuribenzoate, and exhibited comparable sensitivities to a variety of other potential inhibitors. The properties of the two sesquiterpene olefin cyclases, which are the first from a higher plant source to be examined in detail, were very similar to each other and to other monoterpene, sesquiterpene, and diterpene cyclases previously described.

Deoxycytidylate hydroxymethylase: purification, properties, and the role of a thiol group in catalysis

Deoxycytidylate (dCMP) hydroxymethylase from Escherichia coli infected with a T-4 bacteriophage amber mutant has been purified to homogeneity. It is a dimer with a subunit molecular weight of 28,000. Chemical modification of the homogeneous enzyme with N-ethylmaleimide (NEM) and 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) leads to complete loss of enzyme activity. dCMP can protect the enzyme against NEM inactivation, but the dihydrofolate analogues methotrexate and aminopterin alone do not afford similar protection. Compared to dCMP alone, dCMP plus either methotrexate or aminopterin greatly enhances protection against NEM inactivation. DTNB inactivation is reversed by dithiothreitol. For both reagents, inactivation kinetics obey second-order kinetics. NEM inactivation is pH dependent with a pKa for a required thiol group of 9.15 +/- 0.11. Complete enzyme inactivation by both reagents involves the modification of one thiol group per mole of dimeric enzyme. There are two thiol groups in the totally denatured enzyme modified by either NEM or DTNB. Kinetic analysis of NEM inactivation cannot distinguish between these two groups; however, with DTNB kinetic analysis of 2-nitro-5-thiobenzoate release shows that enzyme inactivation is due to the modification of one fast-reacting thiol followed by the modification of a second group that reacts about 5-6-fold more slowly. In the presence of methotrexate, the stoichiometry of dCMP binding to the dimeric enzyme is 1:1 and depends upon a reduced thiol group. It appears that the two equally sized subunits are arranged asymmetrically, resulting in one thiol-containing active site per mole of dimeric enzyme.

Purification and comparison of two forms of S-adenosyl-L-methionine synthetase from rat liver

Only two S-adenosyl-L-methionine synthetase forms exist in rat liver: high-Mr S-adenosyl-L-methionine synthetase and low-Mr S-adenosyl-L-methionine synthetase, which have been purified to apparent homogeneity as judged by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. High-Mr S-adenosyl-L-methionine synthetase had an apparent molecular mass, determined by gel filtration, of 210 kDa and was a tetramer constituted by 48.5-kDa subunits, estimated by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. The apparent molecular mass of low-Mr S-adenosyl-L-methionine synthetase, as estimated by gel filtration, was 110 kDa and was constituted by two subunits of 47 kDa. An antiserum against low-Mr S-adenosyl-L-methionine synthetase cross-reacted with the two forms. Reverse-phase HPLC runs of tryptic digestions of high-Mr and low-Mr S-adenosyl-L-methionine synthetase showed that the peptide maps of the two forms were very similar, if not identical. High-Mr S-adenosyl-L-methionine synthetase activity was inhibited by S-adenosyl-L-methionine and pyrophosphate. Depending on the dose used, S-adenosyl-L-methionine activated or inhibited low-Mr S-adenosyl-L-methionine synthetase and pyrophosphate had no effect on this form. The two synthetases showed a different specific activity at the physiological concentration of methionine. This report shows that even though the two forms are constructed of the same polypeptide chains, they are regulated in a different manner by methionine and by the products of the reaction.

Bacterial expression and isolation of Petunia hybrida 5-enol-pyruvylshikimate-3-phosphate synthase

5-enol-Pyruvylshikimate-3-phosphate synthase (EPSP synthase, EPSPS), an in vivo enzyme target of the herbicide glyphosate (N-phosphonomethyl glycine), was purified from a Petunia hybrida suspension culture line, MP4-G, by a small-scale high-performance chromatographic purification procedure. The cDNA encoding the mature petunia EPSPS (lacking the chloroplast transit sequence) was cloned into a plasmid, pMON342, for expression in Escherichia coli. This clone complemented the EPSPS deficiency of an E. coli aroA- mutant, and the plant enzyme constituted approximately 1% of the total extractable protein. Large-scale purification of the enzyme from E. coli cells resulted in a highly active protein which was homogeneous as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and amino terminal sequencing. Antibodies raised against the purified enzyme also reacted with the E. coli EPSPS in Western analyses. The availability of large quantities of the plant enzyme will significantly facilitate mechanistic investigations as well as a comparative study with EPSPS from bacteria and fungi.

The primary structure of rabbit liver cytosolic serine hydroxymethyltransferase

The complete amino acid sequence of cytosolic serine hydroxymethyltransferase from rabbit liver was determined. The sequence was determined from analysis of peptides isolated from tryptic and cyanogen bromide cleavages of the enzyme. Special procedures were used to isolate and sequence the C-terminal and blocked N-terminal peptides. Each of the four identical subunits of the enzyme consists of 483 residues. The sequence could be easily aligned with the sequence of Escherichia coli serine hydroxymethyltransferase. The primary structural homology between the rabbit and E. coli enzymes is about 42%. The importance of the primary and predicted secondary structural homology between the two enzymes is discussed.

Inhibition of thiaminase I from Bacillus thiaminolyticus. Evidence supporting a covalent 1,6-dihydropyrimidinyl-enzyme intermediate

Thiaminase I from Bacillus thiaminolyticus strain Matsukawa et Misawa is completely and irreversibly inhibited by treatment with 4-amino-6-chloro-2-methylpyrimidine. Inhibition is a time-dependent first-order process, exhibiting a half-time of 4 h at an inhibitor concentration of 5 mM. A specific active-site-directed inactivation is supported by protection of the enzymatic activity in the presence of the substrates thiamin and quinoline as well as by the observation that a stoichiometric amount of inorganic chloride is released during inactivation. 4-Amino-5-(anilinomethyl)-6-chloro-2-methylpyrimidine, which resembles the structure of the product of base exchange of thiamin with aniline, inactivates thiaminase approximately 2 orders of magnitude faster. Inactivation is again complete and irreversible and is a time-dependent first-order process, in this case exhibiting saturation at low inhibitor concentrations (KI = 96 microM). Enzyme inactivation can be explained as the result of displacement of chloride from the chloropyrimidine by a nucleophile at the enzyme active site. The inactivation suggests that the Zoltewicz-Kauffman model of bisulfite-catalyzed thiamin cleavage [Zoltewicz, J. A., & Kauffman, G. M. (1977) J. Am. Chem. Soc. 99, 3134-3142], which calls for the reversible nucleophilic addition of catalyst across the 1,6 double bond of thiamin's pyrimidine ring, may be applicable to thiaminase as well.

Affinity purification and characterization of serine hydroxymethyltransferases from rat liver

A rapid and simple method was developed for the purification of serine hydroxymethyltransferases [EC 2.1.2.1]. The procedure involved ammonium sulfate precipitation, DEAE-cellulose column chromatography and affinity chromatography on an L-adsorbent. Through this procedure the cytosolic enzyme (s-SHMT) was purified 1,650-fold, and the mitochondrial enzyme (m-SHMT) 1,730-fold, with a yield of more than 30% in both cases. Both preparations gave a single band with a Mr of 54,000 on SDS-PAGE. The native enzymes both contained 4 mol of pyridoxal phosphate/mol of enzyme, and showed a Mr value of 220,000 on gel filtration, indicating a tetrameric structure. Several other properties of the enzymes were also studied.

Purification and characterization of a serine hydroxymethyltransferase from an obligate methylotroph, Hyphomicrobium methylovorum GM2

A serine hydroxymethyltransferase was purified to complete homogeneity from a serine-producing obligate methylotroph, Hyphomicrobium methylovorum GM2. The enzyme has a molecular mass of about 98 kDa and consists of two subunits of identical molecular mass. The holoenzyme exhibits absorption maxima at 280 nm, 340 nm and 415 nm in potassium phosphate buffer, pH 7.3, the last of which shifts with a change in pH (6.0-7.5) and contains 2 mol pyridoxal phosphate/mol enzyme. The holoenzyme is converted to the apoenzyme on incubation with phenylhydrazine and reconstituted on the addition of pyridoxal phosphate. The enzyme activity was inhibited on the addition of several sulfhydryl-modifying reagents and then recovered with 2-mercaptoethanol. One sulfhydryl group per subunit was found to be responsible for the activity. Isoelectric focusing showed that the enzyme has a pI of 5.6. The Km values for glycine, L-serine and DL-beta-phenylserine are 0.046 mM, 0.15 mM and 33 mM respectively.

Dehydrodolichyl diphosphate synthetase from rat seminiferous tubules

Homogenates of seminiferous tubules from rat testes catalyzed the incorporation of label from [14C]isopentenyl diphosphate into a variety of polyprenyl products. Long chain polyprenyl mono- and diphosphates were formed as major products when undesirable side reactions were minimized. The long chain polyprenyl diphosphate synthetase was measured as a sum of the mono- and diphosphate derivatives formed and was dependent on the addition of t,t-farnesyl diphosphate, isopentenyl diphosphate, and divalent cation. The highest activity was associated with the membranous fractions, whereas activity was negligible in the cytosolic fraction. The products of this prenyl transferase were labile to acid and yielded petroleum ether soluble products which indicated that the alpha-isoprene unit was unsaturated. Hydrolysis of either the polyprenyl mono-or diphosphates with a testicular phosphatase in the absence of NaF yielded C75, C80, C85, and C90 polyprenols. The chain lengths of the products of the synthetase suggest that this enzyme is responsible for the de novo biosynthesis of dehydrodolichyl diphosphates which are precursors of the dolichyl derivatives found in testes.

The role of formylmethanofuran: tetrahydromethanopterin formyltransferase in methanogenesis from carbon dioxide

Formylmethanofuran: tetrahydromethanopterin formyltransferase was purified to electrophoretic homogeneity from cells of Methanobacterium thermoautotrophicum. The enzyme is a tetramer of similar or identical subunits (Mr = 41,000). The equilibrium favors transfer of the formyl group to tetrahydromethanopterin (H4MPT) at physiological pH. The product of formyl transfer by the purified enzyme was shown by a number of criteria to be 5-formyl-H4MPT, as opposed to 10-formyl-H4MPT or 5,10-methenyl-H4MPT. Reconstitution of a portion of the methanogenic C1 cycle was effected by combining purified formyltransferase, methenyl-H4MPT cyclohydrolase, formylmethanofuran, and H4MPT to give methenyl-H4MPT. Additional reconstitution experiments established that the formyltransferase is an essential enzyme for the conversion of carbon dioxide to methane. In conjunction with previously published data (Donnelly, M.I., Escalante-Semerena, J.C., Rinehart, K. L., Jr., and Wolfe, R.S. (1985) Arch. Biochem. Biophys. 242, 430-439), these data substantiate the role of 5-formyl-H4MPT as an intermediate of methanogenesis.

Purification and characterization of propylamine transferase from Sulfolobus solfataricus, an extreme thermophilic archaebacterium

The enzyme propylamine transferase, catalyzing the transfer of the propylamine moiety from S-adenosyl(5')-3-methylthiopropylamine to several amine acceptors, has been purified 643-fold in 20% yield from Sulfolobus solfataricus, an extreme thermophilic archaebacterium optimally growing at 87 degrees C. The purified enzyme (specific activity 2.05 units/mg protein), is homogeneous by criteria of gel electrophoresis, gel filtration, isoelectric focusing and ultracentrifugation analysis. The molecular mass of the native enzyme was estimated to be about 110 kDa by gel permeation and ultracentrifugation analysis. The protein migrates on SDS/polyacrylamide gel electrophoresis as a single band of 35 kDa, suggesting that the enzyme is a trimer composed by identical subunits. An optimum pH of 7.5 and an acidic isoelectric point of 5.3 have been calculated. The optimum temperature was 90 degrees C and no loss of activity is observable even after exposure of the purified enzyme to 100 degrees C for 1 h. No reducing agents are required for enzymatic activity. Substrate specificity towards the amine acceptors is rather broad in that 1,3-diaminopropane (Km = 1675 microM), putrescine (Km = 3850 microM), sym-norspermidine (Km = 954 microM) and spermidine (Km = 1539 microM) are recognized as substrates. Conversely S-adenosyl(5')-3-methylthiopropylamine is the only propylamine donor (Km = 7.9 microM) and the deamination of the sulfonium compound prevents the recognition by the enzyme. The reaction is irreversible and initial-rate kinetic studies indicate that the propylamine transfer is operated through a sequential mechanism. 5'-Methylthioadenosine, a product of the reaction, acts as a powerful competitive inhibitor with a Ki of 3.7 microM. Enzyme-substrate binding sites have been investigated with the aid of several substrate analogs and products. Among the compounds assayed, 5'-methylthiotubercidin, S-adenosyl(5')-3-thiopropylamine and S-adenosyl-3-thio-1,8-diaminooctane are the most active inhibitors.

Purification and properties of rat lens methionine adenosyltransferase

Methionine adenosyltransferase (MAT) has been partially purified from rat lenses using a combination of ammonium sulfate fractionation and hydrophobic chromatography on phenyl Sepharose columns. The partially purified enzyme resembles purified Type II MAT from non-hepatic tissues. The Km for methionine is 3.0 microM, and the Km for ATP is 80 microM. The enzyme is activated by potassium ions (25-50 mM), and inhibited by higher concentrations of potassium. A divalent cation (magnesium or manganese) is essential for activity. The Vmax with magnesium is about five times higher than with manganese, but the optimal manganese concentration is around 2.0 mM, compared with 10-20 mM for magnesium. The enzyme is active over a broad pH range, with optimal activity between pH 7.0 and 8.0. The enzyme is inhibited by all three of its products, phosphate, pyrophosphate, and S-adenosylmethionine. Individually phosphate and pyrophosphate are weak inhibitors, but in combination they show a marked synergistic inhibitory effect. Tripolyphosphate is also an effective inhibitor. The inhibition of the enzyme by the cataractogenic agent, dimethylsulfoxide, further confirmed the similarity to Type II MAT.

Crystalline serine hydroxymethyltransferase from an obligate methylotroph, Hyphomicrobium methylovorum

Optimal culture conditions of a methylotrophic Hyphomicrobium methylovorum and improved purification of serine hydroxymethyltransferase from the bacterium were established for the large-scale preparation of the enzyme. The first crystalline serine hydroxymethyltransferase from the microbial source was obtained in the apo form and found to be homogeneous. Amino acid analysis revealed that the enzyme had higher value per subunit for acidic and neutral amino acids than that from rabbit liver. The carboxy-terminal amino acid analysis suggested the sequence -Ile-Ala-Tyr.

Isozymes of S-adenosylmethionine synthetase from rat liver: isolation and characterization

S-Adenosylmethionine synthetase exists in at least two distinct forms, alpha- and beta-forms, in adult liver. The beta-form was purified to homogeneity from the soluble fraction of rat liver with a yield of about 10%. An antiserum directed against the purified beta-form from rat liver was prepared by injecting the purified enzyme into a rabbit. Ouchterlony double diffusion analysis and immunochemical titrations revealed that the isozymes, alpha- and beta-forms, are identical. Thus, the alpha-form was isolated from rat liver as a single protein using immunoaffinity chromatography against the beta-form. The molecular weights of the beta- and alpha-forms were determined to be 48,000 each by sodium dodecyl sulfate disc gel electrophoresis, and about 100,000, and 200,000, respectively, by Sephacryl S-200 gel filtration. These results indicate that the beta-form consisted of two subunits of 48,000 daltons and the alpha-form of four subunits of 48,000 daltons. The sedimentation coefficient was calculated to be 5.5S for the beta-form and 8.0S for the alpha-form.

Isoprenoid synthesis in Escherichia coli. Separation and partial purification of four enzymes involved in the synthesis

Isopentenyl pyrophosphate (IPP) isomerase, farnesyl pyrophosphate (FPP) synthetase, octaprenyl pyrophosphate (OPP) synthetase and undecaprenyl pyrophosphate (UPP) synthetase were partially purified from Escherichia coli by DEAE-Toyopearl chromatography. FPP synthetase catalyzed the condensation of IPP with dimethylallyl pyrophosphate (DPP) as well as with geranyl pyrophosphate (GPP) to yield FPP as final product. OPP synthetase and UPP synthetase catalyzed the condensation of IPP with FPP to yield OPP and cis,trans-polyprenyl pyrophosphates (the C45-, C50, and C55-compound), respectively. Neither DPP nor GPP acted as a priming substrate for either enzyme. These four enzymes required Mg2+ or Mn2+ for their activities. UPP synthetase required also Triton X-100 for its activity. The addition of Triton X-100 enhanced OPP synthetase, but it did not affect IPP isomerase and FPP synthetase. It seems possible that the combination of the four enzymes ensures the in vivo synthesis of long-chain isoprenoids in E. coli.

Distribution of alkylglycerone-phosphate synthase in subcellular fractions of rat liver

Subcellular fractions of rat liver were isolated by density-gradient centrifugation on a linear Metrizamide gradient and were assayed for marker enzymes of peroxisomes, lysosomes, microsomes and mitochondria. Alkylglycerone-phosphate synthase catalysing the formation of the ether bond in glycerolipids was also determined along the gradient. The enzyme was found to be enriched in the peroxisomal and the microsomal fractions thus, displaying a bimodal distribution pattern. Two reaction-products each, alkylglycerone phosphate and alkylglycerone were obtained in the enzymic assays performed, the ratio of which was clearly dependent upon the fraction employed. Alkylglycerone phosphate was mainly synthesized by the 'peroxisomal synthase', whereas an inverse proportion was observed assaying the microsomal counterpart. Furthermore, comparing the mean specific activities of both the enzymes the microsomal one was shown to be roughly twice as active in metabolizing 1-O-palmitoylglycerone 3-phosphate, simultaneously displaying a somewhat different sensitivity to NaF. These findings provide a first line of evidence, that two separate synthases, one in microsomes and another one in peroxisomes might be engaged in the biosynthesis of 1-O-alkyl-glycerolipids in rat liver.

Glycine decarboxylase multienzyme complex. Purification and partial characterization from pea leaf mitochondria

The P, H, and T proteins of the glycine cleavage system have been purified separately from pea leaf mitochondria and demonstrate molecular weights of 98,000, 15,500, and 45,000, respectively, by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The molecular weight of P protein by gel filtration was 210,000, indicating that this enzyme has a native homodimer conformation. Reconstitution assays containing purified P, H, and T proteins and yeast lipoamide dehydrogenase catalyze the oxidation of glycine and demonstrate a strict dependence on pyridoxal phosphate, tetrahydrofolate, NAD+, and dithiothreitol. The released CO2, methylamine-H protein intermediate, and methylenetetrahydrofolate are produced in stoichiometric amounts from glycine during the cleavage reaction. H protein acts as co-substrate with glycine during the decarboxylation reaction, demonstrating an apparent Km value of 2.2 microM. P and H protein alone jointly catalyze the glycine carboxyl-14 CO2 exchange reaction in the presence of pyridoxal phosphate and dithiothreitol. L protein of the glycine cleavage system was immunopurified using monoclonal antibodies. Antigenic and molecular weight similarities of the L protein with the lipoamide dehydrogenase component of the pyruvate dehydrogenase complex were shown suggesting the possibility of common isomers of lipoamide dehydrogenase for the two enzyme complexes.

A novel prenyltransferase from Paracoccus denitrificans

A new polyprenyltransferase catalysing the formation of Z-double bonds was found and partially purified from extracts of Paracoccus denitrificans. The enzyme catalysed a consecutive condensation of isopentenyl diphosphate with EE-farnesyl diphosphate as a primer to produce EE-farnesyl-all-Z-hexaprenyl diphosphate (ZE-mixed nonaprenyl diphosphate) as the final product. Not only EE-farnesyl diphosphate but also neryl diphosphate, ZE-farnesyl diphosphate, ZEE-geranylgeranyl diphosphate and ZZEE-pentaprenyl diphosphate were all accepted as substrates. This polyprenyltransferase required detergent such as Triton X-100 for its catalytic activity. The formation of ZE-mixed undecaprenyl diphosphate, which is well known as the precursor of the bacterial sugar-carrier lipid, was not detected in extracts of this bacterium.

Selective overproduction of 5-enol-pyruvylshikimic acid 3-phosphate synthase in a plant cell culture which tolerates high doses of the herbicide glyphosate

Cultured cells of the higher plant Corydalis sempervirens Pers. which had been adapted to growing in the presence of 5 mM glyphosate (N-[phosphonomethyl]-glycine), a herbicide and a potent specific inhibitor of the shikimate pathway enzyme 5-enol-pyruvylshikimate-3-phosphate (EPSP) synthase, had a nearly 40-fold increased level of the extractable activity of EPSP synthase. Activities of five other shikimate pathway enzymes were, however, similar in the adapted and nonadapted cells, and the concentrations of the free aromatic amino acids in the two cell lines were also similar. EPSP synthases purified from glyphosate-adapted, as well as nonadapted cells, had identical physical, kinetic, and immunological properties, which indicated that the glyphosate-sensitive enzyme was overproduced in the adapted culture. Overproduction of EPSP synthase in the adapted culture was unequivocally established by two-dimensional polyacrylamide gel electrophoresis, as well as by one-dimensional sodium dodecyl sulfate-gradient gel electrophoresis and quantitation of EPSP protein by immunoassay after transfer to nitrocellulose membranes. While about 0.06% of the total soluble protein from nonadapted cells was EPSP synthase protein, the proportion was 2.6% in the adapted cells. In vivo pulse-labeling experiments with [35S]methionine established that the adapted cells have an increased rate of EPSP synthase protein synthesis.

Photoaffinity labeling of undecaprenyl pyrophosphate synthetase with a farnesyl pyrophosphate analogue

The prenyl transferase undecaprenyl pyrophosphate synthetase was partially purified from the cytosolic fraction of Escherichia coli. Its enzymic products were characterized as a family of cis-polyprenyl phosphates, which ranged in carbon number from C55 to C25. The enzyme is constituted of two subunits of approximately 30,000 molecular weight. A radiolabeled photolabile analogue of t,t-farnesyl pyrophosphate, [3H]2-diazo-3-trifluoropropionyloxy geranyl pyrophosphate, was shown to label Lactobacillus plantarum and E. coli undecaprenyl pyrophosphate synthetase on UV irradiation in the presence of isopentenyl pyrophosphate and divalent cation. The only labeled polypeptide migrated on electrophoresis in a sodium dodecyl sulfate-polyacrylamide gel at a molecular weight of approximately 30,000. No protein was radiolabeled when the natural substrate, t,t-farnesyl pyrophosphate was included in the irradiation mixture. Irradiation in the presence of MgCl2 without isopentenyl pyrophosphate gave less labeling of the polypeptide. Irradiation with only isopentenyl pyrophosphate gave little labeling of the polypeptide. When the enzyme was irradiated with 3H-photoprobe, [14C]isopentenyl pyrophosphate, and MgCl2, the labeled polypeptide gave a ratio of 14C/3H that indicated the product must also bind to the enzyme on irradiation. These results demonstrate the ability to radiolabel the allylic pyrophosphate binding site and possibly product binding site of undecaprenyl pyrophosphate synthetase by a process which is favored when both cosubstrate and divalent cation are present.

An affinity column for the purification of prenyltransferases

Farnesyl pyrophosphate synthetase (EC 2.5.1.1) from chicken liver, pig liver, and yeast has been purified to homogeneity in a single chromatographic step by affinity chromatography. The affinity ligand, geranylmethylphosphonophosphate, is linked to Affi-Gel 10 through the phosphonophosphate moiety. The affinity gel is stable chemically and the internal phosphonophosphate linkage is not hydrolyzed by nonspecific phosphatases. A single column has been used repeatedly for over a year with no degradation in its performance. A typical purification only requires 2 days and gives a 500- to 600-fold purification of enzyme from a crude ammonium sulfate precipitate.

Solubilization and reconstitution of rat liver mitochondrial carnitine acylcarnitine translocase

Carnitine acylcarnitine translocase has been solubilized from inverted inner membrane vesicles of rat liver mitochondria with octyl glucoside and reconstituted into asolectin liposomes. For both processes, optimization of the detergent to phospholipid ratio was found crucial for obtaining reconstitutively active liposomes. Reassembly of the solubilized carrier into asolectin liposomes was achieved either by the octyl glucoside dilution method or by Extracti-Gel D column chromatography. The reconstituted system catalyzed exchange diffusion of carnitine, exhibited the expected inhibitor and temperature sensitivity, and discriminated between stereoisomers of octanoylcarnitine. The activity of unidirectional import of carnitine was low compared to exchange diffusion. It showed high-temperature sensitivity and a loss of activity on prolonged sonication that was regained by an appropriate freeze-thaw step subsequently.

Isoprenoid enzyme systems of silkworm. I. Partial purification of isopentenyl pyrophosphate isomerase, farnesyl pyrophosphate synthetase, and geranylgeranyl pyrophosphate synthetase

Isopentenyl pyrophosphate isomerase, farnesyl pyrophosphate synthetase, and geranylgeranyl pyrophosphate synthetase were detected in cell-free extracts of Bombyx mori and were partially purified by hydroxyapatite and Sephadex G-100 chromatography. Two forms of farnesyl pyrophosphate synthetase were chromatographically separated. They were designated as farnesyl pyrophosphate synthetases I and II in the order of their elution from hydroxyapatite. Both enzymes catalyzed the exclusive formation of (E,E)-farnesyl pyrophosphate from isopentenyl pyrophosphate and either dimethylallyl pyrophosphate or geranyl pyrophosphate. However, they were not interconvertible, unlike the enzyme from pig liver. These two enzymes resembled each other in pH optima and molecular weights but differed in susceptibility to metal ions. Farnesyl pyrophosphate synthetase II was stimulated by Triton X-100 while synthetase I was inhibited by the same reagent.

Serine hydroxymethyltransferase from Escherichia coli: purification and properties

Serine hydroxymethyltransferase from Escherichia coli was purified to homogeneity. The enzyme was a homodimer of identical subunits with a molecular weight of 95,000. The amino acid sequence of the amino and carboxy-terminal ends and the amino acid composition of cysteine-containing tryptic peptides were in agreement with the primary structure proposed for this enzyme from the structure of the glyA gene (M. Plamann, L. Stauffer, M. Urbanowski, and G. Stauffer, Nucleic Acids Res. 11:2065-2074, 1983). The enzyme contained no disulfide bonds but had one sulfhydryl group on the surface of the protein. Several sulfhydryl reagents reacted with this exposed group and inactivated the enzyme. Spectra of the enzyme in the presence of substrates and substrate analogs showed that the enzyme formed the same complexes and in similar relative concentrations as previously observed with the cytosolic and mitochondrial rabbit liver isoenzymes. Kinetic studies with substrates showed that the affinity and synergistic binding of the amino acid and folate substrates were similar to those obtained with the rabbit liver isoenzymes. The enzyme catalyzed the cleavage of threonine, allothreonine, and 3-phenylserine to glycine and the corresponding aldehyde in the absence of tetrahydrofolate. The enzyme was also inactivated by D-alanine caused by the transamination of the active site pyridoxal phosphate to pyridoxamine phosphate. This substrate specificity was also observed with the rabbit liver isoenzymes. We conclude that the reaction mechanism and the active site structure of E. coli serine hydroxymethyltransferase are very similar to the mechanism and structure of the rabbit liver isoenzymes.

Comparison of two hydrolytic murein transglycosylases of Escherichia coli

Escherichia coli has two murein transglycosylases, which are found in the soluble and the particulate fraction, respectively. The enzymes have been purified and have been shown to differ in some of their molecular properties [Mett, H., Keck, W., Funk, A. & Schwarz, U. (1980) J. Bacteriol. 144, 45-52]. We improved and simplified the purification procedure for the membrane-derived transglycosylase and characterized the two enzymes in more detail by peptide mapping and by immunological procedures. The peptide pattern obtained after tryptic digestion of the purified enzymes differed for the two enzymes. Antisera to the transglycosylases reacted only with their own antigen as shown by specific inhibition of the enzymatic activity, double immunodiffusion and by immunochemical staining of protein blots on nitrocellulose filters. Thus we conclude that the transglycosylases are two distinct proteins and that the one is not a precursor of the other.

Purification and enzymatic characterization of CMP-sialic acid: beta-galactosyl1----3-N-acetylgalactosaminide alpha 2----3-sialyltransferase from human placenta

A CMP-NeuAc:Gal beta 1----3GalNAc-R alpha 2----3-sialyltransferase has been purified over 20,000-fold from a Triton X-100 extract of human placenta by affinity chromatography on concanavalin A-Sepharose and CDP-hexanolamine-Sepharose in a yield of 10%. Sodium dodecyl sulfate-gel electrophoresis under reducing conditions revealed that the enzyme consists of a major polypeptide species with a molecular weight of 41,000 and some minor forms with molecular weights of 40,000, 43,000, and 65,000, respectively, which can be resolved partially by gel filtration on Sephadex G-100. Isoelectric focusing revealed that the enzyme occurs in a major and a minor charged form with pI values of 5.0-5.5 and 6.0, respectively. Acceptor specificity studies indicated that the enzyme catalyzes the incorporation of sialic acid from CMP-NeuAc into glycoproteins, glycolipids, and oligosaccharides which possess a terminal Gal beta----3GalNAc unit. Analysis of the structure of the product chain by high-pressure liquid chromatography and thin layer chromatography as well as methylation analysis revealed that a NeuAc alpha 2----3Gal beta 1----3GalNAc sequence is elaborated. The best glycoprotein acceptors are antifreeze glycoprotein and porcine submaxillary asialo/afucomucin. The disaccharide Gal beta 1----3GalNAc-Thr shows values for Km and V which are close to those of the latter glycoprotein. Lactose as well as oligosaccharides in which galactose is linked beta 1----3 or beta 1----4 to N-acetylglucosamine are less efficient acceptors. Of the glycolipids tested only gangliosides GM1 and GD1b served as an acceptor. The enzyme does not show an absolute aglycon specificity, and attaches sialic acid regardless the anomeric configuration of the N-acetylgalactosaminyl residue in the accepting Gal beta 1----3GalNAc unit. By use of specific acceptor substrates it could be demonstrated that the purified enzyme is free from other known sialyltransferase activities. Studies with rabbit antibodies raised against a partially purified sialyltransferase preparation indicated that the enzyme is immunologically unrelated to a Gal beta 1----4GlcNAc-R alpha 2----3-sialyltransferase, which previously had been identified in human placenta (Van den Eijnden, D.H., and Schiphorst, W. E. C. M. (1981) J. Biol. Chem. 256, 3159-3162). Initial-rate kinetic studies suggest that the sialyltransferase operates through a mechanism involving a ternary complex of enzyme, sugar donor, and acceptor. This is the first report on the extensive purification and characterization of a sialyltransferase from a human tissue.

S-Adenosylmethionine synthetase from human lymphocytes. Purification and characterization

S-Adenosylmethionine synthetase has been purified to apparent homogeneity from human chronic lymphocytic leukemia cells. Equilibrium sedimentation studies and denaturing polyacrylamide gel electrophoresis indicate that the native enzyme has a molecular weight of 185,000 and a subunit composition of either alpha alpha' beta 2, alpha 2 beta 2, or alpha' 2 beta 2, where alpha, alpha', and beta are polypeptide chains of molecular weight 53,000, 51,000, and 38,000. The alpha and alpha' subunits appear to be the same polypeptide and presumably differ by some kind of post-translational modification. Stoichiometric studies show that the expected products S-adenosylmethionine, pyrophosphate, and orthophosphate are generated in equimolar amounts. The enzyme exhibits linear kinetics with respect to substrate dependency and product inhibition, except for orthophosphate which shows parabolic noncompetitive inhibition with respect to ATP. Initial velocity studies of substrate dependence and product inhibition indicate a steady state mechanism that is ordered Bi Ter with ATP adding before L-methionine and S-adenosylmethionine as the first product released. Pyrophosphate and orthophosphate, however, appear to be released by a random mechanism. Free Mg2+ is an essential activator with a half-maximal effect at 1.0 mM. The Km and Kia for ATP are 31 microM and 84 microM, and the Km for L-methionine is 3.3 microM. The enzyme also has tripolyphosphatase activity which is stimulated by S-adenosylmethionine.

Casbene synthetase: regulation of phytoalexin biosynthesis in Ricinus communis L. seedlings. Purification of casbene synthetase and regulation of its biosynthesis during elicitation

Castor bean (Ricinus communis L.) seedlings responded to stress by producing the antifungal diterpene, casbene. Casbene synthetase, the enzyme catalyzing the production of casbene from geranylgeranyl pyrophosphate, was purified 4700-fold to a final specific activity of 4.2 nkat/mg protein by a combination of ion-exchange and dye-ligand chromatographic procedures. Approximately 500 micrograms of purified enzyme was recovered from 1600 seedlings that had been infected with the fungus, Rhizopus stolonifer. The purified enzyme showed a single protein band, by Ag staining, of Mr 59,000 +/- 1000 after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Electrophoretic analysis of the immunoprecipitate obtained from a crude enzyme extract and polyclonal rabbit antibodies raised against the purified enzyme revealed no contaminants or cross-reacting components. In vitro translation of polysomal RNA pools obtained from healthy castor bean seedlings and seedlings at various times after exposure to pectic fragment elicitors coupled with immunoprecipitation showed that healthy seedlings have nondetectable levels of casbene synthetase mRNA and that seedlings exposed to elicitor show a rapid increase in casbene synthetase mRNA which reaches a maximum after 6 h. Casbene synthetase activity increases to a maximum 10 h after elicitation under comparable conditions. These results show that increases in the activity of mRNA for casbene synthetase after elicitation by pectic fragments precede the appearance of casbene synthetase activity as would be expected if the enzyme were being synthesized de novo.

Purification and properties of an ethanolamine-serine base exchange enzyme of rat brain microsomes

The activity of an ethanolamine and serine base exchange enzyme of rat brain microsomes was copurified to near homogeneity. The purification sequence involved detergent solubilization, Sepharose 4B column chromatography, phenyl-Sepharose 4B column chromatography, glycerol gradient sedimentation, and agarose-polyacrylamide gel electrophoresis under non-denaturing conditions. The ratio of the ethanolamine and serine base exchange activities remained almost constant during purification, and both enzyme activities were enriched 25-fold over the initial microsomal suspension. The final enzyme preparation which contained both enzyme activities showed a single protein band on sodium dodecyl sulfate-polyacrylamide gel, having an apparent molecular mass of about 100 kDa. Serine inhibited the ethanolamine incorporation by this preparation and ethanolamine inhibited the serine incorporation. The competitive nature of this inhibition was apparent from Lineweaver-Burk plots, suggesting that the enzyme catalyzes the incorporation of both ethanolamine and serine into their corresponding phospholipids. The Km and Ki values for ethanolamine were quite similar, being 0.02 and 0.025 mM, respectively. The Km and Ki values for serine were also quite similar being 0.11 and 0.12 mM, respectively. The pH optimum was the same at 7.0 with both substrates. The optimum Ca2+ concentration was 8 mM for serine incorporation.