Purification and characterization of an alpha-1,2-mannosidase involved in processing asparagine-linked oligosaccharides

A calcium-dependent alpha-1,2-mannosidase involved in the processing of asparagine-linked oligosaccharides was purified to homogeneity from rabbit liver microsomes. N-terminal amino acid analysis was consistent with the presence of a homogeneous protein. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis, under both reducing and nonreducing conditions, revealed a single protein band with an apparent molecular weight of 52,000. Gel filtration and sedimentation analysis under nondenaturing conditions suggested that the purified enzyme is a monomeric protein. The mannosidase is a glycoprotein based on the presence of protein-linked sugar and specific binding of the enzyme to concanavalin A-Sepharose. Purified mannosidase was optimally active between pH 5.0 and 6.0. The enzyme was inactive with p-nitrophenyl-alpha-D-mannopyranoside and was inhibited by deoxymannojirimycin but not by swainsonine. The enzyme was specifically activated by Ca2+, with half-maximal activation occurring at concentrations of 10 microM or less and was inhibited by Mn2+, Co2+, Ba2+, and Zn2+. Calcium ions protected the enzyme against inactivation by p-chloromercuribenzoate. Rabbit liver mannosidase hydrolyzed alpha-1,2-mannosyl-mannose linkages in a variety of substrates including methyl-2-O-alpha-D-mannopyranosyl-alpha-D-mannopyranoside (Schutzbach, J. S. (1987) Anal. Biochem. 167, 279-283), ovalbumin glycopeptide IV, and the high mannose chains of thyroglobulin and phytohemagglutinin-P. Approximately 70% of the alpha-1,2-linked mannosyl units in the oligosaccharides of thyroglobulin were accessible to rabbit liver alpha-mannosidase, whereas most of the alpha-1,2-mannosyl units in phytohemagglutinin were resistant to digestion prior to heat denaturation of the plant lectin.     

In vitro hydrolysis of oligomannose-type sugar chains by an alpha-1,2-mannosidase from microsomes of developing castor bean cotyledons.

An alpha-1,2-mannosidase involved in the processing of N-linked oligosaccharides was prepared from the microsomal fraction of developing castor bean cotyledons. The processing alpha-mannosidase was solubilized with 1.0% Triton X-100 and purified by ion-exchange chromatography followed by two gel filtration steps. The enzyme obtained could convert Man9GlcNAc2-PA to Man5GlcNAc2-PA, but this enzyme was inactive with Man5GlcNAc2-PA, Man4GlcNAc2-PA, and p-nitrophenyl-alpha-D-mannopyranoside. The enzyme was optimally active between pH 5.5-6.0. The processing mannosidase was inhibited by deoxymannojirimycin, EDTA, and Tris ions but not by swainsonine. Structural analyses of the mannose-trimming intermediates produced by the alpha-mannosidase revealed that specific intermediates were formed during conversion of Man9GlcNAc2-PA to Man5GlcNAc2-PA.     

A fluorescence assay for alpha-1,2-mannosidases involved in glycoprotein processing reactions

A highly specific, sensitive, and convenient fluorescence assay for alpha-1,2-mannosidases involved in glycoprotein processing reactions is described. The assay utilizes a coupled enzyme system to determine the amount of free mannose liberated from the disaccharide O-methyl-2-O-alpha-D-mannopyranosyl-alpha-D-mannopyranoside by the alpha-1,2-mannosidase. The assay was used to determine the substrate specificity of a calcium ion-activated alpha-1,2-mannosidase purified from rabbit liver microsomes. The microsomal mannosidase was specific for hydrolysis of the alpha-1,2 linkage. The mannosyl linkages in alpha-1,3- and alpha-1,6-linked methyl-disaccharides, in methyl-alpha-D-mannopyranoside, and in yeast mannan were hydrolyzed at rates of 2% or less than that noted with the alpha-1,2-linked disaccharide. Mannosidase activity was linear with time and was proportional to enzyme concentration. The Km for the alpha-1,2-linked methyl-disaccharide is 0.5 mM.     

Mannosyltransfer reactions in rabbit liver microsomes

Rabbit liver microsomes catalyzed mannosyltransfer from GDP-[14C]mannose to free $\text{D}$-mannose resulting in the synthesis of α-1,2-, α-1,3-, and α-1,6-mannosyl-mannose. Whereas formation of α-1,2-mannosyl-mannose was stimulated by the addition of manganese chloride or nickel chloride and was inhibited by EDTA, synthesis of α-1,3-mannosyl-mannose was unaffected by manganese or EDTA and was inhibited by nickel. Formation of α-1,6-mannosyl-mannose appeared to be stimulated by manganese and inhibited by nickel. These results suggest that three different mannosyltransferases were involved in the synthesis of mannosyl-mannose glycosidic linkages in rabbit liver.     

Novel tools for the study of class I alpha-mannosidases: a chromogenic substrate and a substrate-analog inhibitor

The use of chromogenic substrates for evaluation of class I alpha-mannosidase is described. 2('),4(')-Dinitrophenyl-alpha-D-mannopyranoside allows rapid and sensitive assays of enzymatic activities, e.g., of heterologously expressed alpha-1,2-mannosidase from Trichoderma reesei. Interaction constants of several ligands with alpha-mannosidases from class I and II could also be determined. Furthermore, novel types of inhibitors derived from D-lyxose are presented. Methyl-alpha-D-lyxopyranosyl-(1(')-->2)-alpha-D-mannopyranoside is a potent inhibitor of the alpha-1,2-mannosidase from T. reesei (K(i)=600 microM) and since it probably spans subsites -1/+1, this disaccharide could be valuable in crystallographic studies of class I alpha-mannosidases.     

Characterization of trimming Man9-mannosidase from pig liver. Purification of a catalytically active fragment and evidence for the transmembrane nature of the intact 65 kDa enzyme.

An alpha 1,2-mannosidase (Man9-mannosidase) involved in N-linked oligosaccharide processing has been purified about 16,000-fold from pig liver crude microsomes (microsomal fractions) by CM-Sepharose and DEAE-Sephacel chromatography, concanavalin A (Con A)-Sepharose chromatography and, as the key step of the procedure, affinity chromatography on immobilized N-5-carboxypentyl-l-deoxymannojirimycin (CP-dMM). On SDS/polyacrylamide-gel electrophoresis under reducing conditions, the isolated enzyme migrated as a single protein band with a molecular mass of 49 kDa. The enzyme does not bind Con A and is not susceptible to glycopeptidase F, indicating that it lacks N-linked oligosaccharides of the high-mannose or complex type. Purified Man9-mannosidase has a pH optimum close to 6.0 and requires bivalent cations for activity, with Ca2+ being most effective. The enzyme is inhibited strongly by basic sugar analogues of mannose such as 1-deoxymannojirimycin (dMM, Ki approximately 5 microM), N-methyl-dMM (Ki approximately 55 microM) and CP-dMM (Ki approximately 150 microM), whereas NN-dimethyl-dMM and the mannosidase II inhibitor swainsonine were hardly or not at all inhibitory. A homogeneous preparation of the 49 kDa enzyme cleaves specifically three of the four alpha 1,2-mannosidic linkages in the natural Man9-GlcNAc2 (M9) substrate. The relative rates by which the parent and intermediate structures are hydrolysed were found to be about 3:2:5 for M9, M8 and M7 respectively. The enzyme displays only marginal activity toward the remaining alpha 1,2-mannosidic linkages in the Man9-GlcNAc2 oligosaccharide (relative rate of M6 hydrolysis approximately 0.02) and is not active against nitrophenyl and methylumbelliferyl alpha-mannosides. This unique substrate specificity suggests that Man9-mannosidase processing differs from that catalysed by other trimming alpha 1,2-mannosidases hitherto reported. A polyclonal antibody raised against the denatured 49 kDa polypeptide not only recognizes a protein band of similar size in Western blots of crude microsomes, but also reacts strongly with a 65 kDa protein species. On trypsin treatment of detergent-solubilized microsomes, the 65 kDa protein is converted specifically into a stable 49 kDa fragment, indicating a precursor-product relationship between the two proteins. We conclude from this observation that the 65 kDa protein represents the intact form of Man9-mannosidase from which the 49 kDa enzyme which we have isolated has been generated, with retention of catalytic activity, by proteolysis during purification. Proteolytic studies with sealed microsomes suggest that the intact 65 kDa enzyme is a protein with a membrane-spanning domain, as well as a cytosolic polypeptide domain of size at least 3 kDa.     

Purification and properties of glucosidase I from mung bean seedlings

The microsomal enzyme fraction from mung bean seedlings was found to contain glucosidase activity capable of releasing [3H]glucose from the glucose-labeled Glc3Man9GlcNAc. The enzymatic activity could be released in a soluble form by treating the microsomal particles with 1.5% Triton X-100. When the solubilized enzyme fraction was chromatographed on DE-52, it was possible to resolve glucosidase I activity (measured by the release of [3H]glucose from Glc3Man9GlcNAc) from glucosidase II (measured by release of [3H]glucose from Glc2Man9GlcNAc). The glucosidase I was purified about 200-fold by chromatography on hydroxylapatite, Sephadex G-200, dextran-Sepharose, and concanavalin A-Sepharose. The purified enzyme was free of glucosidase II and aryl-glucosidase activities. Only a single glucose residue could be released from the Glc3Man9GlcNAc by this purified enzyme and the other product was the Glc2Man9GlcNAc. Furthermore, this enzyme was inhibited in a dose-dependent manner by kojibiose, an alpha-1,2-linked glucose disaccharide, but not by other alpha-linked glucose disaccharides. These data indicate that this glucosidase is a specific alpha-1,2-glucosidase. The pH optimum for the glucosidase I was about 6.3 to 6.5, and no requirements for divalent cations were observed. The enzyme was inhibited strongly by the glucosidase processing inhibitors, castanospermine and deoxynojirimycin, and less strongly by the plant pyrrolidine alkaloid, 2,5-dihydroxymethyl-3,4-dihydroxypyrrolidine. However, the enzyme was not inhibited by the mannosidase processing inhibitors, swainsonine, deoxymannojirimycin or 1,4-dideoxy-1,4-imino-D-mannitol. The stability of the enzyme under various conditions and other properties of the enzyme were determined.     

Purification of soluble alpha1,2-mannosidase from Candida albicans CAI-4

A soluble alpha-mannosidase from Candida albicans CAI-4 was purified by conventional methods of protein isolation. Analytical electrophoresis of the purified preparation revealed two polypeptides of 52 and 27 kDa, the former being responsible for enzyme activity. The purified, 52 kDa enzyme trimmed Man9GlcNAc2, producing Man8GlcNAc2 isomer B and mannose, and was inhibited preferentially by 1-deoxymannojirimycin. These properties are consistent with an endoplasmic reticulum-resident alpha1,2-mannosidase of the glycosyl hydrolase family 47. Moreover, a proteolytic activity responsible for converting the 52 kDa alpha-mannosidase into a polypeptide of 43 kDa retaining full enzyme activity, was demonstrated in membranes of ATCC 26555, but not in CAI-4 strain.     

Evidence for an alpha-mannosidase in endoplasmic reticulum of rat liver

An alpha-mannosidase activity has been identified in a preparation of rat liver endoplasmic reticulum and shown to be distinct from the previously described Golgi alpha-mannosidases I and II and the lysosomal alpha-mannosidase. The enzyme was solubilized with deoxycholate and separated from other alpha-mannosidases by passage over concanavalin A-Sepharose to which it does not bind. The endoplasmic reticulum alpha-mannosidase cleaves alpha-1,2-linked mannoses from high mannose oligosaccharides and, unlike Golgi alpha-mannosidase I, is active against p-nitrophenyl-alpha-D-mannoside (Km = 0.17 mM). It has no activity toward GlcNAc-Man5GlcNAc2 peptide, the specific substrate of the Golgi alpha-mannosidase II. The endoplasmic reticulum alpha-mannosidase activity toward p-nitrophenyl-alpha-D-mannoside is relatively insensitive to swainsonine, an inhibitor of both the lysosomal alpha-mannosidase and Golgi alpha-mannosidase II. We propose that the endoplasmic reticulum alpha-mannosidase is responsible for the removal of mannose residues from asparagine-linked high mannose type oligosaccharides prior to their entry into the Golgi.     

Inhibition of mouse liver sialidase by plant flavonoids

Flavonoids (103 species) were tested for inhibitory activity against mouse liver sialidase using sodium p-nitrophenyl-N-acetyl-alpha-D-neuraminate (PNP-NeuAc) as substrate. Isoscutellarein-8-O-glucuronide from the leaf of Scutellaria baicalensis showed most potent activity (IC50, 40 microM), and this flavone appeared to be a non-competitive inhibitor of the enzyme. This flavone inhibited the lysosomal solubilized sialidase against PNP-NeuAc and sialyllactose effectively, but not microsomal enzyme against gangliosides and colominic acid, whereas, negligible or weak inhibitory activities were observed for influenza virus sialidase, beta-galactosidase, alpha-mannosidase, and alpha-glucosidase tested. These results indicate that this flavone may be useful to elucidate the function of the lysosomal solubilized sialidase.     

Human liver cytosolic epoxide hydrolases

Human liver epoxide hydrolases were characterized by several criteria and a cytosolic cis-stilbene oxide hydrolase (cEHCSO) was purified to apparent homogeneity. Styrene oxide and five phenylmethyloxiranes were tested as substrates for human liver epoxide hydrolases. With microsomes activity was highest with trans-2-methylstyrene oxide, followed by styrene 7,8-oxide, cis-2-methylstyrene oxide, cis-1,2-dimethylstyrene oxide, trans-1,2-dimethylstyrene oxide and 2,2-dimethylstyrene oxide. With cytosol the same order was obtained for the first three substrates, whereas activity with 2,2-dimethylstyrene oxide was higher than with cis-1,2-dimethylstyrene oxide and no hydrolysis occurred with trans-1,2-dimethylstyrene oxide. Generally, activities were lower with cytosol than with microsomes. The isoelectric point for both microsomal styrene 7,8-oxide and cis-stilbene oxide hydrolyzing activity was 7.0, whereas cEHCSO had an isoelectric point of 9.2 and cytosolic trans-stilbene oxide hydrolase (cEHTSO) of 5.7. The cytosolic epoxide hydrolases could be separated by anion-exchange chromatography and gel filtration. The latter technique revealed a higher molecular mass for cEHCSO than for cEHTSO. Both cytosolic epoxide hydrolases showed higher activities at pH 7.4 than at pH 9.0, whereas the opposite was true for microsomal epoxide hydrolase. The effects of ethanol, methanol, tetrahydrofuran, acetonitrile, acetone and dimethylsulfoxide on microsomal epoxide hydrolase depended on the substrate tested, whereas both cytosolic enzymes were not at all, or only slightly, affected by these solvents. Effects of different enzyme modulators on microsomal epoxide hydrolase also depended on the substrates used. Trichloropropene oxide and styrene 7,8-oxide strongly inhibited cEHCSO whereas cEHTSO was moderately affected by these compounds. Immunochemical investigations revealed a close relationship between cEHCSO and rat liver microsomal, but not cytosolic, epoxide hydrolase. Interestingly, cEHTSO has no immunological relationship to rat microsomal, nor to rat cytosolic epoxide hydrolase. cEHTSO from human liver differed also from its counterpart in the rat in that it was only moderately affected by tetrahydrofuran, acetonitrile and trichloropropene oxide. Five steps were necessary to purify cEHCSO. The enzyme has a molecular mass (49 kDa) identical to that of rat liver microsomal epoxide hydrolase.     

Enzymatic Mechanisms Involved in Phenanthrene Degradation by the White Rot Fungus Pleurotus ostreatus

The enzymatic mechanisms involved in the degradation of phenanthrene by the white rot fungus Pleurotus ostreatus were examined. Phase I metabolism (cytochrome P-450 monooxygenase and epoxide hydrolase) and phase II conjugation (glutathione S-transferase, aryl sulfotransferase, UDP-glucuronosyltransferase, and UDP-glucosyltransferase) enzyme activities were determined for mycelial extracts of P. ostreatus. Cytochrome P-450 was detected in both cytosolic and microsomal fractions at 0.16 and 0.38 nmol min(sup-1) mg of protein(sup1), respectively. Both fractions oxidized [9,10-(sup14)C]phenanthrene to phenanthrene trans-9,10-dihydrodiol. The cytochrome P-450 inhibitors 1-aminobenzotriazole (0.1 mM), SKF-525A (proadifen, 0.1 mM), and carbon monoxide inhibited the cytosolic and microsomal P-450s differently. Cytosolic and microsomal epoxide hydrolase activities, with phenanthrene 9,10-oxide as the substrate, were similar, with specific activities of 0.50 and 0.41 nmol min(sup-1) mg of protein(sup-1), respectively. The epoxide hydrolase inhibitor cyclohexene oxide (5 mM) significantly inhibited the formation of phenanthrene trans-9,10-dihydrodiol in both fractions. The phase II enzyme 1-chloro-2,4-dinitrobenzene glutathione S-transferase was detected in the cytosolic fraction (4.16 nmol min(sup-1) mg of protein(sup-1)), whereas aryl adenosine-3(prm1)-phosphate-5(prm1)-phosphosulfate sulfotransferase (aryl PAPS sulfotransferase) UDP-glucuronosyltransferase, and UDP-glucosyltransferase had microsomal activities of 2.14, 4.25, and 4.21 nmol min(sup-1) mg of protein(sup-1), respectively, with low activity in the cytosolic fraction. However, when P. ostreatus culture broth incubated with phenanthrene was screened for phase II metabolites, no sulfate, glutathione, glucoside, or glucuronide conjugates of phenanthrene metabolites were detected. These experiments indicate the involvement of cytochrome P-450 monooxygenase and epoxide hydrolase in the initial phase I oxidation of phenanthrene to form phenanthrene trans-9,10-dihydrodiol. Laccase and manganese-independent peroxidase were not involved in the initial oxidation of phenanthrene. Although P. ostreatus had phase II xenobiotic metabolizing enzymes, conjugation reactions were not important for the elimination of hydroxylated phenanthrene.     

Alpha-1,2-Mannosidase and Hence N-Glycosylation Are Required for Regulatory T Cell Migration and Allograft Tolerance in Mice

Background

Specific immunological unresponsiveness to alloantigens can be induced in vivo by treating mice with a donor alloantigen in combination with a non-depleting anti-CD4 antibody. This tolerance induction protocol enriches for alloantigen reactive regulatory T cells (Treg). We previously demonstrated that alpha-1,2-mannosidase, an enzyme involved in the synthesis and processing of N-linked glycoproteins, is highly expressed in tolerant mice, in both graft infiltrating leukocytes and peripheral blood lymphocytes.

Principal Findings

In this study we have identified that alpha-1,2-mannosidase expression increases in CD25⁺CD4⁺ Treg when they encounter alloantigen in vivo. When alpha-1,2-mannosidase enzyme activity was blocked, Treg retained their capacity to suppress T cell proliferation in vitro but were unable to bind to physiologically relevant ligands in vitro. Further in vivo analysis demonstrated that blocking alpha-1,2-mannosidase in Treg resulted in the migration of significantly lower numbers to the peripheral lymph nodes in skin grafted mice following adoptive transfer, where they were less able to inhibit the proliferation of naïve T cells responding to donor alloantigen and hence unable prevent allograft rejection in vivo.

Significance

Taken together, our results suggest that activation of alloantigen reactive Treg results in increased alpha-1,2-mannosidase expression and altered N-glycosylation of cell surface proteins. In our experimental system, altered N-glycosylation is not essential for intrinsic Treg suppressive capacity, but is essential in vivo as it facilitates Treg migration to sites where they can regulate immune priming. Migration of Treg is central to their role in regulating in vivo immune responses and may require specific changes in N-glycosylation upon antigen encounter.     

Activation of detergent-solubilized rat hepatic 5'-iodothyronine deiodinase by NADPH and nonglutathione cytosolic components

An investigation was made of the possible role of the hepatic microsomal membrane in the activation of 5'-iodothyronine deiodinase (5'-DI) by a cytosolic activating system consisting of fraction A (relative mass (Mr) greater than 60,000), fraction B (Mr, approximately 13,000), and NADPH. Activation of 5'-DI in washed microsomes was compared with that of a microsome extract prepared by solubilization with 3-[(3-cholamidopropyl)dimethylammonio]-1-propane sulphonate and further purification by fractional precipitation with polyethylene glycol and by DEAE-Sephacel chromatography. All 5'-DI preparations exhibited qualitatively similar dependence upon NADPH and cytosolic factors in fractions A and B for 5'-DI activation and were relatively unresponsive to NADH. Activation of solubilized preparations, unlike that of intact microsomes, was more readily inhibited by low concentrations of detergent and not inhibited by NADPH concentrations above 0.25 mM. Attempted purification of 5'-DI failed to produce a substantial increase in specific activity of the enzyme. It is concluded that, while glutathione-independent cytosolic factors and NADPH can activate 5'-DI in the absence of an intact microsomal membrane, some membrane constituents removed during solubilization and purification of the enzyme are required for maximal activation.     

Rat liver microsomal 3 alpha-hydroxysteroid dehydrogenase and dihydrodiol dehydrogenase: solubilization, separation and partial purification

Rat liver microsomes contain 3 alpha-hydroxysteroid dehydrogenase (HSD) (EC 1.1.1.50) and dihydrodiol dehydrogenase (DHD) (EC 1.3.1.20) activities. The two enzyme activities were solubilized by 10% Triton X-100 or 0.4% sodium deoxycholate. Unlike the cytosolic enzyme (Penning & Talalay (1983) Proc. Natl. Acad. Sci. U.S.A., 80, 4505), the microsomal HSD and DHD activities were not inhibited by indomethacin. Chromatography of the microsomal Triton X-100 extract on Affigel Blue and then on Phenyl-Sepharose gave an HSD preparation containing no detectable (less than 3 - 5%) DHD activity, whereas chromatography of the deoxycholate extract on Phenyl-Sepharose provided a DHD preparation that lacked measurable HSD activity. These results are in sharp contrast to the cytosolic enzyme where both HSD and DHD activities could be copurified to homogeneity (Penning et al. (1984) Biochem. J. 222, 601).     

Transient decrease of liver cytosolic glutathione S-transferase activities in rats given 1,2-dibromoethane or CCl4

In vivo treatment of fasted male rats with 1,2-dibromoethane (DBE) (0.4 mmol/kg) or carbon tetrachloride (CCl4) (4 mmol/kg) was found to rapidly alter the activities of liver cytosolic and microsomal glutathione S-transferases. Microsomal activities towards chloro-2,4-dinitrobenzene (CDNB) were increased 2 h after either treatment. Cytosolic activities towards CDNB and 3,4-dichloronitrobenzene (DCNB), but not 1,2-epoxy-3-(p-nitrophenoxy)-propane (ENPP), were selectively and transiently decreased after either treatment. Time course studies in DBE animals indicated that the decrease in cytosolic activity was not evident until 2 h although liver glutathione (GSH) concentrations were diminished within 15 min. In contrast, in CCl4 animals the decrease in cytosolic activity was evident within 15 min and was not accompanied by diminished GSH concentrations. By 4 h, cytosolic activities had rebounded to control levels in both DBE and CCl4-treated animals. Kinetic studies of the enzyme in liver cytosol from animals 2 h after treatment with DBE or CCl4 indicated that both treatments decreased the apparent Vmax while neither treatment altered the apparent Km. This pattern of change allows exclusion of a simple competitive mechanism of enzyme inhibition, but cannot distinguish between reversible non-competitive inhibition and irreversible inhibition. It is possible that the observed decreases in the activities of the abundant cytosal enzyme are due to 'sacrificial' covalent linkages between the enzyme and reactive metabolites of DBE or CCl4.     

Plantagoside, a novel alpha-mannosidase inhibitor isolated from the seeds of Plantago asiatica, suppresses immune response

A hot-water extract from the seed of Plantago asiatica showed a potent inhibitory activity against jack bean alpha-mannosidase, and a flavanone glucoside, plantagoside, was isolated as the inhibitor. Plantagoside was a specific inhibitor for jack bean alpha-mannosidase (IC50 at 5 microM) and appeared to be a non-competitive inhibitor of the enzyme. Whereas, negligible or weak inhibitory activities were observed for beta-mannosidase, beta-glucosidase, and sialidase tested. Plantagoside also inhibited alpha-mannosidase activities in mouse liver lysosomal and microsomal fractions, and the enzyme inhibitory activity in microsomal fraction was enhanced in the presence of glucosidase inhibitor, castanospermine. Plantagoside suppressed antibody response to sheep red blood cells and concanavalin A induced lymphocyte proliferation which was measured by [3H]thymidine incorporation.     

Solubilization and modulation of acyl-CoA:1-acyl-glycerophosphocholine acyltransferase activity in rat liver microsomes

The acylation of 1-acyl-glycerophosphocholine is an important mechanism for the maintenance of the asymmetrical distribution of acyl groups in phosphatidylcholine. The majority of acyl-CoA:1-acyl-glycerophosphocholine acyltransferase is located in the microsomal fraction. In this study, the rat liver microsomes were incubated with various detergents, and the solubilized enzyme was separated from the remainder by centrifugation. Sodium cholate, sodium deoxycholate and octylglucopyranoside caused the Solubilization of 14–25% of the enzyme activity. The acyl specificity of the solubilized enzyme was similar to the insoluble enzyme, indicating that there was no selective solubilization of any acyl specific acyltransferase. The solubilized enzyme did not display any lipid requirement, and its activity was inhibited by phosphatidylcholine, phosphatidylethanolamine and 1,2-diacylglycerol. Kinetic studies with varying concentrations of acyl-CoAs revealed that the inhibition by 1,2-diacylglycerol was essentially uncompetitive. The modulation of acyltransferase activity by 1,2-diacylglycerol may be an important mechanism for controlling the acylation of lysophosphatidylcholine.     

Role of the two catalytic domains of DSR-E dextransucrase and their involvement in the formation of highly alpha-1,2 branched dextran

The dsrE gene from Leuconostoc mesenteroides NRRL B-1299 was shown to encode a very large protein with two potentially active catalytic domains (CD1 and CD2) separated by a glucan binding domain (GBD). From sequence analysis, DSR-E was classified in glucoside hydrolase family 70, where it is the only enzyme to have two catalytic domains. The recombinant protein DSR-E synthesizes both alpha-1,6 and alpha-1,2 glucosidic linkages in transglucosylation reactions using sucrose as the donor and maltose as the acceptor. To investigate the specific roles of CD1 and CD2 in the catalytic mechanism, truncated forms of dsrE were cloned and expressed in Escherichia coli. Gene products were then small-scale purified to isolate the various corresponding enzymes. Dextran and oligosaccharide syntheses were performed. Structural characterization by (13)C nuclear magnetic resonance and/or high-performance liquid chromatography showed that enzymes devoid of CD2 synthesized products containing only alpha-1,6 linkages. On the other hand, enzymes devoid of CD1 modified alpha-1,6 linear oligosaccharides and dextran acceptors through the formation of alpha-1,2 linkages. Therefore, each domain is highly regiospecific, CD1 being specific for the synthesis of alpha-1,6 glucosidic bonds and CD2 only catalyzing the formation of alpha-1,2 linkages. This finding permitted us to elucidate the mechanism of alpha-1,2 branching formation and to engineer a novel transglucosidase specific for the formation of alpha-1,2 linkages. This enzyme will be very useful to control the rate of alpha-1,2 linkage synthesis in dextran or oligosaccharide production.