N-Acetylneuraminic acid biosynthesis in rat liver and kidney

Rat liver and kidney tissue slices incubated withN-acetyl [³H]mannosamine incorporated radioactivity into free and boundN-acetylneuraminic acid and CMP-N-acetylneuraminic acid (CMP-NeuAc). Liver and kidney also incorporated radioactivity from intravenously injected [³H]ManNAc intoN-acetylneuraminic acid and CMP-NeuAc. From the decrease in the specific radioactivity of CMP-NeuAc after a single injection ofN-acetyl[³H]mannosamine the half-life of CMP-NeuAc was determined. From this half-life and the pool size of CMP-NeuAc a synthesis rate of CMP-NeuAc was calculated, being 1.2 nmol/min/g wet weight of kidney. In previous experiments a value of 1.0 nmol/min/g wet weight was determined for liver [Ferwerdaet al. (1983) Biochem J 216: 87–92]. The synthesis rate of CMP-NeuAcin vivo was in the same range as the synthesis rate calculated from the turnover of boundN-acetylneuraminic acid, which was 2.7 and 0.4 nmol/min/g wet weight for liver and kidney respectively.The assay conditions for UDP-N-acetylglucosamine 2-epimerase andN-acetylmannosamine kinase were adapted to measure low activitiesin vitro. It appeared that the kinase activity detected in kidney can synthesizeN-acetylmannosamine6-phosphate at a rate sufficient for the observed production ofN-acetylneuraminic acidin vivo. Also a low, but measurable activity of UDP-N-acetylglucosamine 2-epimerase was detected in kidneyin vitro, suggesting that the biosynthetic pathway ofN-acetylneuraminic acid in kidney is the same as in liver. The synthesis rate ofN-acetylneuraminic acid in liver determinedin vivo is approximately 12 times slower than the maximal potential rate calculated from the activities of theN-acetylneuraminic acid (precursor-) forming enzymes as detectedin vitro. This indicates that in liverin vivo the enzymes are working far below their maximal capacity.     

Low incidence of N-glycolylneuraminic acid in birds and reptiles and its absence in the platypus

The sialic acids of the platypus, birds, and reptiles were investigated with regard to the occurrence of N-glycolylneuraminic (Neu5Gc) acid. They were released from tissues, eggs, or salivary mucin samples by acid hydrolysis, and purified and analyzed by thin-layer chromatography, high-performance liquid chromatography, and mass spectrometry. In muscle and liver of the platypus only N-acetylneuraminic (Neu5Ac) acid was found. The nine bird species studied also did not express N-glycolylneuraminic acid with the exception of an egg, but not tissues, from the budgerigar and traces in poultry. Among nine reptiles, including one turtle, N-glycolylneuraminic acid was only found in the egg and an adult basilisk, but not in a freshly hatched animal. BLAST analysis of the genomes of the platypus, the chicken, and zebra finch against the CMP-N-acetylneuraminic acid hydroxylase did not reveal the existence of a similar protein structure. Apparently monotremes (platypus) and sauropsids (birds and reptiles) cannot synthesize Neu5Gc. The few animals where Neu5Gc was found, especially in eggs, may have acquired this from the diet or by an alternative pathway. Since Neu5Gc is antigenic to man, the observation that this monosaccharide does not or at least only rarely occur in birds and reptiles, may be of nutritional and clinical significance.     

Isolation and properties of a sialidase fromTrypanosoma rangeli

In the culture supernatant ofTrypanosoma rangeli, strain El Salvador, a sialidase was present with an activity of 0.1 U/mg protein as determined with the 4-methylumbelliferyl glycoside of -N-acetylneuraminic acid as substrate. This enzyme was purified about 700-fold almost to homogeneity by gel chromatography on Sephadex G-100 and Blue Sepharose, and affinity chromatographies on 2-deoxy-2,3-didehydroneuraminic acid and horse submandibular gland mucin, both immobilized on Sepharose. The pH optimum is at 5.4–5.6, and the molecular weight was determined by gel chromatography, high performance liquid chromatography and sodium dodecyl sulphate gel electrophoresis to be 70 000. The substrate specificity of the enzyme is comparable to bacterial, viral and mammalian sialidases with cleavage rates for the following substrates in decreasing order: N-acetylneuraminyl-(2–3)-lactose> N-glycoloylneuraminy-(2–3)-lactose> N-acetylneuraminyl-(2–6)-lactose >sialoglycoproteins>gangliosides>9-O-acetylated sialoglycoproteins.4-O-Acetylated derivatives are resistant towards the action of this sialidase. The enzyme activity can be inhibited by 2-deoxy-2,3-didehydro-N-acetylneuraminic acid, Hg²⁺ ions, andp-nitrophenyloxamic acid; it is not dependent on the presence of Ca²⁺ Mn²⁺ or Mg²⁺ ions.Abbreviations BSA bovine serum albumin - BSM bovine submandibular gland mucin - CMP cytidine monophosphate - EDIA ethylenediaminetetraacetic acid - ESM equine submandibular gland mucin - HEPES N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid - HPLC high performance liquid chromatography - Lac lactose - MU-Neu5Ac 4-methylumbelliferyl glycoside of -N-acetylneuraminic acid - Neu5Ac N-acetylneuraminic acid - Neu5Ac2en 2-deoxy-2,3-didehydro-N-acetylneuraminic acid - Neu4Ac5Gc N-glycoloyl-4-O-acetylneuraminic acid - Neu2en 2-deoxy-2,3-didehydroneuraminic acid - Neu5Gc N-glycoloylneuraminic acid - PMSF phenylmethylsulfonyl fluoride - PSM pig submandibular gland mucin - SDS sodium dodecyl sulfate - Tris tris-(hydroxymethyl)aminomethane Dedicated to Professor Dr. Heinz Mühlpfordt on the occasion of his 65th birthday.     

N-acetylneuraminic acid and sialoglycoconjugate metabolism in fibroblasts from a patient with generalized N-acetylneuraminic acid storage disease

Cultured skin fibroblasts from a patient suffering from generalized N-acetylneuraminic acid storage disease were found to accumulate large amounts (approx. 4.0 μmol/g fresh weight) of free N-acetylneuraminic acid in a lysosome-enriched subcellular fraction. However, there were no detectable deficiencies in lysosomal hydrolase activities (including neuraminidase), and the activities of CMP-N-acetylneuraminic acid synthetase and N-acetylneuraminic acid aldolase were within normal limits. The cellular glycoconjugate composition was normal, and pathologic fibroblasts labeled with either [³H]glucosamine-HCl or $\text{N-[}{}^3\text{H]acetylmannosamine}$ showed a marked accumulation of labeled free N-acetylneuraminic acid, along with elevated incorporation into sialoglycoconjugates. Neither normal nor pathologic fibroblasts secreted labeled free N-acetylneuraminic acid into the culture medium. These results are consistent with an inherited defect in N-acetylneuraminic acid reutilization, resulting in the lysosomal accumulation of the free monosaccharide in generalized N-acetylneuraminic acid storage disease.     

Efficient enzymatic synthesis of 4-methylumbelliferyl N-acetyllactosaminide and 4-methylumbelliferyl sialyl N-acetyllactosaminides employing beta-D-galactosidase and sialyltransferases

4-Methylumbelliferyl N-acetyllactosaminide and 4-methylumbelliferyl sialyl N-acetyllactosaminides, which are used for the assay of sialytransferase, neuraminidase and fucosyltransferase, were synthesized, respectively, by the β-D-galactosidase from Bacillus circulans and by a recombinant rat α2,3-(N)-sialyltransferase or rat liver α2,6-(N)-sialyltransferase with CMP-N-acetylneuraminic acid as donor.     

N-Acetyl-4-deoxy-d-neuraminic acid is activated and transferred on to asialoglycoprotein

The sialic acid analogue,N-acetyl-4-deoxy-neuraminic acid, is readily activated by CMP-sialic acid synthase from bovine brain. We also show that sialyl-transfer from CMP-N-acetyl-4-deoxy-neuraminic acid to asialo- ₁-acid glycoprotein is achieved at a high rate using Gal1-4GlcNAc (2.6)-sialyltransferase from rat liver.In contrast toVibrio cholerae sialidase, fowl plague virus sialidase liberates boundN-acetyl-4-deoxy-neuraminic acid from the glycoprotein. Thus, as opposed to the general view, the action of neither synthase nor transferase depends on the presence of the hydroxy group at C-4 ofN-acetylneuraminic acid.Abbrevations BSA bovine serum albumin - DTE dithioerythritol - HPLC high performance liquid chromatography - NeuAc N-acetyl-d-neuraminic acid - 4-deoxy-NeuAc N-acetyl-4-deoxy-d-neuraminic acid - 4-epi-NeuAc 4-acetamido-3,5-dideoxy-d-glycero-d-talononulosonic acid - CMP-NeuAc Cytidine-5-monophospho-N-acetylneuraminic acid - CMP-4-deoxy-NeuAc Cytidine-5-monophospho-N-acetyl-4-deoxy-neuraminic acid - FPV-sialidase Fowl plague virus sialidase - VCN Vibrio cholerae neuraminidase     

Conversion of N-acetylglucosamine to CMP-N-acetylneuraminic acid in a cell-free system of rat liver

A soluble fraction of rat liver converts glucosamine and N-acetylglucosamine in the presence of ATP and UTP to N-acetylneuraminic acid. This system, when supplemented with CTP, forms CMP-N-acetylneuraminic acid in high yield. Nicotinamide was found to enhance the synthesis of UDP-N-acetylglucosamine and N-acetylneuraminic acid. Kinetic analysis reveals N-acetylglucosamine 6-phosphate, UDP-N-acetylglucosamine, N-acetylmannosamine, N-acetylmannosamine 6-phosphate and N-acetylneuraminic acid 9-phosphate as intermediates. Under certain experimental conditions, however, an epimerisation of N-acetylglucosamine to N-acetylmannosamine was seen.     

Bacterial CMP-sialic acid synthetases: production,properties, and applications

Sialic acids are abundant nine-carbon sugars expressed terminally on glycoconjugates of eukaryotic cells and are crucial for a variety of cell biological functions such as cell–cell adhesion, intracellular signaling, and in regulation of glycoproteins stability. In bacteria, N-acetylneuraminic acid (Neu5Ac) polymers are important virulence factors. Cytidine 5′-monophosphate (CMP)-N-acetylneuraminic acid synthetase (CSS; EC 2.7.7.43), the key enzyme that synthesizes CMP-N-acetylneuraminic acid, the donor molecule for numerous sialyltransferase reactions, is present in both prokaryotes and eukaryotic systems. Herein, we emphasize the source, function, and biotechnological applications of CSS enzymes from bacterial sources. To date, only a few CSS from pathogenic bacterial species such as Neisseria meningitidis, Escherichia coli, group B streptococci, Haemophilus ducreyi, and Pasteurella hemolytica and an enzyme from nonpathogenic bacterium, Clostridium thermocellum, have been described. Overall, the enzymes from both Gram-positive and Gram-negative bacteria share common catalytic properties such as their dependency on divalent cation, temperature and pH profiles, and catalytic mechanisms. The enzymes, however, can be categorized as smaller and larger enzymes depending on their molecular weight. The larger enzymes in some cases are bifunctional; they have exhibited acetylhydrolase activity in addition to their sugar nucleotidyltransferase activity. The CSSs are important enzymes for the chemoenzymatic synthesis of various sialooligosaccharides of significance in biotechnology.     

Purification of bovine colostrumβ-galactosideα(2–6)sialyltransferase to near homogeneity by affinity chromatography

Cytidine-5-monophospho-N-acetylneuraminic acid:-galactoside 2-6sialyltransferase was purified from bovine colostrum by two sequential affinity chromatography steps on CDP-ethanolamine-Sepharose and CDP-ethanolamine-(N-caproylamino-)-Sepharose, respectively. While the conditions for elution were those of Paulsonet al. [J Biol Chem (1977) 252:3256–62], the ligand of the second affinity column was coupled to Sepharose by using 6-aminocaproic acid as linker. The ease of this procedure allows rapid synthesis of bulk quantities of ligand.Highly purified preparations of sialyltransferase were obtained which moved on gradient gel electrophoresis as a single band of 76 kDa and on dodecylsulphate electrophoresis as a single band of 54 kDa. The product of the reaction between lactose and CMP-N-acetylneuraminic acid catalyzed by the purified sialyltransferase was identified by high-resolution 500 MHz¹H-NMR spectroscopy as Neu5Ac2-6Gal1-4Glc.     

Sialic acid concentrations in plants are in the range of inadvertent contamination

The long held but challenged view that plants do not synthesize sialic acids was re-evaluated using two different procedures to isolate putative sialic acid containing material from plant tissues and cells. The extracts were reacted with 1,2-diamino-4,5-methylene dioxybenzene and the fluorescently labelled 2-keto sugar acids analysed by reversed phase and normal phase HPLC and by HPLC–electrospray tandem mass spectrometry. No N-glycolylneuraminic acid was found in the protein fraction from Arabidopsis thaliana MM2d cells. However, we did detect 3-deoxy-d-manno-octulosonic acid and trace amounts (3–18 pmol/g fresh weight) of a compound indistinguishable from N-acetylneuraminic acid by its retention time and its mass spectral fragmentation pattern. Thus, plant cells and tissues contain five orders of magnitude less sialic acid than mammalian tissues such as porcine liver. Similar or lower amounts of N-acetylneuraminic acid were detected in tobacco cells, mung bean sprouts, apple and banana. Yet even yeast and buffer blanks, when subjected to the same isolation procedures, apparently contained the equivalent of 5 pmol of sialic acid per gram of material. Thus, we conclude that it is not possible to demonstrate unequivocally that plants synthesize sialic acids because the amounts of these sugars detected in plant cells and tissues are so small that they may originate from extraneous contaminants.     

Preparation ofN-acetylneuraminic acid from delipidated egg yolk

Egg yolk, a large proportion of the egg, was studied for the preparation ofN-acetylneuraminic acid (Neu5Ac). The delipidated hen egg yolk (DEY; 500 kg containing 0.2% w/w, Neu5Ac) was hydrolysed with HCl (pH 1.4) at 80 °C and neutralized with NaOH (pH 6.0). The mixture was filtered and electrodialysed until the conductivity was 240 µS cm^–1. The filtrate was applied on a column of Dowex HCR-W2 (20–50 mesh), followed by a column of Dowex 1-X8 (200–400 mesh). The latter column was washed with water, and then eluted with a linear gradient of HCO₂H (0–2m). The eluates containing Neu5Ac were concentrated using a reverse osmosis membrane and, finally, rotary evaporated at 40 °C. The residue was then lyophilized to yield 500 g Neu5Ac. The purity of Neu5Ac was >98% (TBA method). HPLC, NMR spectroscopy and TLC chromatography of the product obtained from the DEY showed that Neu5Ac was the sole derivative present in egg yolk. The DEY, a byproduct from egg processing plants, was found to be an excellent source for the large-scale preparation of Neu5Ac.Abbreviations Neu5Ac N-acetylneuraminic acid - Neu5Gc N-glycolylneuraminic acid - DEY delipidated egg yolk - HPLC high performance liquid chromatography - TLC thin layer chromatography - NMR nuclear magnetic resonance - IR infrared spectroscopy Presented at the 11th International Symposium on Glycoconjugates, Toronto, Canada.     

Properties and subcellular localization of CMP-N-acetylneuraminic acid hydrolase of calf kidney

The properties and subcellular distribution of CMP-N-acetylneuraminic acid (CMP-NAcNeu) hydrolase were studied in the cortex of calf kidney. The pH optimum was 9.0 in both Tris · HCl and glycine/NaOH buffer. The apparent K_m was 0.47 mM and the apparent V 15.3 μmol/h/g wet wt of calf kidney cortex. A stimulation by divalent metal ions (Ca²⁺ and Mg²⁺) was demonstrated for the hydrolase. In the presence of Triton X-100 an increase in enzyme activity was observed. CMP-NAcNeu hydrolase was inhibited by EDTA, β-mercaptoethanol, nucleoside phosphates and nucleotide-sugars. The inhibition was more pronounced when a sub-optimal CMP-NAcNeu concentration was used, The enzyme appeared to be localized in the plasma membranes. In the plasma membrane preparation of calf kidney cortex, which was derived mainly from the proximal tubule cells, the yield of CMP-NAcNeu hydrolase (13%) and its increase in specific activity (9-fold) was as high as for the plasma membrane marker enzymes. From subcellular distribution studies it appeared that the enzyme was localized mainly at the brush border side of the plasma membrane of the proximal tubule cell.     

Purification and characterization of sialidase fromClostridium sordellii G12

Sialidase secreted by the urease-positiveClostridium sordellii strain G12 was isolated from culture medium and purified to apparent homogeneity as estimated by Fast Protein Liquid Chromatography (FPLC) and sodium dodecylsulphate-polyacrylamide gel electrophoresis (SDS-PAGE). For this purpose, ion-exchange chromatography, gel filtration, isoelectric focusing, and FPLC on ion-exchange resin and gel filtration materials were used. The sialidase was purified 159 300-fold from 5 l of culture medium, yielding 9 g of enzyme protein with a specific activity of 480 U/mg. For the denatured (SDS-PAGE) and native (FPLC) sialidase relative molecular masses of 40 000 and 38 500 Da, respectively, were estimated. The substrate specificity, kinetic data, and pH-optimum of the enzyme are similar to those of other bacterial sialidases. The influences of salt or serum proteins on enzyme activity are of interest.Abbreviations MU-Neu5Ac 4-methylumbelliferyl -d-N-acetylneuraminic acid - Ganglioside GD1a IV³NeuAc, ll³NeuAc-GgOse₄Cer - Neu5Ac2en 2-deoxy-2,3-didehydro-N-acetylneuraminic acid     

CMP-N-acetylneuraminic acid hydroxylase activity determines the wheat germ agglutinin-binding phenotype in two mutants of the lymphoma cell line MDAY-D2

The dominant glycosylation mutants of MDAY-D2 mouse lymphoma cells, designated class 2 (D33W25 and D34W25) were selected for their resistance to the toxic effects of wheat germ agglutinin (WGA) and shown to express elevated levels of Neu5Gc. In accordance with this, the activity of CMP-Neu5Ac hydroxylase was found to be substantially higher in the mutant cells. The hydroxylase in the D33W25 mutant cells exhibited kinetic properties identical to those of the same enzyme from mouse liver. Growth rate experimentsin vivo andin vitro, where the mutant cells grew more slowly at low cell densities in serum-free medium and also formed slower growing tumours in syngeneic mice, indicate that CMP-Neu5Ac hydroxylase expression may be associated with altered growth of the mutant cells.Abbreviations WGA wheat germ agglutinin - Neu5Ac N-acetyl--d-neuraminic acid - Neu5Gc N-glycology--d-neuraminic acid - CMP-Neu5Ac cytidine-5-monophospho-N-acetylneuraminic acid - CMP-Neu5Gc cytidine-5-monophospho-N-glycoloylneuraminic acid - FACS fluorescence-activated cell sorting - buffer A triethylamine hydrogen carbonate, pH 7.6 (concentration given at appropriate points in the text) - SFM serum free medium - IMDM Iscove's modified Dulbecco's medium - CMP-Neu5Ac hydroxylase CMP-N-acetylneuraminate: NAD(P)H oxido-reductase (N-acetyl hydroxylating) (EC 1.14.99.18); CMP-sialate hydrolase (EC 3.1.4.40); sialic acid-pyruvate lyase (EC 4.1.3.3)     

Cloning and expression of human sialic acid pathway genes to generate CMP-sialic acids in insect cells

The addition of sialic acid residues to glycoproteins can affect important protein properties including biological activity and in vivo circulatory half-life. For sialylation to occur, the donor sugar nucleotide cytidine monophospho-sialic acid (CMP-SA) must be generated and enzymatically transferred to an acceptor oligosaccharide. However, examination of insect cells grown in serum-free medium revealed negligible native levels of the most common sialic acid nucleotide, CMP-N-acetylneuraminic acid (CMP-Neu5Ac). To increase substrate levels, the enzymes of the metabolic pathway for CMP-SA synthesis have been engineered into insect cells using the baculovirus expression system. In this study, a human CMP-sialic acid synthase cDNA was identified and found to encode a protein with 94% identity to the murine homologue. The human CMP-sialic acid synthase (Cmp-Sas) is ubiquitously expressed in human cells from multiple tissues. When expressed in insect cells using the baculovirus vector, the encoded protein is functional and localizes to the nucleus as in mammalian cells. In addition, co-expression of Cmp-Sas with the recently cloned sialic acid phosphate synthase with N-acetylmannosamine feeding yields intracellular CMP-Neu5Ac levels 30 times higher than those observed in unsupplemented CHO cells. The absence of any one of these three components abolishes CMP-Neu5Ac production in vivo. However, when N-acetylmannosamine feeding is omitted, the sugar nucleotide form of deaminated Neu5Ac, CMP-2-keto-3-deoxy-D-glycero-D-galacto-nononic acid (CMP-KDN), is produced instead, indicating that alternative sialic acid glycoforms may eventually be possible in insect cells. The human CMP-SAS enzyme is also capable of CMP-N-glycolylneuraminic acid (CMP-Neu5Gc) synthesis when provided with the proper substrate. Engineering the CMP-SA metabolic pathway may be beneficial in various cell lines in which CMP-Neu5Ac production limits sialylation of glycoproteins or other glycans.     

Determination of sialic acids in biological fluids using reversed-phase ion-pair high-performance liquid chromatography

A simple, rapid and sensitive reversed-phase ion-pair high-performance liquid chromatographic method for the determination of N-acetylneuraminic acid and 2-deoxy-2,3-dehydro-N-acetylneuraminic acid in biological fluids is described. Determination of N-acetylneuraminic acid released by acidic hydrolysis, in serum, urine and saliva, and 2-deoxy-2,3-dehydro-N-acetylneuraminic acid in urine, without hydrolysis, was accomplished by injecting the sample without derivatization, into the chromatograph. Measurements were carried out isocratically within 6 min using a C₁₈ column and a mobile phase of aqueous solution of triisopropanolamine, as ion-pair reagent, 60 mM, pH 3.5 at room temperature with UV absorbance detection. The present method is reported for the first time for the determination of sialic acids in biological fluids. Recoveries in serum, urine and saliva ranged from 90 to 102% and the limits of detection were 60 nM and 20 nM for the two sialic acids, respectively. The method has been applied to normal and pathological sera from patients with breast, stomach, colon, ovarian and cervix cancers, to normal urine and urine from patient with sialuria and to normal saliva.     

THE SITE OF SYNTHESIS OF GANGLIOSIDES IN THE CHICK OPTIC SYSTEM

Abstract– In the retinas of 1-day-old chickens that received an intraocular injection of N-[³H]acetylmannosamine the labelling of N-acetylneuraminic acid and CMP-N-acetylneuraminic acid increased for at least 8 h and that of gangliosides for at least 24 h after injection. In the optic tectum contralateral to the injected eye at 8 h after the intraocular injection, the labelling of gangliosides exceeded the labelling of gangliosides in the ipsilateral tectum by approx 20-fold. In the contralateral tectum the highest concentration of labelled gangliosides was in subfractions enriched in synaptosomes and synaptic plasma membranes. No significant contralateral ipsilateral differences were found in the acid soluble substances of the tectum. In the optic tectum, labelled gangliosides appeared earlier in the neuronal perikarya than in synaptosomes when the injection was intracranial. Conversely, when the injection was intraocular the labelling appeared earlier in the synaptosomes than in the neuronal perikarya. The radioactivity pattern of the optic tectum gangliosides resembled the pattern of retina gangliosides when N-[³H]acetylmannosamine was injected intraocularly, but when N-[³H]acetylmannosamine was given intracerebrally the radioactivity pattern resembled that of optic tectum gangliosides. Intraocular injection of colchicine or vinblastine did not affect the labelling of retinal gangliosides from N-[³H]acetylmannosamine injected into the same eye but prevented the appearance of labelled gangliosides in the optic tectum. In vitro the ganglioside glycosylating activity of optic tectum synaptosomes and synaptic plasma membranes was between 6 and 10-fold lower than that found in the optic tectum neuronal perikarya. These findings support the notion that the main subcellular site of synthesis of neuronal gangliosides is in the neuronal perikarya, from which they are translocated to the nerve endings.     

Inhibition of the biosynthesis ofN-acetylneuraminic acid by metal ions and seleniumin vitro

In liver homogenate the biosynthesis ofN-acetylneuraminic acid usingN-acetylglucosamine as precursor can be followed stepwise by applying different chromatographic procedures. In this cell-free system 16 metal ions (Zn²⁺, Mn²⁺, La³⁺, Co²⁺, Cu²⁺, Hg²⁺, VO ₃ ^– , Pb²⁺, Ce³⁺, Cd²⁺, Fe²⁺, Fe³⁺, Al³⁺, Sn²⁺, Cs⁺ and Li⁺) and the selenium compounds, selenium(IV) oxide and sodium selenite, have been checked with respect to their ability to influence a single or possible several steps of the biosynthesis ofN-acetylneuraminic acid. It could be shown that the following enzymes are sensitive to these metal ions (usually applied at a concentration of 1 mmoll^–1):N-acetylglucosamine kinase (inhibited by Zn²⁺ and vandate), UDP-N-acetylglucosamine-2-epimerase (inhibited by zn²⁺, Co²⁺, Cu²⁺, Hg²⁺, VO ₃ ^– , Pb²⁺, Cd²⁺, Fe³⁺, Cs⁺, Li⁺, selenium(IV) oxide and selenite), andN-acetylmannosamine kinase (inhibited by Zn²⁺, Cu²⁺, Cd²⁺, and Co²⁺). Dose dependent measurements have shown that Zn²⁺, Cu²⁺ and selenite are more efficient inhibitors of UDP-N-acetylglucosamine-2-epimerase than vanadate. As for theN-acetylmannosamine kinase inhibition, a decreasing inhibitory effect exists in the following order Zn²⁺, Cd²⁺, Co²⁺ and Cu²⁺. In contrast, La³⁺, Al³⁺ and Mn²⁺ (1 mmoll^–1) did not interfere with the biosynthesis ofN-acetylneuraminic acid. Thus, the conclusion that the inhibitory effect of the metal ions investigated cannot be regarded as simply unspecific is justified.Dedicated to Professor Theodor Günther on the occasion of his 60th birthday     

THE LYSOSOME PERIPHERY: BIOCHEMICAL AND ELECTROKINETIC PROPERTIES OF THE TRITOSOME SURFACE

Normal rat liver lysosomes were isolated by the technique of loading with Triton WR-1339. Purity of the preparation was monitored with marker enzymes; a high enrichment in acid hydrolases was obtained in the tritosome fraction. In 0.0145 M NaCl, 4.5% sorbitol, 0.6 mM NaHCO₃, pH 7.2 at 25°C the tritosomes had an electrophoretic mobility of -1.77 ± 0.02 µm/s/V/cm, a zeta potential of 23.2 mV, a surface charge of 1970 esu/cm², and 33,000 electrons per particle surface assuming a tritosome diameter of 5 x 10^-7 m. Treatment of the tritosomes with 50 µg neuraminidase/mg tritosome protein lowered the electrophoretic mobility of the tritosome to -1.23 ± 0.02 µm/s/V/cm under the same conditions and caused the release of 2.01 µg sialic acid/mg tritosome protein. Treatment of the tritosomes with hyaluronidase did not affect their electrophoretic mobility, while trypsin treatment elevated the net negative electrophoretic mobility of the tritosomes. Tritosome electrophoretic mobilities indicated a homogeneous tritosome population and varied greatly with ionic strength of the suspending media. pH vs. electrophoretic mobility curves indicated the tritosome periphery to contain an acid-dissociable group which likely represents the carboxyl group of N-acetylneuraminic acid; this was not conclusively proven, however, since the tritosomes lysed below a pH of 4 in the present system. Total tritosome carbohydrate (anthrone-positive material as glucose equivalents) was 0.19 mg/mg tritosome protein while total sialic acid was 3.8 µg (11.4 nmol)/mg tritosome protein. A tritosome "membrane" fraction was prepared by osmotic shock, homogenization, and sedimentation. Approximately 25% of the total tritosome protein was present in this fraction. Analysis by gas-liquid chromatography and amino acid analyzer showed the following carbohydrate composition of the tritosome membrane fraction (in microgram per milligram tritosome membrane protein): N-acetylneuraminic acid, 14.8 ± 3; glucosamine, 24 ± 3; galactosamine, 10 ± 2; glucose, 21 ± 2; galactose, 26 ± 2; mannose, 31 ± 5; fucose, 7 ± 1; xylose, 0; and arabinose, 0. The results indicate that the tritosome periphery is characterized by external terminal sialic acid residues and an extensive complement of glycoconjugates. Essentially all the tritosome N-acetylneuraminic acid is located in the membrane and about 53% of it is neuraminidase susceptible.