Characterization of the carbohydrate structures of apolipoprotein H through concanavalin A affinity chromatography

Apo-H (beta-2-glycoprotein) intact N-glycan analysis by MALDI-TOF-MS using sialic acid derivatization

Apo-H is a plasma glycoprotein. Nearly 19% of the molecular weight of this protein is composed of glycans. Up- and down-regulation and structural changes in protein glycans provide diagnostic value for disease detection. Here, an efficient, sensitive, and optimized method is developed for Apo-H N-glycans analysis by MALDI-TOF-MS in positive mode. This bioanalytical method includes sample preparation, sample purification, and detection. An Apo-H enrichment method is developed using standard proteins by anti-Apo-H beads followed by enrichment from plasma samples. SDS-PAGE confirms the Apo-H protein enrichment, which is further verified by LC-MS/MS analysis. The lower ionization efficiency of sialylated glycan hampers their analysis by MALDI-MS. For this, stabilization of sialic acids is done by selective derivatization of carboxyl groups to differentiate between α(2,3)- and α(2,6)-linked sialic acids. Glycans are further purified by HILIC-SPE and analyzed by MALDI-MS. Several branched bi- and tri-antennary glycans with fucosylation and sialylation are identified. The reproducibility of the developed method is tested by analyzing multiple replicates of human plasma, where the same glycans are consistently identified. This method could be applied for the Apo-H glycan profiling of large clinical cohorts for diagnostic purposes.

Anti-phospholipid syndrome: Current opinion on mechanisms involved, laboratory characterization and diagnostic aspects

Anti-phospholipid syndrome is a complex and severe clinical situation, associated with symptoms such as recurrent thrombosis, arterial or venous, at any site, pregnancy loss, and other related syndromes. These clinical burdens, are highly variable from patient to patient, and are associated with biological abnormalities, such as the presence of the Lupus Anticoagulant or phospholipid dependent antibodies, confirmed on two occasions at least 12 weeks apart. From the diagnosis standpoint, both, functional (clotting) or immunological assays, are difficult to standardize and to optimize, due to the absence of reference material, or a characteristic clinical group, and international reference preparations. Large cohort studies are necessary for defining the usefulness of each assay, in terms of specificity, sensitivity, accuracy and for following-up the disease evolution. Clotting assays are based on Activated Partial Thromboplastin Time (APTT) and diluted Russell Viper Venom Time (dRVVT), performed at low and high phospholipid concentration, or on 1:1 mixtures of tested sample and a normal plasma pool. They allow evaluation of the paradoxal effects of LAs, which are pro-thrombotic in vivo, and anticoagulant in vivo. Use of synthetic phospholipids improves assay specificities and sensitivities, especially in patients treated with anticoagulants. Immunoassays can also be used for testing phospholipid dependent antibodies, first identified and measured as anti-cardiolipin antibodies, but now characterized as targeted to phospholipid cofactor proteins: mainly β2GP1 (which exposes cryptic epitopes upon binding to phospholipids), and in some cases prothrombin, and more rarely Protein S, Factor XIII, Protein Z or Annexin V. Use of optimized assays designed with well-characterized anionic phospholipids, then complexed with highly purified phospholipid cofactor protein (mainly β2GP1), offers a better link between reactivity and clinical associations, than the former assays which were empirically designed with cardiolipin. Standardization also remains complicated due to the absence of international standards and harmonized quantitation units. Validation on large cohorts of negative and positive patients remains the key approach for defining assay performance and clinical usefulness. Laboratory practice for all these methods is now greatly facilitated thanks to the use of automated instruments and dedicated software. Along with clinical criteria, laboratory assays are of great usefulness for identification and confirmation of the anti-phospholipid syndrome and they allow disease follow-up when appropriate patient management is in place.

Enrichment of glycoproteins using nanoscale chelating concanavalin A monolithic capillary chromatography

Immobilized lectin chromatography can be employed for glycoprotein enrichment, but commonly used columns have limitations of yield and resolution. To improve efficiency and to make the technique applicable to minimal sample material, we have developed a nanoscale chelating Concanavalin A (Con A) monolithic capillary prepared using GMA-EDMA (glycidyl methacrylate-co-ethylene dimethacrylate) as polymeric support. Con A was immobilized on Cu(II)-charged iminodiacetic acid (IDA) regenerable sorbents by forming a IDA:Cu(II):Con A sandwich affinity structure that has high column capacity, as well as stability. When compared with conventional Con A lectin chromatography, the monolithic capillary enabled the better reproducible detection of over double the number of unique N-glycoproteins in human urine samples. Utility for analysis of minimal biological samples was confirmed by the successful elucidation of glycoprotein profiles in mouse urine samples at the microliter scale. The improved efficiency of the nanoscale monolithic capillary will impact the analysis of glycoproteins in complex biological samples, especially where only limited material may be available.

Preparation of a monolithic capillary column with immobilized α-mannose for affinity chromatography of lectins

A simple method for the preparation of an affinity monolithic (also called continuous bed) capillary column for alpha-mannose-specific lectins is described. 2-Hydroxyethyl methacrylate in combination with (+)-N,N -diallyltartardiamide (DATD) and piperazine diacrylamide (PDA, 1,4-bisacryloyl-piperazine) as crosslinkers, were used as monomers for the monolith. After oxidation of DATD with periodate, alpha-mannose with spacer was bound to the aldehyde groups of the polymeric skeleton via reductive amination to form an affinity column for the separation, enrichment or binding studies of mannose-specific lectins. The permeability of the column was excellent. The porosity of the monolith was investigated by scanning electron microscope (SEM) and inverse size exclusion chromatography (ISEC). The affinity of the monolith was evaluated by frontal analysis (FA) and fluorescence microscopy (FM) using fluorescently labeled concanavalin (Con A). Frontal affinity chromatography showed a specific interaction of two different lectins with the alpha-mannose-modified monolith. According to FM the affinity sites were evenly distributed over the monolithic bed.

Enzymatic Polymerization of Phenolic Compounds Using Laccase and Tyrosinase from Ustilago maydis

Flavonoids are a big group of polyphenols of low molecular weight with in vitro antioxidant properties. In this study, the laccase and tyrosinase from Ustilago maydis were partially characterized and their effect on the antioxidant activity of some phenolic compounds was investigated. Since enzymatic polymerization of the phenolic compounds was detected, the size of the aggregates was determined and related to their antioxidant activity. Morphology of the polymers was analyzed by atomic force microscopy. The results showed that the laccase- and tyrosinase-catalyzed polymerization of quercetin produced aggregates with relatively low molecular weight and higher antioxidant activity than the monomeric quercetin. In the case of kaempferol, the aggregates reached higher sizes in the first 2 h of reaction and their antioxidant activity was increased. In the last case, the aggregates adopted fractal-ordered shapes similar to coral in the case of the kaempferol-laccase system and to fern in the case of the kaempferol-tyrosinase system. The kaempferol and quercetin polymers at low concentration had strong scavenging effect on Reactive oxygen species (ROS) and inhibition of lipoperoxidation in human hepatic cell line WRL-68.

Purification and characterization of sea squirt α-N-acetylgalactosaminidase

Sea squirt alpha-N-acetylgalactosaminidase was purified to homogeneity. Its molecular weight was estimated to be approximately 160,000 by gel filtration and 40,000 by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) under reducing condition. The chromatographic and electrophoretic behaviors indicated that the enzyme was composed of four subunits. The optimum pH of the enzyme reaction was about 4.0 at 37 degrees C, while the enzyme was stable in the range of pH 5.0 to 6.0 during 4 h preincubation at 37 degrees C. Although the enzyme (0.1 unit) was stable at 0 degrees C for 30 min in the presence of 7.5 mM metal ions (Al3+, Ba2+, Ca2+, K+, Mn2+, Pb2+, Sr2+, and Zn2+), almost 40% of the enzyme activity was lost in the presence of Cu2+, Hg2+, monoiodoacetic acid, and EDTA. The enzyme hydrolyzed aryl N-acetyl-alpha-D-galactosaminide as well as GalNAcalpha1(-->4GalNAcalpha1-->)n 4GalNAc-p-aminobenzoic acid ethyl ester (ABEE) (n = 1-4), but GalNAcalpha1-->4GalNAc-ABEE only scarcely. Furthermore, an allergenic pentasaccharitol ABEE derivative, GalNAcalpha1-->2Fucalpha1-->3(GalNAcbeta1-->4) GlcNAcbeta1-->2(3-acetoamido-3-deoxy)L-threose-ABEE, the minimum structural unit for the sea squirt allergenicity was hydrolyzed to 95 mol% for 72 h incubation with the enzyme. The enzyme could be utilized as a powerful tool for the structural analyses of the carbohydrate epitopes of the sea squirt allergen molecules.

Study of the glycosylation of apolipoprotein H

Apolipoprotein H is a single chain polypeptide composed of 326 amino acids highly glycosylated. Its carbohydrate content is approximately 19% of the molecular weight. We show that it is rich in sialic acid linked alpha (2-6) to galactose or N-acetylgalactosamine. Sialic acid is not alpha (2-3) linked to galactose. Galactose is beta (1-4) linked to N-acetylglucosamine and beta (1-3) linked to N-acetylgalactosamine. Carbohydrate O-linked chains (mainly sialic acid) are alpha (2-6) linked to galactose or N-acetylgalactosamine. Galactose is also organised in O-linked chains and beta (1-4) linked to N-acetylglucosamine and beta (1-3) linked to acetylgalactosamine. Concanavalin A lectin was used to isolate two groups of apolipoprotein H molecules bearing biantennary and truncated hybrids and high mannose and hybrid oligosaccharides. Apolipoprotein H fails to bind lysine-Sepharose. Our results thus show that it presents truncated hybrid or hybrid-type carbohydrate chains which bear few unmasked mannose residues as a terminal sugar. Biochemical analysis of carbohydrate structures conducted on single isoforms separated through IEF revealed that no specific carbohydrate complex is bound to a single isoform.