Some questions and suggestions on the type references of the official nomenclature (IUB) for sialidase(s) and endosialidase

Introduction of Carbonyl Groups into Antibodies

Antibodies and their derivatives (scFv, Fabs, etc.) represent a unique class of biomolecules that combine selectivity with the ability to target drug delivery. Currently, one of the most promising endeavors in this field is the development of molecular diagnostic tools and antibody-based therapeutic agents, including antibody-drug conjugates (ADCs). To meet this challenge, it is imperative to advance methods for modifying antibodies. A particularly promising strategy involves the introduction of carbonyl groups into the antibody that are amenable to further modification by biorthogonal reactions, namely aliphatic, aromatic, and α-oxo aldehydes, as well as aliphatic and aryl-alkyl ketones. In this review, we summarize the preparation methods and applications of site-specific antibody conjugates that are synthesized using this approach.

New dihydrobenzoxanthone derivatives with bacterial neuraminidase inhibitory activity isolated from Artocarpus elasticus

Artocarpus elasticus is a popular fruit tree in the tropical regions. Primary screenings of methanol extracts of the root bark confirmed its potent inhibition of bacterial neuraminidase (BNA), which plays an essential role in the pathogenesis of many microbial diseases. Assessments of the responsible phytochemicals were conducted by isolating eight compounds (1-8) and two of them (6 and 8) were identified as new compounds. Among the isolates, the dihydrobenzoxanthones attained the highest BNA inhibition with IC₅₀ values of 0.5 ∼ 3.9 µM. Further investigation of the inhibitory mechanism by Lineweaver-Burk plots revealed the phytochemicals to function as reversible noncompetitive inhibitors. Fluorescence quenching showed their binding affinities were highly correlated with their inhibitory potential dose-dependently. Molecular docking experiments suggested the dihydrobenzoxanthones (4 and 6) as noncompetitive inhibitors of BNA with unique interaction with Tyr435 of BNA in comparison with the mother flavonoid (7).

Mass Spectrometry Imaging for Glycome in the Brain

Glycans are diverse structured biomolecules that play crucial roles in various biological processes. Glycosylation, an enzymatic system through which various glycans are bound to proteins and lipids, is the most common and functionally crucial post-translational modification process. It is known to be associated with brain development, signal transduction, molecular trafficking, neurodegenerative disorders, psychopathologies, and brain cancers. Glycans in glycoproteins and glycolipids expressed in brain cells are involved in neuronal development, biological processes, and central nervous system maintenance. The composition and expression of glycans are known to change during those physiological processes. Therefore, imaging of glycans and the glycoconjugates in the brain regions has become a “hot” topic nowadays. Imaging techniques using lectins, antibodies, and chemical reporters are traditionally used for glycan detection. However, those techniques offer limited glycome detection. Mass spectrometry imaging (MSI) is an evolving field that combines mass spectrometry with histology allowing spatial and label-free visualization of molecules in the brain. In the last decades, several studies have employed MSI for glycome imaging in brain tissues. The current state of MSI uses on-tissue enzymatic digestion or chemical reaction to facilitate successful glycome imaging. Here, we reviewed the available literature that applied MSI techniques for glycome visualization and characterization in the brain. We also described the general methodologies for glycome MSI and discussed its potential use in the three-dimensional MSI in the brain.

Sialidase substrates for Sialdiase assays - activity, specificity, quantification and inhibition

Sialidases are glycosidases responsible for the removal of sialic acid (Sia) residues (desialylation) from glycan portions of either glycoproteins or glycolipids. By desialylation, sialidases are able to modulate the functionality and stability of the Sia-containing molecules and are involved in both physiological and pathological pathways. Therefore, evaluation of sialidase activity and specificity is important for understanding the biological significance of desialylation by sialidases and its function and the related molecular mechanisms of the physiological and pathological pathways. In addition, it is essential for developing novel mechanisms and approaches for disease treatment and diagnosis and pathogen detection as well. This review summarizes the most recent sialidase substrates for evaluating sialidase activity and specificity and screening sialidase inhibitors, including (i) general sialidase substrates, (ii) specific sialidase substrates, (iii) native sialidase substrates and (iv) cellular sialidase substrates. This review also provides a brief introduction of recent instrumental methods for quantifying the sialidase activity, such as UV, fluorescence, HPLC and LC-MS methods.

Exploration of the Sialic Acid World

Sialic acids are cytoprotectors, mainly localized on the surface of cell membranes with multiple and outstanding cell biological functions. The history of their structural analysis, occurrence, and functions is fascinating and described in this review. Reports from different researchers on apparently similar substances from a variety of biological materials led to the identification of a 9-carbon monosaccharide, which in 1957 was designated "sialic acid." The most frequently occurring member of the sialic acid family is N-acetylneuraminic acid, followed by N-glycolylneuraminic acid and O-acetylated derivatives, and up to now over about 80 neuraminic acid derivatives have been described. They appeared first in the animal kingdom, ranging from echinoderms up to higher animals, in many microorganisms, and are also expressed in insects, but are absent in higher plants. Sialic acids are masks and ligands and play as such dual roles in biology. Their involvement in immunology and tumor biology, as well as in hereditary diseases, cannot be underestimated. N-Glycolylneuraminic acid is very special, as this sugar cannot be expressed by humans, but is a xenoantigen with pathogenetic potential. Sialidases (neuraminidases), which liberate sialic acids from cellular compounds, had been known from very early on from studies with influenza viruses. Sialyltransferases, which are responsible for the sialylation of glycans and elongation of polysialic acids, are studied because of their significance in development and, for instance, in cancer. As more information about the functions in health and disease is acquired, the use of sialic acids in the treatment of diseases is also envisaged.

Endosialidases: Versatile Tools for the Study of Polysialic Acid

Polysialic acid is an α2,8-linked N-acetylneuraminic acid polymer found on the surface of both bacterial and eukaryotic cells. Endosialidases are bacteriophage-borne glycosyl hydrolases that specifically cleave polysialic acid. The crystal structure of an endosialidase reveals a trimeric mushroom-shaped molecule which, in addition to the active site, harbors two additional polysialic acid binding sites. Folding of the protein crucially depends on an intramolecular C-terminal chaperone domain that is proteolytically released in an intramolecular reaction. Based on structural data and previous considerations, an updated catalytic mechanism is discussed. Endosialidases degrade polysialic acid in a processive mode of action, and a model for its mechanism is suggested. The review summarizes the structural and biochemical elucidations of the last decade and the importance of endosialidases in biochemical and medical applications. Active endosialidases are important tools in studies on the biological roles of polysialic acid, such as the pathogenesis of septicemia and meningitis by polysialic acid-encapsulated bacteria, or its role as a modulator of the adhesion and interactions of neural and other cells. Endosialidase mutants that have lost their polysialic acid cleaving activity while retaining their polysialic acid binding capability have been fused to green fluorescent protein to provide an efficient tool for the specific detection of polysialic acid.

Comparative enzymology, biochemistry and pathophysiology of human exo-alpha-sialidases (neuraminidases)

This review summarizes the current research on human exo-alpha-sialidase (sialidase, neuraminidase). Where appropriate, the properties of viral, bacterial, and human sialidases have been compared. Sialic acids are implicated in diverse physiological processes. Sialidases, as enzymes acting upon sialic acids, assume importance as well. Sialidases hydrolyze the terminal, non-reducing, sialic acid linkage in glycoproteins, glycolipids, gangliosides, polysaccharides, and synthetic molecules. Therefore, a variety of assays are available to measure sialidase activity. Human sialidase is present in several organs and cells. Its cellular distribution could be cytosolic, lysosomal, or in the membrane. Human sialidase occurs in a high molecular-mass complex with several other proteins, including cathepsin A and beta-galactosidase. Multi-protein complexation is important for the in vivo integrity and catalytic activity of the sialidase. However, multi-protein complexation, the occurrence of isoenzymes, diverse subcellular localization, thermal instability, and membrane association have all contributed to difficulties in purifying and characterizing human sialidases. Human sialidase isoenzymes have recently been cloned and sequenced. Even though crystal structures for the human sialidases are not available, the highly conserved regions of the sialidase from various organisms have facilitated molecular modeling of the human enzyme and raise interesting evolutionary questions. While the molecular mechanisms vary, genetic defects leading to human sialidase deficiency are closely associated with at least two well-known human diseases, namely sialidosis and galactosialidosis. No therapy is currently available for either disease. A thorough investigation of human sialidases is therefore crucial to human health.

Evaluation of the activities of eight lysosomal hydrolases in sera of humans, rats and pigs of different ages

The activities of 21 enzymes (belonging to four classes of enzymes) involved in different metabolic pathways were assayed in blood sera of healthy young and adult/elderly groups of humans, rats and pigs, to determine whether activity changes coinciding with changes in age and aging could be detected. In all three species analysed, measurable activities (performed by highly specific and sensitive techniques, generally spectrofluorimetric procedures) were found, usually following a decreasing order of: among glycosidases, beta-N-acetylglucosaminidase (NAG) > alpha-L-fucosidase > alpha-mannosidase > beta-glucuronidase > beta-galactosidase > alpha-galactosidase. In addition, among esterases very high values were found for arylesterase and acid phosphatase. By contrast, no measurable activity was found for the remaining enzymes assayed (8 hydrolases, 1 oxidoreductase, 3 transferases and 1 lyase). In the elderly group of humans, significantly higher activities (P < or = 0.05) were found for NAG, alpha-mannosidase and beta-glucuronidase in comparison to the adult and young groups. However, several activities in rats and all activities in pigs decreased with age. In conclusion, differences in the activities of 6 lysosomal glycosidases and 2 esterases (but no significant differences for another 13 enzymes belonging to several enzyme classes) are found in the sera of healthy humans, rats and pigs. These differences coincide with changes observed in aging.

Human erythrocyte sialidase is linked to the plasma membrane by a glycosylphosphatidylinositol anchor and partly located on the outer surface

Treatment of human erythrocyte ghosts with phosphatidylinositol-phospholipase C (PIPLC) from Bacillus cereus liberated the ghost-linked sialidase. Maximal release of sialidase (about 70% of total) was achieved by incubating ghosts at 37 degrees C for 60 min, at pH 6.0, with PIPLC (PIPLC total units/ghost protein ratio, 4.5 each time) added at the beginning of incubation and every 15 min (four subsequent additions). Liberated sialidase was fully resistant to at least four cycles of rapid freezing and thawing and to storage at 4 degrees C for at least 48 h. The liberated enzyme had an optimal activity at pH 4.2, degraded ganglioside GD1a better than methylumbelliferyl N-acetylneuraminic acid (about fourfold), and gave a Km value of 2.56 x 10(-4) M and an apparent Vmax of 2.22 mU per mg protein on GD1a. Treatment of intact erythrocytes with PIPLC (PIPLC total units/erythrocyte protein ratio, 8), under conditions where haemolysis was practically negligible, caused liberation of 10-12% of membrane linked sialidase, indicating that the enzyme is, at least in part, located on the outer surface of the erythrocyte membrane. It is concluded that the erythrocyte membrane sialidase is anchored by a glycosylphosphatidylinositol structure sensitive to PIPLC action, and is partly located on the outer surface.

Increased influenza A virus sialidase activity with N-acetyl-9-O-acetylneuraminic acid-containing substrates resulting from influenza C virus O-acetylesterase action

Influenza virus type C (Johannesburg/1/66) was used as a source for the enzyme O-acetylesterase (EC 3.1.1.53) with several natural sialoglycoconjugates as substrates. The resulting products were immediately employed as substrates using influenza virus type A [(Singapore/6/86) (H1N1) or Shanghai/11/87 (H3N2)] as a source for sialidase (neuraminidase, EC 3.2.1.18). A significant increase in the percentage of sialic acid released was found when the O-acetyl group was cleaved by O-acetylesterase activity from certain substrates (bovine submandibular gland mucin, rat serum glycoproteins, human saliva glycoproteins, mouse erythrocyte stroma, chick embryonic brain gangliosides and bovine brain gangliosides). A common feature of all these substrates is that they contain N-acetyl-9-O-acetylneuraminic acid residues. By contrast, no significant increase in the release of sialic acid was detected when certain other substrates could not be de-O-acetylated by the action of influenza C esterase, either because they lacked O-acetylsialic acid (human glycophorin A, alpha 1-acid glycoprotein from human serum, fetuin and porcine submandibular gland mucin) or because the 4-O-acetyl group was scarcely cleaved by the viral O-acetylesterase (equine submandibular gland mucin). The biological significance of these facts is discussed, relative to the infective capacity of influenza C virus.