The characteristic separation of purified glycolipids into doublets, triplets, etc. by thin layer chromatography barely raises an eyebrow in these days of recombinant chimeras, gene transfer, signal transduction, phosphorylation cascades etc. However, evidence is beginning to accrue that the mundane complexity which provides the base for membrane glycolipids may modulate the function of the carbohydrate moiety. The assay of glycolipid function on artificial surfaces and the ease and propensity with which carbohydrate sequences are cleaved from their natural conjoiners and coupled to more (or less) inert foreign carriers may result in the loss of the fine, control in carbohydrate receptor function. Some recent experiments suggest that the fatty acid makes glycolipid carbohydrate receptor function at least several fold more complex then is apparent from consideration of the sugar sequence alone.
Sialic acids are acidic, 9-carbon carboxylated monosaccharides, typically expressed on the outermost end of the sugar chains of membrane and secreted glycoconjugates. This particular location provides it accessibility reflected in its regulation of a multitude of cellular and molecular interactions. Amongst the diverse structural variations that occur, the commonest is O-acetylation at C-4, 7, 8 and9 carbon positions. This diversity in O-acetylated sialic acid (O-AcSA) derivatives is attributed to their linkage to the underlying sugar chain, the carbon position that gets O-acetylated and nature of the glycoconjugates to which they are attached. Steady refinement of analytical techniques has allowed for identification of O-AcSA derivatives in several diseases leading to a shift in focus towards understanding the biological importance of this modification. The present review deals primarily with the recently evolved information regarding detection of altered O-acetylation in various pathophysiological conditions and their biological relevance. Particular attention is focused on two diverse diseases, namely, Acute Lymphoblastic Leukemia (ALL) and Visceral Leishmaniasis (VL) in which 9-O acetylated sialoglycoconjugates (9-OAcSGs) have proved to be effective biomarkers for monitoring their clinical status.
Important progress has been made recently in the development of analytical technology for O-linked oligosaccharide structure determination. Methods were developed for the chemical release (and subsequent labeling) of O-linked glycans from glycoproteins, e.g. by β-elimination or by hydrazinolysis. Different HPLC-methods and mass-spectrometry have become available as sensitive and sophisticated tools for the structural analysis of O-linked sugars. Site-specific glycan analysis is now technically achievable. Although functional analysis of O-linked glycans is still difficult, various roles have been established and some generalizations have emerged. O-linked glycans may play important roles maintaining the three-dimensional structure of the glycoprotein through intramolecular interactions, leading to the extension of the glycoprotein (O-linked). In some cases such domains function as molecular spacers and may fulfil the same role in membrane-bound receptors where the functional part of the receptor is oriented to the extracellular space because of the presence of glycans. Also O-linked sugars are essential in intermolecular interactions, e.g. in the recognition between glycoproteins. They can influence the activities of signaling molecules and enzymes, and are essential for cellular glycoprotein expression and processing. The alternative splicing of O-glycosylated Ser/Thr-rich domains confers an additional regulation level for the properties and activity of different glycoproteins. Additionally, involvement of alterations of O-linked glycosylation in health and disease is also relevant for further developments in human medicine.
DBGET/LinkDB is an integrated molecular biology database retrieval system developed through collaboration between the Institute for Chemical Research (ICR), Kyoto University and the Human Genome Center (HGC), University of Tokyo. The main server is located at ICR, and can be accessed via GenomeNet from all over the world. Mirror servers are located at HGC and Japan Advanced Institute of Science and Technology (JAIST). DBGET/LinkDB allows users not only to retrieve molecular biology databases, such as GenBank and SWISS-PROT by keywords, but also to retrieve information related to the result through the feature of the LinkDB, which is a compilation of link information between database entries. Because the easiest and the most popular usage of DBGET/LinkDB is to use WWW interface, we will explain here the usage of the system by WWW.