Lipid-associated sialic acid

Monitoring of the levels of total sialic acid and lipid-associated sialic acid in serum is an important diagnostic tool for the detection of precancerous lesions [224], Transition-state analogues derived from sialic acid are potent inhibitors of sialidases from various influenza virus strains, and these have been evaluated clinically as influenza remedies [225], A detailed review of our current knowledge regarding the occurrence, specificity and function of sialic acid-specific lectins, particularly those that occur in viruses, bacteria and non-vertebrate eukaryotes has been recently published [226],... 

Other tumor markers—including catecholamines, polyamines, lipid-associated sialic acid, and receptors— have been used clinically with various degrees of success (Table 23-14). Receptors are probably the most successful of this group of markers. The catecholamines and their metabolites are discussed in Chapter 29. 

A unique, membrane-associated, sialyltransferase system in E. coli has been described that catalyzes the synthesis of sialic acid polymers with the aid of sialyl-monophosphonoundecaprenol.293 In mammalian systems, no evidence was obtained lor the involvement of lipid intermediates in sialic acid transfer, or in the fonnation of sialyl-(2— -8) link-... 

In addition to the lipid bilayer, enveloped viruses generally have two or more distinct layers of protein that are organized across the membrane. Thus, most viruses have an outer layer of proteins, usually glycoproteins, which are anchored in the membrane as integral membrane proteins. These proteins function to attach the virion to target host cell receptors and facilitate the entry or fusion of the viral membrane with that of the host cell. In addition, some viruses also contain enzymatic activities associated with this outer layer of protein. For example, influenza virus carries with it a neuraminidase that is responsible for cleaving sialic acid residues on host cells. 

Although lipid barriers are common outside the capsids of viruses (and inside as well for poxviruses), no lipids are associated with adenoviruses or reoviruses. Enzymes are frequently present. Neuraminidase, in myxoviruses, releases sialic acid 5.15) from a glycopeptide widely present in mucus. Vaccinia virus (a poxvirus) contains the enzyme RNA polymerase which enters the host cell at the time of infection. A herpesvirus has adenosine triphosphatase. Lysozyme is common in phages. Carbohydrates, other than the pentose that forms part of the nucleic acids, have often been found. Analysis of purified herpes simplex virus showed proteins (70%), phospholipids (22%), DNA (6%), and carbohydrates (1.6%). 

The lipoprotein complexes which are involved in lipid transport (MoRRisErret al. 1975, ScANU et al. 1975) consist of well-defined apoproteins which in part contain sialic acids in addition to gangliosides in smaller amounts. These functionally important sialic acids appear to be associated with complex type oligosaccharide chains linked to asparagine (Swaninathan and Aladjem 1976), and study of the different apoproteins has revealed a specific distribution with 0, 1 or 2 sialic acid molecules per mole of apoprotein (Scanu et al. 1975). 

---