Chemical diversity in the sialic acids and related alpha-keto acids: an evolutionary perspective

Gangliosides with O-acetylated sialic acids in tumors of neuroectodermal origin

Gangliosides, carrying an O-acetylated sialic acid in their carbohydrate moiety, are often found in growing and developing tissues, especially of neuro-ectodermal origin. The most prominent one is 9-O-Ac-GD3, which is considered as an oncofetal marker in animal and human tumors like neuronal tumors, melanoma, basalioma or breast cancer, as well as in psoriatic lesions. Also other gangliosides like GD2 or GT3 were found to be O-acetylated in their terminal sialic acid. In this review we are summarising the occurrence of such gangliosides in normal and transformed tissues and delineate a more general theory that O-acetylated sialic acids in gangliosides are a universal marker for growing cells and tissues.

To sialylate, or not to sialylate: that is the question

Most oropharyngeal pathogens express sialic acid units on their surfaces, mimicking the sialyl-rich mucin layer coating epithelial cells and the glycoconjugates present on virtually all host cell surfaces and serum proteins. Unlike the host's cells, which synthesize sialic acids endogenously, several microbial pathogens use truncated sialylation pathways. How microorganisms regulate sialic acid metabolism to ensure an adequate supply of free sugar for surface remodeling is a new area of research interest to basic scientists and those focused on the clinical outcome of the host-pathogen interaction.

Sialylglycoconjugates and sialyltransferase activity in the fungus Cryptococcus neoformans

Cryptococcus neoformans is a fungal pathogen associated with systemic mycoses in up to 10% of AIDS patients. C. neoformans yeasts express sialic acids on the cell wall, where they play an anti-phagocytic role, and may represent a virulence factor at the initial phase of infection. Since the nature of the sialic acid-carrying components is undefined in C. neoformans, our aim in the present work was to identify sialylated molecules in this fungus and study the sialylation process. C. neoformans yeast forms were cultivated in a chemically defined medium free of sialic acids, to search for autologous sialylglycoconjugates. Sialylated glycolipids were not detected. Two glycoproteins with molecular masses of 38 and 67 kDa were recognized by Sambucus nigra agglutinin, an alpha2,6-sialic acid-specific lectin. The 67 kDa glycoprotein also interacted with Influenza C virus, but not with Limax flavus agglutinin, suggesting the presence of the 9-O-acetylated sialic acid derivative as a constituent of the oligosaccharide chains. A partially purified protein fraction from cryptococcal yeast forms was able to transfer sialic acid from CMP-Neu5Ac to both N-(acetyl-1-(14)C)-lactosamine and asialofetuin. Additional evidence for a sialyltransferase in C. neoformans was obtained through the reactivity of fungal proteins with rabbit anti-rat alpha2,6 sialyltransferase polyclonal antibody. Our results indicate that sialic acids in C. neoformans are linked to glycoproteins, which are sialylated by the action of a fungal sialyltransferase. This is the first demonstration of this biosynthetic step in pathogenic fungi.

Loss of N-glycolylneuraminic acid in humans: Mechanisms, consequences, and implications for hominid evolution

The surface of all mammalian cells is covered with a dense and complex array of sugar chains, which are frequently terminated by members of a family of molecules called sialic acids. One particular sialic acid called N-glycolylneuraminic acid (Neu5Gc) is widely expressed on most mammalian tissues, but is not easily detectable on human cells. In fact, it provokes an immune response in adult humans. The human deficiency of Neu5Gc is explained by an inactivating mutation in the gene encoding CMP-N-acetylneuraminic acid hydroxylase, the rate-limiting enzyme in generating Neu5Gc in cells of other mammals. This deficiency also results in an excess of the precursor sialic acid N-acetylneuraminic acid (Neu5Ac) in humans. This mutation appears universal to modern humans, occurred sometime after our last common ancestor with the great apes, and happens to be one of the first known human-great ape genetic differences with an obvious biochemical readout. While the original selection mechanisms and major biological consequences of this human-specific mutation remain uncertain, several interesting clues are currently being pursued. First, there is evidence that the human condition can explain differences in susceptibility or resistance to certain microbial pathogens. Second, the functions of some endogenous receptors for sialic acids in the immune system may be altered by this difference. Third, despite the lack of any obvious alternate pathway for synthesis, Neu5Gc has been reported in human tumors and possibly in human fetal tissues, and traces have even been detected in normal human tissues. One possible explanation is that this represents accumulation of Neu5Gc from dietary sources of animal origin. Finally, a markedly reduced expression of hydroxylase in the brains of other mammals raises the possibility that the human-specific mutation of this enzyme could have played a role in human brain evolution.

Oxidative deamination of amino sugars: recent advances

An overview of the oxidative deamination of N-acetylneuraminic acid derivatives (Neu5Ac) leading to the formation of ketodeoxynonulosonic acid (KDN), its stereoisomers and glycosides is presented. A brief historical introduction to the deamination is given, followed by a description of recent advances in reaction conditions, which have allowed application of the process to Neu5Ac thioglycosides, and that have enabled the range of nucleophiles incorporated in the course of the reaction to be extended beyond the original acetate and azide. Recent advances resulting in derivatization of the Neu55Ac 4-position concomitant with replacement of the acetamido group, via the presumed intermediacy of a vinyl diazonium ion, are then described. The literature on the mechanism of the deamination reaction is next considered leading to the presentation of an overall mechanistic framework that accounts for all observations to date. Finally, the application of the deamination reaction to complex Neu5Ac-based oligosaccharides and other aminosugars is presented.