Sialic acid

Abstract

Sialic acids are acid monosaccharides mainly present as terminal sugars of oligosaccharide chains of glycoproteins and glycolipids. Their strong negative charge plays an important role in a wide variety of biological processes, like cell adhesion and protection. Three clinically different rare human genetic defects in sialic acid metabolism are known, (1) free sialic acid storage disease (SASD), a lysosomal membrane transporter defect, (2) sialuria, a feedback inhibition defect in biosynthesis and (3) sialidosis, a lysosomal breakdown defect of sialyloligosaccharides. All these defects are characterized by increased amounts of sialic acid (free or oligosaccharide-bound) in tissues or body fluids. Their biochemical diagnosis is based on the demonstration and quantification of free sialic acid (in SASD and sialuria) or oligosaccharide-bound sialic acid (in Sialidosis) in urine and cultured fibroblasts. This chapter describes the diagnostic workflow in the metabolic laboratory upon the clinical suspicion of a sialic acid metabolism defect. First, qualitative screening for free or bound sialic acid is performed on urine samples using Thin-Layer Chromatography (TLC). The next required step is quantitative sialic acid determination in urine and cultured fibroblasts. The last step is enzyme activity measurement or molecular analysis. © 2008 Springer-Verlag.