Sialuria sialic acid metabolism

Three different rare genetic metabolic defects in sialic acid metabolism are known, as indicated in Fig. 4.3.2 [3, 21] (1) free sialic acid storage disease (SASD Online Mendelian Inheritance in Man, OMIM 604369, 269920), a lysosomal membrane transporter defect (2) sialuria (OMIM 269921), a feedback inhibition defect in sialic acid biosynthesis (3) sialidosis (OMIM 256550), a breakdown defect of sialyloli-gosaccharides caused by a defect of lysosomal sialidase. In all these genetic defects, an increased amount of sialic acid can be found in tissues and or body fluids, either bound to OGSs as in (3), or in its free state as in (1) and (2). 

Fig. 22.3. Metabolism of free N-acetylneuraminic acid (= sialic acid) UDP-acetylglucos-amine 2-epimerase is inhibited by the activated sialic acid donor, CMP-N-acetylneur-aminic acid, which also provides N-acetylneuraminic acid for glycoconjugate synthesis. In sialuria (22.6) loss of feedback inhibition leads to overproduction of sialic acid. In sialic acid storage disorders (22.5.1 and 22.5.2) N-acetylneuraminic acid accumulates in the lysosomes due to a defect of a specific transporter (sialin)...

Procedures have been described for the assay of protein-bound carbohydrate in sera and urine from normal children and from patients with sialuria. It was concluded that sialuria is not associated with an anomalous metabolism of glycoproteins, but with an anomalous biosynthesis of sialic acid. 

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