The Disialosyl Group of Glycoproteins

Abstract

A sensitive method for the determination of disialosyl (α‐N‐acetylneuraminyl‐(2‐8)‐N‐acetyl‐neuraminyl) groups was developed. The method is based on the quantitation of the amount of 8‐O‐substituted neuraminic acid by methylation analysis using mass fragmentographic detection with two different m/e values. Lability of the 8‐O‐substituent of neuraminic acid to neuraminidase is taken as an indication for the presence of a disialosyl sequence.Considerable differences in the amount of disialosyl groups were observed between glycopeptides from different tissues of the rat. The proportion of 8‐O‐substituted neuraminic acid as compared to total glycopeptide‐bound neuraminic acid was 8.5% in brain, 3.1% in liver, 2.6% in diaphragmatic muscle and 3.0% in small intestinal mucosa, whereas glycopeptides derived from kidney, gastric mucosa, erythrocytes and plasma did not contain disialosyl groups.The disialosyl groups were enriched in glycoproteins of the membranous fractions of liver and brain, whereas only small amounts were observed in the soluble fractions. The proportion of glycopeptide‐bound 8‐O‐substituted neuraminic acid in the brain of young rats (13.9%) was higher than in the brain of adult animals. The highest proportion of 8‐O‐substituted neuraminic acid (19.8%) was observed in glycopeptides of the plasma membrane fraction of the young rat brain.