Presence in human erythrocyte membranes of a novel form of sialidase acting optimally at neutral pH.

The feature of intact human erythrocytes and erythrocyte white ghosts is a unique sialidase activity with acidic optimal pH (acidic sialidase). The treatment of white ghosts with mildly alkaline isotonic solutions at 37 degrees C, like that used to produce resealed ghosts, is accompanied by the expression, together with the acidic sialidase, of a novel sialidase with a pH optimum of 7.2 (neutral sialidase) that remained masked in the inside-out vesicles prepared from white ghosts. Exhaustive treatment of resealed ghosts with Bacillus Thuringiensis phosphatidylinositol-specific phospholipase C causes an almost complete release of the acidic sialidase, with the neutral enzyme remaining totally unaffected. The treatment of resealed ghosts with 1.2% Triton X-100 resulted in the solubilization of only the neutral sialidase, whereas 3.6% octylglucoside also solubilized the acidic sialidase. The neutral enzyme affected not only the artificial substrate but also any sialoderivatives of a ganglioside, glycoprotein, and oligosaccharide nature; the acidic enzyme did not affect sialoglycoproteins. Erythrocyte endogenous gangliosides were hydrolyzed by both sialidases, whereas the endogenous sialoglycoproteins responded to only the neutral enzyme. It was definitely proved that the acidic sialidase is located on the outer erythrocyte membrane surface, so presumably the neutral enzyme has the same location. It could be that the newly discovered neutral sialidase has a physiologic role in the releasing of sialic acid from erythrocytes during the erythrocyte aging process, leading to eventual phagocytosis by macrophages.