Red cell aging. I. Surface charge density and sialic acid content of density-fractionated human erythrocytes.

It has been suggested that decreases in red cell surface-charge density accompany aging in vivo and reflect alterations in the cell surface which play a critical role in the recognition and elimination of effete erythrocytes by macrophages in the reticuloendothelial system. The bases for this suggestion are reports that the surfacecharge density progressively decreases for red cell subpopulations sampled from regions of increasing density within the whole population where the least dense fractions are enriched in young erythrocytes and the most dense in old erythrocytes. We have attempted to reproduce these results and have examined the relationship between cell surface sialic acid and cell surface-charge density for the extreme density fractions of fresh human red cells. Contrary to the earlier reports, we observed no differences in net surfacecharge density for the extreme 5% fractions based on the electrophoretic behavior of the cells. However, slightly lower levels of sialic acid were consistently observed in the densest red cell subpopulations. These observations and other cited evidence are consistent with the view that during its life span in vivo, portions of the red cell membrane are lost as the result of innumerable cell-cell and cell-vessel wall contacts during the passage of the cells through the circulatory system. Such losses would account for decreases in the level of membrane constituents per cell, such as sialic acid, but would not require that the concentration of those constituents be altered in the remaining membrane. Thus such features as surface-charge density could remain unchanged even though the total sialic acid content per cell was reduced.