Determination of the charge content at the surface of cells using a colloid titration technique

A colloid titration technique has been used to estimate the surface charge content of three distinct cell types of differing surface charge characteristics, i.e., human red blood cells, the surface of which is studded with sialic acid residues, endothelial cells which are surrounded by a thick glycocalyx, and chondrocytes which, when grown at high cell density for several weeks, synthesize a dense pericellular matrix similar to that observed in cartilaginous tissues. Estimates of the charge content obtained for human erythrocyte ghosts and cultured endothelial cells are in good agreement with the charge determined by chemical analyses. On the other hand, the charge at the surface of chondrocytes represented only a fraction of that calculated from measurements of the glycosaminoglycan content (15% by Day 12 in culture); close correlation between charge and chemical amount could be obtained only after digestion of the cell layer with collagenase and papain indicating the possible electrostatic involvement of the negative groups of the glycosaminoglycan chains with basic proteins. Thus, the colloid titration technique may provide a simple and sensitive tool to study the interactions occurring between the extracellular matrix components while the matrix is being formed and to establish the chemical nature of the molecules contributing to the cell surface charge.