Surface charge distribution on the endothelial cell of liver sinusoids.

Abstract

The topography of the charged residues on the endothelial cell surface of liver sinusoid capillaries was investigated by using EM tracers of different size and charge. The tracers used were native ferritin (pI [isoelectric point] 4.2-4.7), and its cationized (pI 8.4) and anionized (pI 3.7) derivatives, BSA [bovine serum albumin] coupled to colloidal Au (pI of the complex 5.1), hemeundecapeptide (pI 4.85), and alcian blue (pI > 10). The tracers were either injected in vivo or perfused in situ through the portal vein of the mouse liver. In some experiments, 2 tracers of opposite charge were sequentially perfused with extensive washing in between. The liver was processed for EM and the binding pattern of the injected markers was recorded. The electrostatic nature of the tracer binding was assessed by perfusion with high ionic strength solutions, by aldehyde quenching of the plasma membrane basic residues and by substituting the cell surface acidic moieties with positively charged groups. Results indicate that the endothelial cells of the liver sinusoids expose on their surface both cationic and anionic residues. The density distribution of these charged groups on the cell surface is different. While the negative charge is randomly and patchily scattered all over the membrane, the cationic residues seem to be accumulated in coated pits. The charged groups coexist in the same coated pit and bind the opposite charged macromolecule. It appears that the fixed positive and negative charges of the coated pit glycocalyx are mainly segregated in space. The layer of basic residues is located at 20-30 nm distance of the membrane, while most of the negative charges lie close to the external leaflet of the plasmalemma.