Intracellular fate of a multivalent ligand covalently bound to cell surface components. An electron microscopic study.

The electron dense tracer ferritin hydrazide (FH) was covalently attached to sialoglycoconjugates of the luminal surface of capillary endothelium in rat pancreas, after chemical modification of sialyl residues. Mild oxidation of the vasculature by perfusion with 1 mM sodium periodate was followed by a 30-90 min incubation with the tracer, at 37 degrees C. More than 60% of coated pits and coated vesicles were marked by FH, although in only approximately 25% of these structures the label density was higher than on adjacent domains of plasmalemma proper. Intracellularly, even after prolonged (90 min) perfusion of the vasculature with FH, the tracer was detected only in coated vesicles and in a few smooth-surfaced vesicles, but not in multivesicular bodies and endosomes. Cationized ferritin (CF), pI approximately 8.4, which binds to cell surface acidic sites (including sialoglycoconjugates) via an electrostatic noncovalent interaction, was rapidly internalized by the oxidized endothelium in coated vesicles and vacuoles. Unlike FH, after 30 min CF was already found in some multivesicular bodies and large vacuoles. At 60 to 90 min, most of multivesicular bodies contained the tracer. Similarly, a conjugate of wheat germ agglutinin with ferritin, which binds noncovalently to cell surface glycoconjugates containing N-acetylneuraminyl and N-acetylglucosaminyl residues, was internalized by the capillary endothelium in coated vesicles and transported to multivesicular bodies. It is concluded that FH, being unable to dissociate from its binding sites, is stopped on the endocytotic route in a compartment prior to its delivery to multivesicular bodies, thus escaping lysosomal degradation.