In situ 125I-labelling of endosome proteins with lactoperoxidase conjugates.

Asialoorosomucoid was conjugated to lactoperoxidase and bound specifically to the asialoglycoprotein receptor on the human cell line Hep G2 at 4 degrees C. The bound conjugates incorporated 125I into cell surface proteins in the presence of H2O2. When Hep G2 cells were allowed to endocytose the prebound conjugates by warming to 37 degrees C for 10 min or were incubated for 1 h at 23 degrees C in the presence of conjugate, addition of 125I and H2O2 at 4 degrees C now resulted in labelling of endocytic vesicle proteins. The cell surface labelling pattern and the endosome labelling pattern were compared and found to be distinct. A major component labelled by the endocytosed asialoorosomucoid conjugate is shown to be the transferrin receptor. This protein and a component of 230 000 daltons are enriched in the endosome relative to the cell surface. The endocytosed lactoperoxidase conjugate was also visualised at the morphological level. Characteristic endosome tubules and vesicles contained electron-dense peroxidase reaction product as did cell surface coated pits. Selective capture of some cell surface proteins but not others by coated pits presumably gives rise to the distinct polypeptide composition of the endosome.