Fluid-phase marker transport in rat liver: free-flow electrophoresis separates distinct endosome subpopulations.

Free-flow electrophoresis (FFE) was used to investigate the intracellular compartments involved in fluid-phase marker, fluoresceine isothiocyanate (FITC)-dextran, transport in the isolated perfused rat liver. One to 2 min after uptake at 37 degrees C, FITC-dextran was found in endosomes with the same electrophoretic mobility as early sorting endosomes labeled either by the hepatocyte-specific marker asialoorosomucoid (ASOR) or by transferrin that enters all liver cells. Labeling at low temperature (16 degrees C) blocked transport of ASOR and dextran in early endosomes. With increasing internalization time (3-13 min) at 37 degrees C, FITC-dextran-labeled compartments co-localized with late, ASOR-containing endosomes. Since localization of FITC-dextran in late transcytotic compartments was not observed upon FFE separation, it is concluded that the majority of internalized markers is directed to lysosomes. The FITC-label did not account for the predominant lysosomal targeting of the dextran, since [3H]dextran-labeled endosomes exhibited an identical FFE pattern. Taken together, these data indicate that the fluid-phase marker dextran is transported through intracellular compartments with identical characteristics as endosome subcompartments of the receptor-mediated lysosomal route.