FLUID-PHASE MARKER TRANSPORT IN RAT-LIVER - FREE-FLOW ELECTROPHORESISSEPARATES DISTINCT ENDOSOME SUBPOPULATIONS

Free-flow electrophoresis (FFE) was used to investigate the intracellu lar compartments involved in fluid-phase marker, fluoresceine isothioc yanate (FITC)dextran, transport in the isolated perfused rat liver. On e to 2 min after uptake at 37 degrees C, FITC-dextran was found in end osomes with the same electrophoretic mobility as early sorting endosom es 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 ea rly endosomes. With increasing internalization time (3-13 min) at 37 d egrees C, FITC-dextran-labeled compartments co-localized with late, AS OR-containing endosomes. Since localization of FITC-dextran in late tr anscytotic compartments was not observed upon FFE separation, it is co ncluded that the majority of internalized markers is directed to lysos omes. The FITC-label did not account for the predominant lysosomal tar geting of the dextran, since [H-3]dextran-labeled endosomes exhibited an identical FFE pattern. Taken together, these data indicate that the fluid-phase marker dextran is transported through intracellular compa rtments with identical characteristics as endosome subcompartments of the receptor-mediated lysosomal route.