Subcellular distribution of human asialotransferrin type 3 in the rat liver

A small quantity of 125I-labeled human asialotransferrin type 3 (2 to 4 microgram/100 g) was injected in intact rats and the distribution of the hepatic radioactivity analyzed by fractionation of liver homogenates on continuous sucrose density gradient. The ligand rapidly partitioned between plasma membrane and the interior of the cell at an approximate ratio of 1 to 4. The ratio remained constant between 3 min and 1 h. Intracellular 125I was encapsulated in a particle that was of a median equilibrium density of 1.11 (1.109 to 1.114) g/cm3 at 20 degrees C. The ligand recovered from the particles showed no sign of proteolytic digestion and was bound by the immobilized asialoglycoprotein-binding lectin from rabbit liver. The electron microscopic appearance of the subfractions containing of the entrapped ligand closely resembled that of an intermediate Golgi preparation. Various attempts were made to separate the ligand-containing particles from sialyltransferase and phosphodiesterase I activities, but complete separation could not be accomplished. 125I-Asialoorosomucoid studied in the same quantities and under the same conditions as asialotransferrin, yielded a subcellular distribution which was distinct from that of asialotransferrin type 3. Increasing the dose of asialotransferrin, to a level at which rapid catabolism of this asialoprotein occurs, profoundly changed the subcellular distribution of radioactivity. The subcellular distribution thus obtained was comparable with that found for asialoorosomucoid. These findings suggest that asialotransferrin type 3 is associated with different intracellular vehicles (different endosomes?) depending on whether the protein is simply diacytosed or is en route to lysosomes.