ABSTR^Cr Hepatocytes of estradiol-treated rats, which express many low density lipoprotein receptors, rapidly accumulate intravenously injected low density lipoprotein in multivesicular bodies (MVBs). We have isolated MVBs and Golgi apparatus fractions from livers of estradioltreated rats. MVB fractions were composed mainly of large vesicles, ~0.55 tam diam, filled with remnantlike very low density lipoproteins, known to be taken up into hepatocytes by receptor-mediated endocytosis. MVBs also contained numerous small vesicles, 0.05-0.07 tam in diameter, and had two types of appendages: one fingerlike and electron dense and the other saclike and electron lucent. MVBs contained little galactosyltransferase or arylsulfatase activity, and content lipoproteins were largely intact. Very low density lipoproteins from Golgi fractions, which are derived to a large extent from secretory vesicles, were larger than those of MVB fractions and contained newly synthesized triglycerides. Membranes of MVBs contained much more cholesterol and less protein than did Golgi membranes. We conclude that two distinct lipoprotein-filled organelles are located in the bile canalicular pole of hepatocytes. MVBs, a major prelysosomat organelle of low density in the endocytic pathway, contain remnants of triglyceride-rich lipoproteins, whereas secretory vesicles of the Golgi apparatus contain nascent very low density lipoproteins.Many different types of macromolecules, including hormones, growth factors, plasma glycoproteins and lipoproteins, and some viruses, are taken up by cells after binding to specific receptors in plasma membranes. The ligand-receptor complexes are concentrated in clathrin-coated pits and taken into the cell by endocytosis, forming primary endocytotic vesicles 0.05-0.08 um in diameter. The complexes soon become associated near the cell surface with large, smooth-surfaced, polymorphic structures (1-4) which have been called sorting vesicles (5), endosomes (3), receptosomes (6), or compartment of uncoupling of receptor and ligand (CURL) (7). Later, in hepatocytes ligands are found in larger vesicular structures in the bile canalicular region of the cell, near the Golgi apparatus. These organelles, most of which contain small internal vesicles, are called multivesicular bodies (MVBs).t Like secretory vesicles of the Golgi apparatu~ in this cell, they contain 1Abbreviation used in this paper. MVB, multivesicular body. structures with diameters of 300-800 A that resemble plasma lipoproteins (8, 9).In earlier research we identified MVBs as an intermediate compartment in the receptor-mediated catabolic pathway for low density lipoprotein (LDL) in hepatocytes (8,9). We now report a method for isolating MVBs from livers of estradioltreated rats. We have isolated MVBs and intact Golgi apparatus fractions from the same livers and have found that they can be distinguished by morphological and enzymatic criteria. Furthermore, the properties of the very low density lipoprotein (VLDL) content of these adjacent organdies are distinct a...
