Detailed investigations by quantitative centrifugal fractionation were conducted to determine the subcellular distribution of protein-bound sialic acid in rat liver. Homogenates obtained from perfused livers were fractionated by differential centrifugation into nuclear fraction, large granules, microsomes, and final supernate fraction, or were used to isolate membrane preparations enriched in either plasma membranes or Golgi complex elements . Large granule fractions, microsome fractions, and plasma membrane preparations were subfractionated by density equilibration in linear gradients of sucrose . In some experiments, microsomes or plasma membrane preparations were treated with digitonin before isopycnic centrifugation to better distinguish subcellular elements related to the plasma membrane or the Golgi complex from the other cell components ; in other experiments, large granule fractions were obtained from Triton WR-1339-loaded livers, which effectively resolve lysosomes from mitochondria and peroxisomes in density gradient analysis . Protein-bound sialic acid and marker enzymes were assayed in the various subcellular fractions. The distributions obtained show that sialoglycoprotein is restricted to some particular domains of the cell, which include the plasma membrane, phagolysosomes, and possibly the Golgi complex. Although sialoglycoprotein is largely recovered in the microsome fraction, it has not been detected in the endoplasmic reticulum-derived elements of this subcellular fraction . In addition, it has not been detected either in mitochondria or in peroxisomes. Because the sialyltransferase activities are associated with the Golgi complex, the cytoplasm appears compartmentalized into components which biogenetically involve the Golgi apparatus and components which do not.Sialic acid is found within rat liver cells mainly as the terminal sugar of the saccharide chains of glycoproteins and glycolipids. These constituents are abundant at the cell surface, with their sugar residues externally disposed (45) .Several publications have reported that protein-bound sialic acid (sialoglycoprotein) is not confmed to the pericellular membrane but also occurs within the cell, in the nuclear membrane (21, 22, 26), mitochondria (13, 14, 35), lysosomal membrane (20), Golgi complex (5, 23), endoplasmic reticulum (5,7,19,24,28), and cytosol (7) (see also reference 41 for a review) . However, in most of these publications it has not been unequivocally established that sialoglycoprotein belongs to the main cell component of the preparation analyzed rather than to a sialoglycoprotein-rich contaminant, for instance the plasma membrane . Whether sialoglycoproteins are confmed to the cell surface or also occur in cell organelles raises key questions concerning both their function and their route of biosynthesis . It is generally accepted that after passage through the Golgi complex, which is the sole subcellular organelle unambiguously established as being endowed with sialyltransferase activities (33, 43), glycoprot...