Golgi fractions prepared from rat liver homogenates. II. Biochemical characterization

Cellular binding sites for insulin in rat liver

A study of the sites of insulin binding in subcellular fractions of rat liver is reported. A method for the isolation of liver plasma membranes, which permits one to follow quantitatively the distribution of all the parameters of interest, was modified and applied to the study of the cellular topography of insulin binding. The insulin binding capacity did not follow closely the enzyme marker (5'-nucleotidase) for plasma membranes when differential centrifugation schemes were used, and the divergence from this marker was more prominent when separations were performed on discontinous sucrose gradients. A significant amount of insulin binding capacity was always present in fractions with higher density than those containing the majority of 5'-nycleotidase. Results of studies on linear sucrose gradients have disclosed in some of the purified membrane fractions small but consistent differences in density of the insulin binding, and plasma membrane particles. It is suggested that there may be several types of intracellular membranes to which insulin can bind besides the plasma membranes.

A micro method for simultaneous determination of galactosyltransferase and 5'-nucleotidase activities in cell fractions

Galactosyltransferase and 5'-nucleotidase were assayed in the same reaction mixture, with ovalbumin as exogenous acceptor of (14-C)galactose and with (3-H)AMP as the substrate for the 5'-nucleotidase assay. The substrates and reaction products of either assay had no significant effect on the activity of the other enzyme.

Differential solubilization of proteins, phospholipids, free and esterified cholesterol of rat liver cellular membranes by sodium deoxycholate

1. Smooth microsomes, Golgi-rich fractions, and light and heavy plasmalemmal subfractions from rat liver were isolated and their purity assessed using enzymic, chemical and morphological criteria. 2. Membranes were prepared by Tris-EDTA washing combined with sonication treatment of the different subcellular fractions. 3. Washed membranes were submitted to differential solubilization with 0.26% sodium deoxycholate. When the deoxycholate/phospholipid molar ratio (R) is raised, all the membranes showed a maximum protein solubilization occurring at R approximately equal TO 2. The higher the membrane neutral lipid to phospholipid molar ratio is, the lower the solubilized protein plateau lies. 4. Phospholipids are solubilized in slightly greater amounts than proteins and their solubilization is complete at R equals 14-16. 5. For R smaller than 2, sterols are solubilized in slightly greater amounts than phospholipids. At maximum protein solubilization, cholesterol and cholesterol esters completely differ in their behaviour. The whole membrane cholesterol goes into solution for R equals 14-16 while the solubilization of esterified cholesterol is never complete. The higher the protein plateau is, the lower the cholesterol esters solubilization curve asymptote lies.

Functional polarity of the rat hepatocyte surface membrane. Isolation and characterization of plasma-membrane subfractions from the blood-sinusoidal, bile-Canalicular and contiguous surfaces of the hepatocyte

1. Six rat liver plasma-membrane subfractions of different density and morphological, enzymic and chemical properties were prepared from homogenates by a combination of differential, rate-zonal and density-gradient centrifugation. They consisted of three vesicular 'light' subfractions of density 1.12-1.13 and three 'heavy' subfractions of density 1.16-1.18 containing membrane strips and intercellular junctions. 2. All six subfractions contained a basal adenylate cyclase activity. One of the 'light' subfractions that showed the highest glucagon-stimulated adenylate cyclase activity was identified as deriving form the blood-sinusoidal face of the hepatocyte. This subfraction, unlike the others, was contaminated by Golgi components, as indicated by its morphological properties and the presence of galactosyl- and sialyl-transferase activities. 3. All the six subfractions showed high activities of the following plasma-membrane marker enzymes: 5'-nucleotidase, alkaline phosphodiesterase (nucleotide pyrophosphatase), alkaline phosphatase, leucine naphthylamidase and Mg2+-activated adenosine triphosphatase. A 'light' subfraction that showed the highest specific activities of all the above marker enzymes, but lacked a glucagon-stimulated adenylate cyclase activity, was identified as deriving from the bile-canalicular face of the hepatocyte. 4. The 'heavy' subfractions, which showed generally the lowest activities of the above plasma-membrane enzyme markers, and were characterized by the presence of desmosomes and gap junctions, were taken to originate from the contiguous faces of the hepatocyte. 5. The protein composition of the six subfractions was generally similar, as shown by polyacrylamide-gel electrophoresis. Differences in the amounts of various protein and glycoprotein bands among the subfractions correlated with their morphology, enzymic composition and sialic acid content. 6. Hormonal and histochemical evidence supporting the identification of a bile-canalicular subfraction, a blood-sinusoidal subfraction and contiguous-face subfractions is discussed.

A proposed pathway of plasma glycoprotein synthesis

Plasma glycoprotein synthesis in the liver occurs in a stepwise fashion. The first sugar, N-acetyl-glucosamine, is attached to the protein during the growth of the polypeptide chain on the membrane-bound ribosomes. Subsequent carbohydrates are incorporated after the completion of the protein in the lumen of the endoplasmic reticulum and Golgi apparatus. The reactions are carried out by enzymes strongly bound to the membranes. Because the glycosylation reaction occurs in the interior of the cytoplasmic tubules a permeability problem for the nucleotide sugar exists. Recent studies indicate that sugar-lipids are formed on the cytoplasmic site of the membrane and these complexes transfer the sugars across the membrane. Experimental evidence for this pathway is presented in this article.

Analysis of the pinocytic process in rat kidney. I. Isolation of pinocytic vesicles from rat kidney cortex

Pinocytosis was induced in rat kidney by exposure to horseradish peroxidase (HRP). Pinocytic vesicle preparations were enriched after homogenization of kidney cortex by differential centrifugation and free-flow electrophoresis with HRP as an exogenous marker. Vesicles were identified by enzymatic analysis and by electron microscopy, including specific staining procedures. Typical brush-border enzymes such as alkaline phosphatase, aminopeptidase, 5'-nucleotidase, lysosomal acid phosphatase, and mitochondrial succinic dehydrogenase were reduced in the vesicular fraction, compared to the kidney cortex homogenate. Glucose-6-phosphatase and Na(+)-K(+)-ATPase were only slightly increased in the fraction. These results indicate that preparations of pinocytic vesicles from rat kidney cortex can be enriched. They have biochemical characteristics that differ from those of the cell organelles and membranes previously purified from renal tissue.

Heterogeneous distribution of enzymes among plasma-membrane fragments sedimenting with the microsomal fraction of rat liver

Plasma-membrane fragments recovered in the microsomal fraction of rat liver homogenates were shown to be heterogeneous in density. It was demonstrated that 5'-nucleotidase, the most commonly used plasma-membrane marker, is concentrated in the lightest subfraction. Two of the published procedures for the isolation of plasma-membrane fragments from the microsomal fraction (Touster et al., 1970; Hinton et al., 1971) are shown to give products which are not representative of all the plasma-membrane fragments of microsomal size, and it is argued that a third procedure (House & Weidemann, 1970) is likely to give a similar product.

Analytical study of microsomes and isolated subcellular membranes from rat liver. IV. Biochemical, physical, and morphological modifications of microsomal components induced by digitonin, EDTA, and pyrophosphate

Isopycnic equilibration and sedimentation rate studies of rat liver microsomes led previously to the assignment of microsomal constituents into group a1 (monoamine oxidase), group a2 (5'-nucleotidase, alkaline phosphodiesterase I, alkaline phosphatase and cholesterol), group a3 (galactosyltransferase), group b (NADH cytochrome c reductase, NADPH cytochrome c reductase, aminopyrine demethylase, cytochrome b(5) and P 450), and group c (glucose 6-phosphatase, esterase, nucleoside diphosphatase, beta-glucuronidase and glucuronyltransferase). Confirmation and extension of the assignment into groups has been obtained by studying the differential effect of the reagents digitonin, EDTA, and PPi. Digitonin specifically affected the equilibrium density only of the group a2 and (to a lesser extent) group a3, and not of groups b and c under conditions which preserved the structure-linked latency of nucleoside diphosphatase and galactosyltransferase. Within experimental error the rate of sedimentation of all microsomal constituents was unaffected. The morphological appearance under the electron microscope was indistinguishable from that of nondigitonin-treated microsomes, except that a few smooth membranes (< 10%) exhibited broken-looking profiles. Treatment of microsomes with EDTA or PPi detached a substantial part of RNA and released protein in excess over the amount accountable for by detachment of ribosome constituents. This detachment was confirmed by electron microscopy. EDTA and PPi decreased markedly the equilibrium density and the density dispersion of groups b and c, due mainly to the uncoating of rough elements. EDTA and PPi shifted slightly the distribution profiles of groups a towards lower densities, possibly as a result of the release of adsorbed proteins. The combination of EDTA and digitonin, used subsequently, rendered the average equilibrium density of group a2 higher than that of groups b and c. Dense subfractions were thus enriched in constituents of group a2 and showed mainly broken-looking vesicles under the electron microscope. The import of our results on the biochemical and enzymic properties of the subcellular components of the microsome fractions is discussed.

Selective release of content from microsomal vesicles without membrane disassembly. II. Electrophoretic and immunological characterization of microsomal subfractions

Rough and smooth microsomes were shown to have similar sets of polypeptide chains except for the proteins of ribosomes bound to the rough endoplasmic reticulum (ER). More than 50 species of polypeptides were detected by acrylamide gel electrophoresis, ranging in molecular weight from 10,000 to approximately 200,000 daltons. The content of rough and smooth microsomes was separated from the membrane vesicles using sublytic concentrations of detergents and differential centrifugation. A specific subset of proteins which consisted of approximately 25 polypeptides was characteristic of the microsomal content. Some of these proteins showed high rates of in vivo incorporation of radioactive leucine or glucosamine, but several others incorporated only low levels of radioactivity within short labeling intervals and appeared to be long-term residents of the lumen of the ER. Seven polypeptides in the content subfractions, including serum albumin, contained almost 50% of the leucine radioactivity incorporated during 5 min and cross-reacted with antiserum against rat serum. Almost all microsomal glycoproteins were at least partly released with the microsomal content. Smooth microsomes contained higher levels of albumin than rough microsomes, but after short times of labeling with [(3)H]leucine the specific activity of albumin in the latter was higher, supporting the notion that newly synthesized serum proteins are transferred from rough to smooth portions of the ER. On the other hand, after labeling for 30 min with [(3)H]glucosamine, smooth microsomes contained higher levels of radioactivity than rough microsomes. This would be expected if glycosidation of newly synthesized polypeptides proceeds during their transit through ER cisternae. The labeling pattern of membrane proteins in microsomes obtained from animals which received three daily injections of [(3)H]leucine, the last administered 1 day before sacrifice, followed the intensity of bands stained with Coomassie blue, with a main radioactive peak corresponding to cytochrome P 450. After the long-term labeling procedure most content proteins had low levels of radioactivity; this was especially true of serum proteins which were highly labeled after 30 min.

Cytochemistry of Golgi fractions prepared from rat liver

Cytochemical tests for several marker enzymes were applied to liver tissue and to the three Golgi fractions (GF(1), GF(2), GF(3)) separated by the procedure of Ehrenreich et al. from liver homogenates of alcohol-treated rats. 5'-Nucleotidase (AMPase) reaction product was found in all three fractions but in different locations: It occurred along the inside of the membrane of VLDL-filled vacuoles in GF(1) and GF(2), and along the outside of the cisternal membranes in GF(3). In the latter it was restricted to the dilated cisternal rims and was absent from the cisternal centers. The AMPase activity found in the fractions by biochemical assay is therefore indigenous to Golgi components and is not due to contamination by plasma membrane. Acid phosphatase (AcPase) reaction product was detected within lysosomal contaminants in GF(1) and within many VLDL-filled vacuoles in GF(1) and GF(2), indicating that AcPase activity is due not only to contaminating lysosomes, but also to enzyme indigenous to Golgi secretory vacuoles. G-6-Pase reaction product was present in GF(3) and within contaminating endoplasmic reticulum fragments, but not in other fractions. Thiamine pyrophosphatase (TPPase) was localized to some of the VLDL-filled vacuoles and cisternae in GF(1) and GF(2), and was not found in the cisternae in GF(3). The results demonstrate the usefulness of cytochemical methods in monitoring the fractionation procedure: They have (a) allowed a reliable identification of contaminants, (b) made possible a distinction between indigenous and contaminating activities, and (c) shown, primarily by the results of the TPPase test, that the procedure achieves a meaningful subfractionation of Golgi elements, with GF(1) and GF(3), representing primarily trans-Golgi elements from the secretory Golgi face, and GF(3) consisting largely of cis-Golgi components from the opposite face.

Golgi fractions prepared from rat liver homogenates. I. Isolation procedure and morphological characterization

In devising a new procedure for the isolation of Golgi fractions from rat liver homogenates, we have taken advantage of the overloading with very low density lipoprotein (VLDL) particles that occurs in the Golgi elements of hepatocytes approximately 90 min after ethanol is administered (0.6 g/100 g body weight) by stomach tube to the animals. The VLDLs act as morphological markers as well as density modifiers of these elements. The starting preparation is a total microsomal fraction prepared from liver homogenized (1:5) in 0.25 M sucrose. This fraction is resuspended in 1.15 M sucrose and loaded at the bottom of a discontinuous sucrose density gradient. Centrifugation at approximately 13 x 10(6)g.min yields by flotation three Golgi fractions of density >1.041 and <1.173. The light and intermediate fractions consist essentially of VLDL-loaded Golgi vacuoles and cisternae. Nearly empty, often collapsed, Golgi cisternae are the main component of the heavy fraction. A procedure which subjects the Golgi fractions to hypotonic shock and shearing in a French press at pH 8.5 allows the extraction of the content of the Golgi elements and the subsequent isolation of their membranes by differential centrifugation.