The in vivo effect of colchicine on the addition of galactose and sialic acid to rat hepatic serum glycoproteins

Membrane recycling in secretory cells: pathway to the Golgi complex

The pathway taken by membrane that is recovered by endocytosis from the surface of secretory cells was investigated with electron-dense tracers 9dextrans and cationized ferritin). The cell types examined included exocrine cells of the parotid and lacrimal glands, endocrine cells of the anterior pituitary gland, and immunoglobulin-secreting cells from lymph nodes or myeloma cell lines. In all cases, when the cells were incubated at 37 degrees C the tracers were initially taken up by endocytosis and they later appeared in the stacked Golgi cisternae, in immature secretion granules or vacuoles and in lysosomes. Similar results were obtained after covalent labelling of surface membrane constituents when myeloma cells were radioiodinated and the fate of the labelled components was followed by autoradiography. Initially only the cell surface was labelled, and the autoradiographic grains were concentrated over the plasmalemma. After incubation at 37 degrees C some of the labelled components were internalized (by endocytosis), and the majority of the internal autoradiographic grains were found over Golgi cisternae and over associated secretory vacuoles, which were the only organelles significantly labelled. The findings indicate the existence of considerable membrane traffic from the plasmalemma to the stacked Golgi cisternae and forming secretion granules or vacuoles in all these cell types. Membrane is thus continually recovered from the cell surface of secretory cells and funnelled through the Golgi complex; moreover, the plasmalemma-to-Golgi traffic appears to represent a major route of membrane traffic in secretory cells. A large portion of this traffic appears to be associated with the recycling of the membrane containers used in the packaging of secretory products.

The Golgi apparatus: roles for distinct 'cis' and 'trans' compartments

The Golgi apparatus seems to consist of distinct cis and trans compartments that are proposed to act sequentially to refine the protein export of the endoplasmic reticulum by removing escaped endoplasmic reticulum proteins. Refinement may be a multi-stage process that employs a principle akin to fractional distillation; the stack of cisternae comprising the cis Golgi may be the plates in this distillation tower. The trans Golgi, consisting of the last one or two cisternae, may be the receiver that collects from the cis Golgi only its most refined fraction for later distribution to specific locations throughout the cell.

Cytoskeletal requirements in Chlamydia trachomatis infection of host cells

Infection of genital epithelial cells by the closely related sexually transmitted pathogens Chlamydia trachomatis serovars E and L2 results in different clinical disease manifestations. Following entry into target host cells, individual vesicles containing chlamydiae fuse with one another to form one large inclusion. At the cellular level, the only obvious difference between these serovars is the time until inclusion maturation, which is 48 h for the invasive serovar L2 and 72 h for serovar E. To begin to define the intracellular events of these pathogens, the effect of cytoskeletal disruption on early endosome fusion and inclusion development in epithelial (HEC-1B) and fibroblast (McCoy) cells was analyzed by fluorescence microscopy. Disruption of microfilaments with cytochalasin D markedly reduced serovar E, but not serovar L2, infection of both cell lines. Conversely, microfilament as well as microtubule disruption, with colchicine or nocodazole, had no effect on serovar E inclusion development but resulted in the formation of multiple serovar L2 inclusions per cell during early and mid-development. Later in serovar L2 inclusion development (> 36 h postinfection), vesicles containing chlamydiae fused to form one large inclusion in the absence of an intact cytoskeleton. These results imply that (i) C. trachomatis serovar E may utilize a different pathway for uptake and development from serovar L2; (ii) these differences are consistent in both epithelial cells and fibroblasts; and (iii) the cytoskeleton plays a unique role in the infection of host cells by these two genital pathogens.

Mitotic arrest with anti-microtubule agents or okadaic acid is associated with increased glycoprotein terminal GlcNAc's

The two major intermediate filament glycoproteins in human simple epithelia are keratins 8 and 18 (K8/18). A dramatic increase in terminal N-acetylglucosamine (GlcNAc) residues in K8/18 was previously noted after arresting cells in G2/M using anti-microtubule agents. Here we use in vitro galactosylation to show that increased terminal GlcNAc's is a general phenomenon that occurs in glycoproteins isolated from nuclear and plasma membrane fractions after cells are arrested in mitosis using colcemid, nocodazole, or okadaic acid. All three agents also resulted in a hyperphosphorylated form of K8 as determined by phosphatase treatment and tryptic phosphopeptide mapping. The altered glycosylation was found to be independent of microtubule disassembly, and was not directly related to the G2/M phase of the cell cycle after aphidicolin synchronization. Staurosporine (1 microM) inhibited K8/18 phosphorylation in okadaic acid- or nocodazole-treated cells, and inhibited the increase in K8/18 glycosylation without inhibiting the increase in terminal GlcNAc's of membrane-associated glycoproteins. In contrast, brefeldin A resulted in a dramatic increase in terminal GlcNAc's of membrane-associated but not intermediate filament proteins. Golgi complex-related staining using anti-beta-COP antibody showed significant fragmentation under conditions associated with altered membrane protein glycosylation. Our results suggest that Golgi disruption may be involved in the observed increase in terminal GlcNAc's of membrane but not intermediate filament glycoproteins. The mechanism of increased glycoprotein terminal GlcNAc's in association with mitotic arrest appears to be distinct for intermediate filaments and membrane-associated proteins, and in the case of intermediate filament proteins, phosphorylation may play an important role. Some of the effects of agents that induce mitotic arrest may be mediated by glycosylation changes.

Influence of colchicine on the addition of a sugar to the enamel protein in secretory ameloblasts of cultured germs of rat molar tooth by 3H-galactose radioautography

The influence of colchicine on the addition of 3H-galactose to the enamel protein in secretory ameloblasts of cultured germs of rat molar tooth was investigated by light- and electron-microscopic radioautography. In tooth germs cultured without colchicine, the reaction products of 3H-galactose were observed over Golgi cisternae at early chase times and then localized over the enamel with time. In tooth germs cultured with colchicine, the silver grains were seen over the Golgi cisternae, condensing granules and accumulated secretory granules. Some grains also appeared with time over the pale granular material precipitated in the intercellular space with colchicine treatment. In quantitative analysis with light microscopic radioautography, values of silver grain counts over the unit area (100 microns2) on ameloblasts and enamel of colchicine-treated tooth germs were significantly lower at both 0 min and 30 min chase after 30 min pulse than those of control tooth germs, respectively. This finding indicates that colchicine diminished the incorporation of 3H-galactose into the secretory ameloblast of cultured tooth germs. It is suggested that colchicine decreases the activity of the Golgi apparatus with regard to the addition of sugar to the synthesizing glycoprotein in the secretory ameloblast.

Differential secretion of proteins and glycoproteins by livers of immature and adult rats. Effect of antimicrotubule drugs

This study was initiated to re-examine reported differences in the action of antimicrotubule agents on plasma protein secretion from livers of immature versus adult rats. The aim was (1) to determine the composition and to monitor the secretion of various plasma proteins and glycoproteins from liver slices labeled in vitro with specific amino acids and sugar residues, and (2) to correlate observed differences in secretion of these proteins with structural changes in the hepatocytes of the different aged animals. For the most part, slices of liver from fetal (term), neonatal (4- to 5 days old), and adult rats (70 days old) were incubated with radioactive amino acids or various tritiated sugars specific for N-linked core oligosaccharide and/or N-linked terminal oligosaccharide chains. Our findings indicate that liver slices of fetal and neonatal rats are efficient in synthesizing plasma proteins including fully glycosylated glycoproteins. The secretion of glycosylated and nonglycosylated proteins believed to be processed through Golgi complexes was inhibited to the same extent (approximately 70-80%) by antimicrotubule agents, regardless of the age of the host animal. However, other proteins and glycoproteins secreted by livers of immature rats were found to be relatively insensitive (i.e. inhibited to only 30-40%) to the action of various antimicrotubule drugs. The glycoproteins were found to lack N-linked terminal sugars (although the glycoproteins did contain N-linked core sugars), and it is likely that the drug-insensitive proteins bypassed critical glycosylating sites in the Golgi compartment prior to release. Overall, these findings support earlier data showing that antimicrotubule drugs have a special impact on Golgi-associated events in liver cells. To what extent these findings are related to the action of microtubules remains to be seen.

Cytochemical study of the Golgi apparatus and related organelles of the secretory ameloblasts of rat molar tooth germs cultured with and without colchicine

The Golgi apparatus and Golgi-associated endoplasmic reticulum lysosome (GERL) were examined in the ameloblasts with a cytochemical marker, osmium impregnation, and two enzyme markers, thiamine pyrophosphatase (TPPase) and acid phosphatase (ACPase). In control cultured germs, osmium deposit appeared in one to two immature side cisternae of Golgi stacks; TPPase activity was restricted in a few mature side cisternae and condensing vacuoles. ACPase activity existed in the GERL and, sometimes, in the mature side-cisternae and condensing vacuoles. These findings show that Golgi stacks of ameloblasts consist of several distinct compartments. In colchicine-treated tooth germs, there were morphological and cytochemical changes in both Golgi stacks and GERL. The Golgi apparatus was fragmented and its stacks were scattered throughout the supranuclear region. In some stacks, the number of osmium-positive cisternae was greater than normal; in others they were absent. TPPase and ACPase activity was absent or diminished. These findings suggest the importance of microtubules in the organization of Golgi complex and GERL in the secretory ameloblast.

Subcellular location of secretory proteins retained in the liver during the ethanol-induced inhibition of hepatic protein secretion in the rat

Ethanol administration inhibits the secretion of proteins by the liver, resulting in their hepatocellular retention. Experiments were designed in this study to determine the subcellular location of the retained secretory proteins. Ethanol was administered acutely to nonfasted rats by gastric intubation, whereas control animals received an isocaloric dose of glucose. Two hours after intubation, when maximum blood ethanol levels (45 mM) were observed, [3H]leucine and [14C]fucose were injected simultaneously into the dorsal vein of the penis. The labelling of secretory proteins was determined in the liver and plasma at various time periods after label injection. Ethanol treatment decreased the secretion of both leucine- and fucose-labeled proteins into the plasma. This inhibition of secretion was accompanied by a corresponding increase in the hepatic retention of both leucine- and fucose-labeled immunoprecipitable secretory proteins. At the time of maximum inhibition of secretion, leucine labeled secretory proteins located in the Golgi apparatus represented about 50% of the accumulated secretory proteins in the livers of the ethanol-treated rats, whereas the remainder was essentially equally divided among the rough and smooth endoplasmic reticulum and cytosol. Because fucose is incorporated into secretory proteins almost exclusively in the Golgi complex, fucose-labeled proteins accumulated in the livers of the ethanol-treated rats mainly in the Golgi apparatus, with the remainder located in the cytosol. These results show that ethanol administration causes an impaired movement of secretory proteins along the secretory pathway, and that secretory proteins accumulate mainly, but not exclusively, in the Golgi apparatus.

The effects of cytotropic compounds on the resialylation of human asialotransferrin type 3 in the rat

Effects of chloroquine, colchicine, leupeptin, taxol and vinblastine on the resialylation and degradation of human [125I]asialotransferrin type 3 were studied in rats. An improved experimental technique was applied that permitted the quantification of resialylated ligand produced by individual animals over 3 h by using deconvolution. All three microtubule inhibitors increased the proportion of the dose undergoing resialylation by 35-39%. In addition, colchicine, and, especially, vinblastine enhanced the overall recovery of the dose as protein-bound 125I. The dose recovery was also augmented by leupeptin without any concomitant change in resialylation. Chloroquine suppressed resialylation and this effect could only be partially lifted by the administration of colchicine. The blood of colchicine-treated rats possessed no resialylating activity toward the ligand even when supplemented with additional alkaloid in vitro. The observations support the view that the respective fractions of the ligand destined for resialylation and degradation can, to a certain extent, be varied independently of each other. The effects of short-term starvation (20 h) and refeeding (4 h) on these processes are also presented.

Influence of colchicine and vinblastine on the intracellular migration of secretory and membrane glycoproteins: III. Inhibition of intracellular migration of membrane glycoproteins in rat intestinal columnar cells and hepatocytes as visualized by light and electron-microscope radioautography after 3H-fucose injection

In the first paper of this series (Bennett et al., 1984), light-microscope radioautographic studies showed that colchicine or vinblastine inhibited intracellular migration of glycoproteins out of the Golgi region in a variety of cell types. In the present work, the effects of these drugs on migration of membrane glycoproteins have been examined at the ultrastructural level in duodenal villous columnar cells and hepatocytes. Young (40 gm) rats were given a single intravenous injection of colchicine (4.0 mg) or vinblastine (2.0 mg). At 10 min after colchicine and 30 min after vinblastine administration, the rats were injected with 3H-fucose. Control rats received 3H-fucose only. All rats were sacrificed 90 min after 3H-fucose injection and their tissues processed for radioautography. In duodenal villous columnar cells, 3H-fucose labeling of the apical plasma membrane was reduced by 51% after colchicine and by 67% after vinblastine treatment; but there was little change in labeling of the lateral plasma membrane. Labeling of the Golgi apparatus increased. This suggests that labeled glycoproteins destined for the apical plasma membrane were inhibited from leaving the Golgi region, while migration to the lateral plasma membrane was not impaired. In hepatocytes, labeling of the sinusoidal plasma membrane was reduced by 83% after colchicine and by 85% after vinblastine treatment. Labeling of the lateral plasma membrane also decreased, although not so dramatically. Labeling of the Golgi apparatus and neighboring secretory vesicles increased. This indicates that the drugs inhibited migration of membrane glycoproteins from the Golgi region to the various portions of the plasma membrane. Accumulation of secretory vesicles at the sinusoidal front suggests that exocytosis may also have been partially inhibited. In both cell types, microtubules almost completely disappeared after drug treatment. Microtubules may, therefore, be necessary for intracellular transport of membrane glycoproteins, although the possibility of a direct action of these drugs on Golgi or plasma membranes must also be considered.

Influence of colchicine and vinblastine on the intracellular migration of secretory and membrane glycoproteins: I. Inhibition of glycoprotein migration in various rat cell types as shown by light microscope radioautography after injection of 3H-fucose

Previous studies have shown that colchicine and vinblastine inhibit secretion in many cell types by interrupting the normal intracellular migration of secretory products. In the present work, radioautography has been used to study the effects of these drugs on migration of membrane and secretory glycoproteins in a variety of cell types. Young (40 gm) rats were given a single intravenous injection of colchicine (4.0 mg) or vinblastine (2.0 mg). At 10 min after colchicine and 30 min after vinblastine administration, the rats were injected with 3H-fucose. Control rats received 3H-fucose only. All rats were sacrificed 90 min after 3H-fucose injection and their tissues processed for light microscope radioautography. Examination of secretory cell types such as ameloblasts and thyroid follicular cells in control animals revealed reactions of approximately equal intensity over the Golgi region and over extracellular secretion products, while in drug-treated rats most of the reaction was confined to the Golgi region. In a variety of other cell types, including endocrine cells (e.g., hepatocytes) and cells generally considered as nonsecretory (e.g., intestinal columnar cells), reaction in control animals occurred both over the Golgi region and over various portions of the cell surface. In drug-treated animals, a strong Golgi reaction was present, but reaction over the cell surface was weak or absent. These results indicate that in many cell types, colchicine and vinblastine inhibit migration out of the Golgi region not only of secretory glycoproteins, but also of membrane glycoproteins destined for the plasma membrane.

Effect of ethanol on hepatic secretory proteins

Both acute and chronic ethanol administration inhibit the secretion of albumin and glycoproteins from the liver. Impairment of posttranslational steps of the secretory process are mainly involved in this secretory defect, although in some instances altered synthesis of the protein moiety may be a factor. Decreased secretion following ethanol administration results in the intrahepatic retention of export proteins. The secretory defect is a consequence of the metabolism of ethanol and is likely mediated via acetaldehyde, although more conclusive proof is still required. The manner by which acetaldehyde impairs the secretory process is unknown, but may be related to its high reactivity with hepatocellular proteins. The specific posttranslational steps or processes involved in the secretory defect are still unclear; however, it appears that the final steps of secretion (post-Golgi events) may be the primary site of impairment. Impaired secretion of proteins from the liver could contribute to altered levels of plasma proteins and hepatomegaly as well as to the liver injury observed in the alcoholic.

Effect of colchicine on rat small intestinal absorptive cells. II. Distribution of label after incorporation of [3H]fucose into plasma membrane glycoproteins

By means of radioautography the influence was tested of various periods (5, 15, 30, 40 min, 2 hr) of pretreatment with colchicine, administered intraperitoneally to rats at a dosage of 0.5 mg/100 g of body weight, on the intracellular pathway of [3H]fucose in absorptive cells of the small intestine. Administration of colchicine for 30 min and longer time intervals causes delay in the insertion of [3H]fucose into the oligosaccharide chains of glycoconjugates in the Golgi apparatus, and results in redistribution of the label apparent over the different portions of the plasma membrane. In controls, at 2 and 4 hr after administration of [3H]fucose the apical plasma membrane is strongly labeled; 53.7 +/- 3.2% of the silver grains are recorded over apical regions of the plasma membrane that contrast to basolateral portions comprising 25.4 +/- 3.2% of the label. Colchicine causes equalization of the reaction of apical and basolateral regions of the plasma membrane: the number of silver grains attributable to the apical plasma membrane is reduced; following treatment with colchicine, apical portions of the plasma membrane comprise 31.6 +/- 1.8% of the silver grains, 38.6 +/- 3.8% are attributable to basolateral membrane regions. The colchicine-induced equalization of the density of label of apical and basolateral regions of the plasma membrane, in addition to the occurrence of basolateral microvillus borders (demonstrated in the companion paper), suggests microtubules to be important in the maintenance of the polar organization of small intestinal absorptive cells.

Disparate effects of monensin and colchicine on intracellular processing of secretory proteins in cultured rat hepatocytes

We have studied the biosynthesis and intracellular processing of three major secretory proteins, albumin, alpha 1-protease inhibitor and alpha 2u-globulin, in cultured rat hepatocytes. The effect of secretion-blocking agents, monensin, a monovalent ionophore, and the microtubule-affecting agents colchicine and taxol was determined. In the control cells, alpha 1-protease inhibitor, a glycoprotein, was first synthesized as an endoglycosidase-H-sensitive form with Mr 51 000, and then processed to two endoglycosidase-H-resistant forms having Mr 51 000 and 56 000, the latter of which was secreted into the medium. Initially synthesized proalbumin was converted with chase to serum-type albumin, while no pro-type precursor was identified for alpha 2u-globulin. In the cells treated with colchicine or taxol, in which secretion was greatly inhibited, the fully processed alpha 1-protease inhibitor and albumin accumulated and were finally secreted into the medium. In the monensin-treated cells, however, most of the newly synthesized alpha 1-protease inhibitor and albumin were not processed to the final mature forms, resulting in accumulation of two 51 000-Mr forms and proalbumin, respectively. Moreover in treated cells, proalbumin and the endoglycosidase-H-resistant alpha 1-protease inhibitor were finally secreted into the medium. Such an effect was not caused by NH4Cl which also inhibited the secretion and is known to exert the similar effect as monensin on the receptor-mediated endocytosis pathway. Based on these results, the use of monensin may prove valuable for more detailed analysis of intracellular processing of various proteins.

Biosynthesis and secretion of alpha 1-antitrypsin in primary cultures of rat hepatocytes. Characterization of differently glycosylated intracellular and extracellular forms

The biosynthesis and secretion of alpha 1-antitrypsin was studied in rat hepatocyte primary cultures. After labeling with [35S]methionine an alpha 1-antitrypsin with an apparent molecular weight of 49000 estimated by sodium dodecyl sulfate/polyacrylamide slab gel electrophoresis was immunoprecipitated from the cell homogenate. This intracellular form of alpha 1-antitrypsin could be deglycosylated by endoglycosidase H treatment indicating that its oligosaccharide chains were of the high-mannose type. Pulse-chase experiments showed that about 30 min after its synthesis the transformation of the 49000-Mr alpha 1-antitrypsin to a protein with an apparent molecular weight of 54000 began. Only this 54000-Mr protein was secreted by the hepatocytes. The 54000-Mr alpha 1-antitrypsin was not sensitive to endoglycosidase H, but sensitive to neuraminidase, and it incorporated [3H]galactose and [3H]fucose indicating that its oligosaccharide chains were of the complex type. In the presence of tunicamycin, which blocks the formation of N-asparagine-linked oligosaccharide chains, an unglycosylated alpha 1-antitrypsin with an apparent molecular weight of 41000 was found in the cells as well as in the medium. However, tunicamycin decreased the secretion of alpha 1-antitrypsin by 60-70%, whereas the secretion of albumin remained unaffected. In the presence of colchicine the secretion of both alpha 1-antitrypsin and albumin was impaired. The results demonstrate the importance of glycosylation for the secretion of alpha 1-antitrypsin.

Effect of ethanol on the synthesis and secretion of hepatic secretory glycoproteins and albumin

The effects of ethanol on the synthesis and secretion of serum glycoproteins and albumin, a nonglycosylated protein, were studied in rat liver slices. Serum glycoproteins and albumin were determined by immunoprecipitation from either the incubation medium or from the washed slices. When ethanol (10 mM) was present in the incubation medium, [14C]glucosamine incorporation in secretory glycoproteins was decreased. This inhibitory effect was, however, much greater in the secretory proteins released into the medium than in those retained in the liver slices. Similar inhibitions by ethanol were also observed when leucine or valine were used as a label for either total export proteins or albumin. Since ethanol impaired protein synthesis, and inhibitor of protein synthesis, cycloheximide, was used so that both the control and ethanol-treated slices had identical pools of protein acceptors available for glycosylation. When cycloheximide alone was added to the slices, glucosamine radioactivity of secretory glycoproteins was equally reduced in both the medium and the liver. When cycloheximide and ethanol were both present, decreased appearance of glucosamine-labeled proteins in the medium was observed when compared to the slices containing cycloheximide alone; however, radioactivity of secretory glycoproteins retained in the liver was elevated. Ethanol also decreased the appearance of fucose-labeled glycoproteins in the medium without altering fucose incorporation into the total pool of secretory glycoproteins. The effects of ethanol on hepatic protein secretion independent of its effect on synthesis were further determined by prelabeling proteins with either [14C]leucine or [14C]fucose. Ethanol impaired the secretion of these prelabeled proteins into the medium. The results of this study show that ethanol impairs both the synthesis and secretion of secretory glycoproteins and albumin.

The golgi apparatus: two organelles in tandem

The Golgi apparatus consists of distinct cis and trans compartments that may act sequentially to refine the protein export of the endoplasmic reticulum by removing escaped endoplasmic reticulum proteins. Refinement may be a multistage process akin to fractional distillation; the stack of cisternae comprising the cis Golgi may be the plates in this distillation tower. The trans Golgi, consisting of the last one or two cisternae, may be the receiver that collects from the cis Golgi only its most refined fraction for later distribution to specific locations throughout the cell.

An effect of colchicine on galactosyl- and sialyltransferases of rat liver Golgi membranes

Colchicine inhibited the activity of the galactosyl- and sialyltransferases of rat liver Golgi membranes. The sialyltransferase was more sensitive to the drug than galactosyltransferase since it was inhibited to a greater extent and at lower concentrations of colchicine than the galactosyltransferase. Two soluble enzymes, i.e. that from rat serum and that isolated from bovine milk, were not inhibited by colchicine. Even with very high concentrations of colchicine a marked stimulation of activity was observed. The data suggest that the inhibition observed in the Golgi membranes is in some way related to the arrangement of the enzymes in the lipid bilayer. In support of this hypothesis, the milk galactosyltransferase became very sensitive to colchicine after incorporation of the enzyme into lipid vesicles. The incorporation of colchicine into Golgi membranes was shown to decrease the order parameter as determined by electron spin resonance which reflects an increased fluidity of the Golgi membranes. A change in fluidity may be responsible for the inhibition of enzyme activity at least in part.

Impairment of glycoprotein secretion by phenobarbital in rat liver slices

The effects of phenobarbital on protein and glycoprotein synthesis and secretion were studied in rat liver slices. Phenobarbital (2 mM) decreased [14C]glucosamine and [14C]leucine incorporation into liver proteins and markedly inhibited their incorporation into medium (secretory) proteins. The inhibitory effect of phenobarbital was dose dependent and not reversible under the conditions of this study. In the presence of cycloheximide, an inhibitor of peptide synthesis, phenobarbital still inhibited the release of glycoproteins into the medium; however, the specific activity of liver glycoproteins was increased. The effects of phenobarbital on hepatic macromolecular secretion, independent of its effects on synthesis, were determined by prelabeling proteins in a liver slice system with either [14C]leucine or [14C]glucosamine. When phenobarbital was present, the secretion of these prelabeled proteins into the medium was inpaired. 12 h after intraperitoneal injections of phenobarbital, glycoprotein secretion was inhibited from liver slices prepared from the pretreated rats. This inhibition of secretion occurred even though protein synthesis was stimulated and intracellular glycosylations unaffected. The results of this study indicate that phenobarbital impairs the secretion of glycoproteins by the liver.

Organ cultures of human fetal hepatocytes in the study of extra-and intracellular alpha1-antitrypsin

The rate of synthesis of alpha 1-antitrypsin has been studied in organ cultures of fetal human liver. By de novo synthesis, alpha 1-antitrypsin of the same electrophoretic mobility and molecular size as plasma alpha 1-antitrypsin was produced. Synthetic rate was comparable to in vivo conditions and was suppressed by cycloheximide, colchicine and neuraminidase. By increasing alpha 1-antitrypsin levels in cultre medium, suppression of alpha 1-antitrypsin release from the intra-to the extracellular site was achieved, i.e., synthesis does not proceed autonomously. This suppression was preceded by a temporary enhancement of synthesis. Both effects were found to be independent of degree of sialylation of add-d alpha 1-antitrypsin. In contrast to alpha 1-antitrypsin released in tissue culture, the intracellular protein, as analyzed by crossed immunoelectrophoresis of Triton X-100 extracts from fetal liver, was found to occur partly as slowly moving peaks. Whether these peaks represent proforms or incompletely glycosylated precursors of export alpha 1-antitrypsin or complexes with proteases remains unsettled. A variety of other plasma proteins are released in organ cultures making the system suitable for study of factors regulating plasma protein synthesis.

Elimination of asialofetuin and asialoorosomucoid by the intact rat. Quantitative aspects of the hepatic clearance mechanism

The capacity of the liver to eliminate asialofetuin and asialoorosomucoid was investigated in intact rats. From plasma radioactivity curve measurements and assays on tissue homogenates the liver is shown to be able to dispose of an average of 19.8 microgram of asialofetuin/min per 100 g body weight. No other major route is identified for the disappearance of asialofetuin from the plasma, although trace amounts of the protein were detectable in the urine. From analyses of the plasma radioactivity curves the elimination process for asialoorosomucoid appears to be comparatively complex because of the existence of extrahepatic disposal routes. Quantification of labelled asialoorosomucoid in liver homogenates indicates, however, that the hepatic clearance rate for asialoorosomucoid is similar to that for asialofetuin. Urinary excretion significantly contributes to the disappearance of asialoorosomucoid from the plasma but the hepatic and renal routes do not account for all the protein lost from this compartment. At plasma concentrations above the maximal eliminative capacity of the liver, the hepatic clearance of asialofetuin obeys zero-order kinetics and is remarkably constant. Elimination of a quantity of asialoglycoprotein which exceeds the calculated total number of binding sites in the liver does not reduce the efficiency of the pathway, and studies of [3H]leucine incorporation indicate that the lectin, unlike the bound asialoglycoprotein, is not destroyed in the elimination process. Cytochalasin B (80 microgram/100 g body wt.) had no measureable effect on the hepatic clearance of asialofetuin. Administration of colchicine (10 mg/100 g body wt.) resulted in transitory accumulations of asialoorosomucoid in the liver, presumably due to interference with the intracellular transport of the endocytised protein.

Fusion of secretory vesicles isolated from rat liver

Secretory vesicles isolated from rat liver were found to fuse after exposure to Ca2+. Vesicle fusion is characterized by the occurrence of twinned vesicles with a continuous cleavage plane between two vesicles in freeze-fracture electron microscopy. The number of fused vesicles increases with increasing Ca2+-concentrations and is half maximal around 10(-6)M. Other divalent cations (Ba2+, Sr2+, and Mg2+) were ineffective. Mg2+ inhibits Ca2+-induced fusion. Therefore, the fusion of secretory vesicles in vitro is Ca2+ specific and exhibits properties similar to the exocytotic process of various secretory cells. Various substances affecting secretion in vivo (microtubular inhibitors, local anesthetics, ionophores) were tested for their effect on membrane fusion in our system. The fusion of isolated secretory vesicles from liver was found to differ from that of pure phospholipid membranes in its temperature dependence, in its much lower requirement for Ca2+, and in its Ca2+-specificity. Chemical and enzymatic modifcations of the vesicle membrane indicate that glycoproteins may account for these differences.