Isolation of highly purified fractions of plasma membrane and tonoplast from the same homogenate of soybean hypocotyls by free-flow electrophoresis

A procedure is described whereby highly purified fractions of plasma membrane and tonoplast were isolated from hypocotyls of dark-grown soybean (Glycine max L. var Wayne) by the technique of preparative free-flow electrophoresis. Fractions migrating the slowest toward the anode were enriched in thick (10 nanometers) membranes identified as plasma membranes based on ability to bind N-1-naphthylphthalamic acid (NPA), glucan synthetase-II, and K(+)-stimulated, vanadate-inhibited Mg(2+) ATPase, reaction with phosphotungstic acid at low pH on electron microscope sections, and morphological evaluations. Fractions migrating farthest toward the anode (farthest from the point of sample injection) were enriched in membrane vesicles with thick (7-9 nanometers) membranes that did not stain with phosphotungstic acid at low pH, contained a nitrate-inhibited, Cl-stimulated ATPase and had the in situ morphological characteristics of tonoplast including the presence of flocculent contents. These vesicles neither bound NPA nor contained levels of glucan synthetase II above background. Other membranous cell components such as dictyosomes (fucosyltransferase, latent nucleosidediphosphate phosphatase), endoplasmic reticulum vesicles (NADH- and NADPH- cytochrome c reductase), mitochondria (succinate-2(p-indophenyl)-3-p-nitrophenyl)-5-phenyl tetrazolium-reductase and cytochrome oxidase) and plastids (carotenoids and monogalactosyl diglyceride synthetase) were identified on the basis of appropriate marker constituents and, except for plastid thylakoids, had thin (<7 nanometers) membranes. They were located in the fractions intermediate between plasma membrane and tonoplast after free-flow electrophoretic separation and did not contaminate either the plasma membrane or the tonoplast fraction as determined from marker activities. From electron microscope morphometry (using both membrane measurements and staining with phosphotungstic acid at low pH) and analysis of marker enzymes, both plasma membrane and tonoplast fractions were estimated to be about 90% pure. Neither fraction appeared to be contaminated by the other by more than 3%.

NADP phosphatase as a marker in free-flow electrophoretic separations for cisternae of the Golgi apparatus midregion

Based on cytochemical analysis, the enzyme NADP phosphatase is most concentrated in the so-called intercalary cisternae from the mid-region of the Golgi apparatus stack. Using free-flow electrophoresis to separate different Golgi regions of rat liver Golgi apparatus, the NADP phosphatase activity, based on estimation of the rate of release of inorganic phosphate from NADP under standard conditions, was similarly localized to membrane fractions from the center of electrophoretic separations. Peak specific activities for both a putative cis marker (NADH-cytochrome c reductase) and an established trans marker (galactosyltransferase) coincided with minima in NADP phosphatase activity, in agreement with the cytochemical observations. The pattern of distribution of enzyme activity for NADP phosphatase differed from that of both acid phosphatase and glucose-6-phosphatase. The pH optimum was 5.0, the Km for NADP was 0.6 mM and a corresponding production of NAD and inorganic phosphorus was shown. Taken together with other markers for free-flow electrophoresis separation, the NADP phosphatase will provide considerable utility as a specific marker to help identify intercalary cisternae of the mammalian Golgi apparatus and to monitor electrophoretic separations.

Decrease of NADH in HeLa cells in the presence of transferrin or ferricyanide

The short-term incubation of HeLa cells in the presence of diferric transferrin or ferricyanide, which are reduced externally by the transplasma membrane reductase, produces a stoichiometric decrease in NADH and increase in NAD+, which is stimulated by insulin. The NADP/NADPH ratio does not change during 15 min incubation with the oxidants. The total pyridine nucleotide pool of HeLa cells is not affected. Incubation with apotransferrin and ferrocyanide, which cannot act as oxidants for transmembrane electron transport, does not change the pyridine nucleotide concentrations in the cells. Our results show that NADH can act as the internal electron donor for the reduction of external oxidants by the transmembrane reductase. It appears that oxidation of NADH by the transmembrane electron transport using ferricyanide or iron transferrin as external electron acceptors is sufficient to stimulate growth in HeLa cells.

Fibronectin binding to gangliosides and rat liver plasma membranes

Binding of fibronectins to gangliosides was tested directly using several different in vitro models. Using an enzyme-linked immunoabsorbent assay (ELISA), gangliosides were immobilized on polystyrene tubes and relative binding of fibronectin was estimated by alkaline phosphatase activity of conjugated second antibody. Above a critical ganglioside concentration, the gangliosides bound the fibronectin (GT1b congruent to GD1b congruent to GD1a greater than GM1 much greater than GM2 congruent to GD3 congruent to GM3) in approximately the same order of efficiency as they competed for the cellular sites of fibronectin binding in cell attachment assays (Kleinman et al., Proc natl acad sci US 76 (1979) 3367). Alternatively, these same gangliosides bound to immobilized fibronectin. Rat erythrocytes coated with gangliosides GM1, GD1a or GT1b bound more fibronectin than erythrocytes not supplemented with gangliosides. Using fibronectin in which lysine residues were radioiodinated, an apparent Kd for binding to mixed rat liver gangliosides of 7.8 X 10(-9) M was determined. This value compared favorably with the apparent Kd for attachment of fibronectin to isolated plasma membranes from rat liver of 3.7 X 10(-9) M for fibronectin modified on the tyrosine residue, or 6.4 X 10(-9) M for fibronectin modified on lysine residues. As shown previously by Grinnell & Minter (Biochem biophys acta 550 (1979) 92), fibronectin modified on tyrosine residues did not promote spreading and attachment of CHO cells. It did, however, bind to cells. In contrast, lysine-modified fibronectin both bound to cells and promoted cell attachment. Plasma membranes isolated from hepatic tumors in which the higher gangliosides that bind fibronectin were depleted bound 43-75% less [125I]fibronectin than did plasma membranes from control livers. The findings were consistent with binding of fibronectins to gangliosides, including the same gangliosides depleted from cell surfaces during tumorigenesis in the rat.

Oxidation of reduced pyridine nucleotides by plasma membranes of soybean hypocotyl

Highly purified plasma membranes isolated from soybean hypocotyls by free-flow electrophoresis or by a two-phase polymer separation system oxidize reduced pyridine nucleotides, NADH or NADPH, at rates of 2-5 nanomoles/mg protein/min. These rates are not influenced by mitochondrial inhibitors or by inhibitors of the alternate respiratory pathway. The NADH oxidase has a Km of 200 microM NADH. The enzyme activity is stimulated by Ca2+ and Mg2+ ions. The function of this enzyme is unknown at present, but it may represent a redox-controlled proton pump linked to acidification.

Kinetics of monensin-induced swelling of Golgi apparatus cisternae of H-2 hepatoma cells

In H-2 hepatoma cells grown in culture, a pattern of response to the sodium-selective ionophore, monensin, was observed in which cisternal swelling commenced at one pole of the Golgi apparatus and was accompanied by the accumulation of swollen vacuoles in the cytoplasm in the absence of a decrease in the number of cisternae per stack. The results suggest that the swollen vacuoles are formed from Golgi apparatus cisternae that are replaced by formation of new cisternae. Depending on monensin concentration, between 2 and 4 such vacuoles accumulated per Golgi apparatus stack within 12.5 min after monensin addition. The rate of vacuole formation was greatest at 10(-6) M monensin and least at 10(-8) M monensin over the range 10(-5) to 10(-8) M monensin. The general pattern of early response to the ionophore is that formation of normal secretory vesicles is blocked, after which the Golgi apparatus continues to function for a time to release membrane as swollen cisternal units that accumulate in the cytoplasm at or near the trans Golgi apparatus face. These phenomena all precede the general swelling response normally observed in glutaraldehyde-fixed animal material and illustrate that the early dynamic responses to monensin are not confined to plant cells.

Dictyosome-like structures from guinea-pig testes lack galactosyltransferase, a Golgi apparatus marker

More than twenty different enzyme activities of fractions containing dictyosome-like structures (DLS) as a dominant cell component were monitored. Plasma membrane vesicles were a major contaminant of the DLS fractions, which, presumably as a consequence, were enriched somewhat in plasma membrane markers. The lysosomal enzymes arylsulfatase and latent acid phosphatase were present in the DLS fractions as were the Golgi apparatus activities thiamine pyrophosphatase and nucleoside diphosphatase. The presence of the latter two enzymes in DLS, plus NADH-ferricyanide reductase, has been verified from cytochemistry. On the other hand, the Golgi apparatus marker, galactosyltransferase, was not enriched in DLS fractions and appeared to be absent. This latter finding, verified from cytochemistry with isolated DLS fractions and, in situ, from [3H]galactose incorporation by testis tubules with analysis by autoradiography, provides the first clear biochemical characteristic that serves unequivocally to distinguish DLS from conventional Golgi apparatus.

Decreased NADH-oxidoreductase activities as an early response in rat liver to the carcinogen 2-acetylaminofluorene

Reduced nicotinamide adenine dinucleotide (NADH):ferricyanide reductase and DT-diaphorase specific activity in total homogenates of rat liver are markedly decreased as a very early biochemical event of hepatocarcinogenesis induced by the carcinogen 2-acetylaminofluorene (AAF). A 50 to 75% decrease in NADH:ferricyanide reductase was observed after 1 day of AAF (0.025% in the diet) feeding and persisted throughout a 7-week continuum of AAF administration. Carcinogen added directly to cell extracts had no effect. Similar results were obtained with single injections of either AAF or diethylnitrosamine. Xanthine dehydrogenase was also reduced in liver following AAF administration to nearly the same extent as NADH:ferricyanide reductase and DT-diaphorase. Total NADH-cytochrome c reductase and mitochondrial activity as estimated from succinic dehydrogenase were not affected by carcinogen administration relative to basal dietary controls. The reduced nicotinamide adenine dinucleotide phosphate:cytochrome c reductase that functions in drug detoxification was elevated. With livers of animals fed 4-acetamidophenol, a hepatotoxin chemically related to AAF, small decreases were noted in NADH:ferricyanide reductase, but not in xanthine dehydrogenase nor in DT-diaphorase. Initial lowering of these activities in the livers of the carcinogen-treated animals is preceded by or concomitant with a reduction in the levels of extramitochondrial pyridine nucleotides known from other studies to result from DNA damage.

Phosphatidylinositol turnover in isolated soybean membranes stimulated by the synthetic growth hormone 2,4-dichlorophenoxyacetic acid

Phospholipids of plant membranes isolated from homogenates of dark-grown hypocotyls of soybean (Glycine max L.) undergo rapid and specific degradative changes. The degradation of phosphatidylinositol (PI) in such membranes is enhanced in the presence of the synthetic auxin, 2,4-dichlorophenoxyacetic acid (2,4-D), measured as the hydrolysis of PI or by an enhancement of [3H]inositol incorporation into membrane-associated PI stimulated by Mn2+, but not dependent upon added CTP, Mg2+, or diglyceride. The response is rapid and enhanced by auxin throughout the physiological range of growth-promoting concentrations (optimum at about 7 X 10(-7) M). The growth-inactive 2,4-D analogue, 2,3-dichlorophenoxyacetic acid (2,3-D), is without effect. These findings suggest a cell-free response of isolated membranes to the hormone mediated by a definable enzymatic reaction.

An electrogenic proton pump associated with the Golgi apparatus of mouse liver driven by NADH and ATP

Golgi-apparatus membranes, isolated from mouse liver, pump protons inwards, when supplied with NADH or ATP. The acidification of Golgi-apparatus cisternae and vesicles was detected with neutral red, a permeant dye, as a difference in absorbance at 550 nm minus that at 600 nm. The maximum rates detected with NADH and ATP were between 0.0006-0.0009 and 0.0030-0.0050 delta OD units/mg of protein/min, respectively, at pH 7.5. The outside buffer used was a bovine serum albumin suspension. The acidification of Golgi apparatus was inhibited from 45 to 100% by ionophores and from 22 to 100% by uncouplers. The results implicate both ATP and a redox system coupled to NADH oxidation in the acidification of Golgi-apparatus membranes.

Immunocytochemical localization of class I H-2 histocompatibility antigens of mouse liver

The subcellular location of class I H-2 histocompatibility antigens was determined for mouse liver using immunocytochemical techniques and correlated with information determined by cell fractionation and analysis in situ. Surface antigens first were localized by standard procedures involving surface labeling with ferritin-labeled antibody. This approach could not be used for internal membranes either in situ or in fractions since the antigens are not expressed at the cytoplasmic surface. For this purpose, thin sections of tissues embedded in Lowicryl were analyzed and quantitated. The in situ analysis confirmed the presence of H-2 antigens on internal membrane compartments as well as on the cell surface and helped rule out the possibility that distributions based on analyses by immunoprecipitation of fractions of internal membranes were influenced greatly by plasma membrane contamination. Quantitation was provided by immunoprecipitation of H-2 antigens from radioiodinated or metabolically labeled isolated and highly purified cell fractions. The findings establish the presence of class I H-2 histocompatibility antigens in endoplasmic reticulum, Golgi apparatus and plasma membrane in the approximate ratios of 1:3:7. No class I H-2 histocompatibility antigens could be detected in mitochondria, salt extracts of isolated membranes or NP-40-insoluble membrane material.

Flow kinetics of fibronectin secretion by livers of young preweanling rats

Fibronectin is a high molecular weight adhesive glycoprotein present both on the cell surface (cellular fibronectin) and circulating free in the plasma (plasma fibronectin). To investigate the cellular origins of plasma fibronectin, monoclonal antibodies prepared against purified plasma fibronectin of the rat were used in both in vivo and in vitro pulse-chase experiments comparing livers of animals of different ages. For in vitro analyses, slices were pulsed with 3H-leucine and purified fractions of endoplasmic reticulum, Golgi apparatus and plasma membrane were prepared at various times post-chase and characterized morphologically. The total protein was solubilized with NP-40, and incorporation into fibronectin was determined from PAGE analysis of immunoprecipitates. Incorporation was greatest in 5-day-old animals, representing greater than 0.3% of the total radioactivity and was least in adults animals. Label appeared in fibronectin associated with the plasma membrane within 60 min. At earlier times, labeled fibronectin appeared first in the endoplasmic reticulum (30 min) and then in the Golgi apparatus (45 min). The findings demonstrate synthesis of fibronectin by rat liver and evidence for participation of liver endomembranes in its secretion.

Cyclic modulation of enzymes of pyrimidine nucleotide biosynthesis precedes sialoglycoconjugate changes during 2-acetylaminofluorene-induced hepatocarcinogenesis in the rat

Three enzymatic activities associated with pyrimidine nucleotide biosynthesis were monitored at weekly or bi-weekly intervals during 2-acetylaminofluorene- (0.025% in a Farber Basal Carcinogenic diet) induced hepatocarcinogenesis in the rat. Dihydroorotate dehydrogenase, the fourth of six enzymes in de novo pyrimidine biosynthesis, declined in activity while UDP kinase and CTP synthetase showed sequential increases in activity. The alterations in activity appeared to be cyclic, followed by a full or partial return to control values. Three full cycles were monitored. The first cycle preceded nodule formation. The second cycle accompanied nodule formation and preceded sialoglycoconjugate changes reported previously. The third cycle accompanied the early glycoconjugate changes. The cyclic pattern was reproducible in three separate experiments. In each cycle, the order of events was as follows: decrease in dihydroorotate dehydrogenase, sequential increases in UDP kinase, CTP synthetase and CMPsialic acid synthase, and finally increases in the enzyme lactosylceramide: CMPsialic acid sialyltransferase, lipid-soluble sialic acid and total sialic acid. In livers of animals fed 1.87% of the hepatotoxin, 4-acetamidophenol, no biochemical alterations resembling those induced by 2-acetylaminofluorene were obtained, despite acute centrilobular necrosis of the livers. The findings point to a biochemical cascade beginning with administration of carcinogen and continuing through the development of hyperplastic nodules and of frank carcinomas resulting not from hepatotoxicity but as events associated with the hepatocarcinogenic progression.

Monensin-induced swelling of Golgi apparatus cisternae mediated by a proton gradient

Monensin, a monovalent ionophore, caused swelling of mature cisternae of plant Golgi apparatus. The appearance of swollen cisternae was time-dependent and linear over a period of 1 h with an estimated maximum rate of production of one swollen cisterna every 3 to 4 min. Implicit in these observations was a need for the uptake of osmotically active monovalent cations to have occurred accompanied by a concomitant efflux of H+ and the entry of water. Furthermore, to sustain the H+ efflux, a source of H+ influx also would be required. To test for the latter, cisternal swelling, as visualized by electron microscopy, was monitored by treatment of wild carrot cells in suspension culture with drugs and inhibitors known to interfere with proton gradients. Swelling was inhibited by the protonophore, FCCP, by the inhibitor of lysosomal acidification, quercetin, and by the lysosomotropic amines, chloroquine and ammonia. While antimycin A, an inhibitor of mitochondrial oxidative phosphorylation, was ineffective, cyanide dramatically decreased swelling. The numbers of swollen cisternae produced could be reduced by prolonged treatment with arsenate, such that an ATP requirement is indicated, at least, for cisternal formation. Swelling was promoted by citrate, representative of a permeant organic anion. Reductions in numbers of monensin-induced swollen cisternae in the presence of quercetin, vanadate, and chloroquine could be compensated for by the addition of citrate. We conclude that the monensin-induced swelling of Golgi apparatus cisternae may involve a mechanism generating a proton gradient at or near the mature Golgi apparatus face.

Phosphorylation of membrane-located proteins of soybean in vitro and response to auxin

Isolated membranes of soybean incorporate (32)P from gamma-[(32)P]ATP in vitro. The incorporation was rapid and did not require added calcium. When displayed on 10% sodium dodecyl sulfate-polyacrylamide gels, several protein bands were revealed. An apparent auxin (2,4-dichlorophenoxyacetic acid) stimulation of (32)P incorporation into material from membrane vesicles insoluble in trichloroacetic acid-perchloric acid may be reflected partly in enhanced incorporation into protein bands with apparent molecular weights of 45,000 and 50,000. Additionally, a low molecular weight component was sometimes observed where incorporation was stimulated 2- to 3-fold by auxin. However, protein-bound radioactivity represented only a small fraction of the total radioactivity of the acid-insoluble material. Other labeled constituents, not retained on the gels, may contribute to the apparent, rapid (10 s or less) auxin response of the isolated membranes. Stimulation of incorporation into the low molecular weight component was given by diglyceride plus calcium, constituents known to augment protein kinase activities in other systems.

Sialoglycoconjugate changes during 2-acetylaminofluorene-induced hepatocarcinogenesis in the rat

Previous studies indicated a reproducible pattern of altered glycosphingolipid biosynthesis accompanying late stages of liver tumorigenesis in the rat induced by the carcinogen 2-acetylaminofluorene. The sequence began with a dramatic elevation in CMP-sialic acid:lactosylceramide sialyltransferase and was followed by sequential elevations and eventual depressions in other enzymes catalyzing sugar transfers to glycolipid acceptors. The present study focused on the early events of glycolipid biosynthesis during the first 11 weeks of 2-acetylaminofluorene administration according to the same feeding schedule as used previously. Transient elevations in CMP-sialic acid synthetase and elevations in neutral glycosphingolipid precursors to gangliosides were found to precede the major elevations in CMP-sialic acid:lactosylceramide sialyltransferase (GM3 synthetase) noted earlier. Two cycles of response were observed prior to the initiation of the sustained enhancement of biosynthesis of precursor ganglioside, GM3, and/or a significant increase in total or lipid-soluble sialic acid. In vitro rates of sialyl transfer from CMP-sialic acid to endogenous protein acceptors were not altered. The results suggest that the previous observations of altered ganglioside biosynthesis following 2-acetylaminofluorene administration are not an isolated occurrence but may represent late events in a sequence or 'cascade' of biochemical change involving, as well, biosynthesis of ganglioside precursors, CMP-sialic acid and neutral glycosphingolipids.

Isolation of plasma membrane, golgi apparatus, and endoplasmic reticulum fractions from single homogenates of mouse liver

Procedures to isolate plasma membrane, Golgi apparatus, and endoplasmic reticulum from a single homogenate of mouse liver are described. Fractions contain low levels of contaminating membranes as determined from morphometry and analyses of marker enzymes. The method requires only 2-3 gm of liver as starting material and yields approximately 0.7, 0.7, and 0.5 mg protein/gm liver, respectively, for endoplasmic reticulum, Golgi apparatus, and plasma membrane. Golgi apparatus fractions show high levels of galactosyltransferase activity and consist of cisternal stacks and associated secretory vesicles and tubules. Endoplasmic reticulum fractions are enriched in both glucose-6-phosphatase and nicotinamide adenine dinucleotide phosphate (reduced) (NADPH)-cytochrome c reductase and contain membrane vesicles with attached ribosomes. K+-stimulated p-nitrophenyl phosphatase and (Na+K+) adenosine triphosphatase activity are enriched in the plasma membrane fraction. This fraction consists of membrane sheets, many with junctional complexes, and bile canaliculi that are representative of the total hepatocyte plasma membrane. The fractionation procedure is designed to utilize small amounts of tissue (e.g., with liver slices), to reduce the total time required for fractionation, and to permit comparisons of constituents of plasma membrane, Golgi apparatus, and endoplasmic reticulum prepared from the same starting homogenates.

Coated vesicles of liver Golgi apparatus increased by ascorbate

Cultured hepatocytes and hepatoma cells when treated for 10-30 min with 30 mM ascorbate (vitamin C) responded by a 2- to 3- fold increase in the numbers of coated vesicles of the Golgi apparatus zone as determined by quantitation from electron micrographs. At a near optimal concentration of 30 mM ascorbate, the effect was reproduced under a variety of conditions of fixation and methods of specimen preparation.

Dictyosome-like structures present in human testicular cells

Dictyosome-like structures (DLS) are present in human spermatocytes and spermatids. They display morphological and developmental characteristics similar to those of other species that contain DLS.

Monodehydroascorbate as an electron acceptor for NADH reduction by coated vesicle and Golgi apparatus fractions of rat liver

Coated vesicles were isolated from rat liver in about 80% fraction purity as determined from electron microscopy and analyses of marker enzymes and compared with Golgi apparatus and other membrane fractions isolated in parallel. The fractions were enriched in NADH-monodehydroascorbate reductase, ascorbate oxidase and ascorbic acid. The NADH-monodehydroascorbate reductase and ascorbate oxidase of the Golgi apparatus and coated vesicles differed from that of the endoplasmic reticulum in being inhibited by the sodium selective ionophore, monensin, at physiological concentrations while these activities were stimulated by ethylenediaminetetraacetic acid in coated vesicles but not in Golgi apparatus. Activities of both coated vesicles and Golgi apparatus fractions depleted in the coat protein, clathrin, were activated by the addition of clathrin-rich supernatant fractions. The results are discussed in the context of monodehydroascorbate as an acceptor for electron transport-mediated transfer of electrons from NADH by coated vesicles as part of a possible mechanism to drive membrane translocations or to acidify the interiors of vesicles.

Laminated figures of the intercisternal regions of dictyosome-like structures from guinea-pig spermatocytes fixed with glutaraldehyde-tannic acid

Dictyosome-like structures (DLS) of guinea pig spermatocytes, when prefixed in mixtures of glutaraldehyde and tannic acid, exhibited laminated figures with a repeating periodicity of about 4.5 nm in the spaces between DLS saccules or in association with the surfaces of the DLS saccules. These laminated figures were similar to those figures derived from saturated lipids in other tissues. Alternatively, spaces between saccules were collapsed leaving only thin, electron-dense material separating adjacent saccules. These changes were not observed when the DLS were prefixed in glutaraldehyde before exposure to tannic acid. The presence of laminated figures following fixation with tannic acid and osmium tetroxide suggests that saturated lipids are present in, or associated with, the intersaccular regions of the DLS. The distribution of laminated figures in other membrane structures was not affected by post fixation with tannic acid nor were laminated figures comparable to those of the DLS observed between cisternae of the Golgi apparatus. These results support previous conclusions that DLS are distinct from Golgi apparatus and are a unique component of the germ cell cytoplasm.

Coupling of uridine-5'-diphosphate (UDP) formation and nicotinamide adenine dinucleotide (NAD+) reduction for cytochemical localization of glycosyltransferases

A technique applicable to the cytochemical localization of glycosyltransferases through a series of coupled enzyme reactions is described. Uridine-5'-diphosphate (UDP) formed by glycosyltransferases is first phosphorylated to uridine-5'-triphosphate (UTP) by nucleoside 5'-diphosphate kinase. The UTP plus exogenous glucose-1-phosphate is converted into UDP-glucose by uridine-5'-diphosphoglucose pyrophosphorylase. UDP-glucose is then oxidized by uridine-5'-diphosphoglucose dehydrogenase to form UDP-glucuronic acid and reduced nicotinamide adenine dinucleotide (NADH). The NADH is utilized by membrane-located NADH-ferricyanide oxidoreductases in the presence of a copper salt to form electron-dense deposits of cupric ferrocyanide (Hatchett's brown). Using this technique, galactosyltransferase has been localized in cisternae (including the central midregions of the cisternae) of Golgi apparatus isolated from rat liver. Reactivity is absent from the cis-most cisternae and membrane elements. The reaction is dependent on UDP-galactose and inhibited by ethylene diaminetetraacetic acid and puromycin. the latter is a known inhibitor of galactosyltransferase of rat liver Golgi apparatus. The reaction is adaptable by varying the sugar nucleotide donor and acceptor to any glycosyltransferase utilizing UDP-sugars (except UDP-glucose). Presently it is restricted to isolated membrane fractions and permeabilized cells due to the need for accessibility of reagents and coupling enzymes.

Calmodulin-NADH semidehydroascorbate oxidoreductase interactions of clathrin coated vesicles

NADH semidehydroascorbate oxidoreductase activity is present in clathrin coated vesicles isolated from rat liver. The activity of the enzyme on the coated vesicles and Golgi apparatus but not that of endoplasmic reticulum is stimulated by calmodulin and is inhibited by three different drugs which are known inhibitors of calmodulin function including trifluoperizine, pimozide and R24571. Extraction of clathrin from the vesicles causes a decrease in activity which can be partially restored when the extracted clathrin is added back. Added calmodulin also restores much of the activity which is lost when the clathrin is removed and the specific activity of added pure calmodulin is similar to that of the crude clathrin on a protein basis. There is a decrease in enzyme activity if coated vesicles or Golgi apparatus are treated with a calcium antagonist (8-[N,N-diethylamino]-octyl-3,4,5-trimethoxybenzoate) (TMB-8). However, the enzyme activity can be recovered to that of the untreated control if calcium (6.0 mM) is added. An additive stimulatory effect on enzyme activity is also observed when both calcium (1.0 mM) and calmodulin (40 micrograms/ml) are present in the vesicles simultaneously. The results show that the NADH-semidehydroascorbate oxidoreductase of coated vesicles and Golgi apparatus have regulatory properties different from those of the microsomal electron transport system. Calmodulin-calcium control mediated through the semidehydroascorbate reductase, may be among the components that regulate Golgi apparatus secretion.

Subfractionation of rat liver Golgi apparatus by free-flow electrophoresis

Using the technique of preparative free-flow electrophoresis, cisternae of unstacked rat liver Golgi apparatus were separated into a series of fractions of increasing content of sialic acid, thiamine pyrophosphatase and 5'-nucleotidase, markers regarded as being concentrated toward the mature Golgi apparatus face. These same fractions showed a decreasing content of nucleoside diphosphatase, an endoplasmic reticulum marker. Fractions enriched in sialic acid also were enriched in cisternae from the mature or trans face of the Golgi apparatus as deduced from cytochemical criteria. Those fractions least enriched in sialic acid contained cisternae that accumulated deposits of reduced osmium under standard conditions, a test used to mark the opposite, forming or cis-face. Thus subfractionation along the functional polarity axis of the Golgi apparatus with separation of cis and trans face cisternae has been achieved.

Kinetics of Golgi apparatus membrane flux following monensin treatment of embryogenic carrot cells

Ultrastructural changes resulting from treatment with the sodium selective ionophore, monensin, were studied in embryogenic suspension cultures of carrot, Daucus carota (L.) in the presence of 10 microM monensin, an early change in the Golgi apparatus was an increase in the number of cisternae per stack (dictyosome). An average of one additional cisterna per stack was formed within the first 2 to 4 min of monensin treatment; in some experiments a second cisterna was formed within about 8 min. Thereafter, large vacuoles began to appear in the cytoplasm adjacent to the Golgi apparatus with a return of the number of cisternae per dictyosomal stack to the control number of about 5. Cells treated comparable but in the absence of monensin showed no ultrastructural changes during the entire observation period. By 1 h of monensin treatment, the regions of the cells containing dictyosomes were populated by large number of vacuoles (up to 20 or more per electron microscope section). These vacuoles were interpreted as swollen dictyosome cisternae that separated from the stack but had not migrated from the Golgi apparatus zone in the monensin-treated cells. The results permitted an estimation of the average time for formation of a new dictyosome cisterna of 2 to 4 min. This range of values agreed with estimates for mammalian cells from short time labeling and turnover experiments of 3 to 4 min assuming a dynamic model for Golgi apparatus function in which cisternae are released from a maturing face and new cisternae are built up at an opposite or forming face.

Mannosyl carrier functions of retinyl phosphate and dolichyl phosphate in rat liver endoplasmic reticulum

Of the subcellular fractions of rat liver the endoplasmic reticulum was the most active in GDP-mannose: retinyl phosphate mannosyl-transfer activity. The synthesis of retinyl phosphate mannose reached a maximum at 20-30 min of incubation and declined at later times. Retinyl phosphate mannose and dolichyl phosphate mannose from endogenous retinyl phosphate and dolichyl phosphate could also be assayed in the endoplasmic reticulum. About 1.8 ng (5 pmol) of endogenous retinyl phosphate was mannosylated per mg of endoplasmic reticulum protein (15 min at 37 degrees C, in the presence of 5 mM-MnCl2), and about 0.15 ng (0.41 pmol) of endogenous retinyl phosphate was mannosylated with Golgi-apparatus membranes. About 20 ng (13.4 pmol) of endogenous dolichyl phosphate was mannosylated in endoplasmic reticulum and 4.5 ng (3 pmol) in Golgi apparatus under these conditions. Endoplasmic reticulum, but not Golgi-apparatus membranes, catalysed significant transfer of [14C]mannose to endogenous acceptor proteins in the presence of exogenous retinyl phosphate. Mannosylation of endogenous acceptors in the presence of exogenous dolichyl phosphate required the presence of Triton X-100 and could not be detected when dolichyl phosphate was solubilized in liposomes. Dolichyl phosphate mainly stimulated the incorporation of mannose into the lipid-oligosaccharide-containing fraction, whereas retinyl phosphate transferred mannose directly to protein.

Neutral lipid precursors for gangliosides are not formed by rat liver homogenates or by purified cell fractions

As part of an ongoing investigation into the ganglioside and neutral glycosphingolipid biosynthetic pathways in rodent liver, the synthesis of ceramide, glucosylceramide and lactosylceramide was examined in homogenates and purified membrane fractions of rat liver. Ceramide synthesis and its stimulation by exogenously added D-sphingosine was demonstrated in homogenates and isolated membrane fractions, with activity concentrated in the endoplasmic reticulum. In contrast, significant D-sphingosine- or ceramide-dependent formation of glucosylceramide, or glucosylceramide-dependent formation of lactosylceramide (in the presence of labelled UDP-[14C]glucose and UDP-[14C]galactose, respectively,) could not be demonstrated. The possibility is raised that liver may be dependent on extra-hepatic sources of neutral glycosphingolipids to support ganglioside biosynthesis.

Dictyosome polarity and membrane differentiation in outer cap cells of the maize root tip

Outer rootcap cells of maize produce large numbers of secretory vesicles that ultimately fuse with the plasma membrane to discharge their product from the cell. As a result of the fusion, these vesicles contribute large quantities of membrane to the cell surface. In the present study, this phenomenon has been investigated using sections stained with phosphotungstic acid at low pH (PACP), a procedure in plant cells that specifically stains the plasma membrane. In the maize root tip, the PACP also stains the membranes of the secretory vesicles derived from Golgi apparatus to about the same density that it stains the plasma membrane. Additionally, the membranes of the secretory vesicles acquire the staining characteristic while still attached to the Golgi apparatus. The staining progresses across the dictyosome from the forming to the maturing pole, thus confirming the marked polarity of these dictyosomes. Interestingly, the PACP staining of Golgi apparatus is confined to the membranes of the secretory vesicles. It is largely absent from the central plates or peripheral tubules and provides an unambiguous example of lateral differentiation of membranes orthogonal to the major polarity axis. In the cytoplasm we could find no vesicles other than secretory vesicles bearing polysaccharide that were PACP positive. Even the occasional coated vesicle seen in the vicinity of the Golgi apparatus did not stain. Thus, if exocytotic vesicles are present in the maize root cap cell, they are formed in a manner where the PACP-staining constituent is not retained by the internalized membrane. The findings confirm dictyosome polarity in the maize root cap, provide evidence for membrane differentiation both across and at right angles to the major polarity axis, and suggest that endocytotic vesicles, if present, exclude the PACP-staining component.

Changes in protein kinase activities during 2-acetylaminofluorene-induced hepatocarcinogenesis in the rat

Livers of rats fed the carcinogen 2-acetylaminofluorene (AAF) at a concentration of 0.025% were analyzed for protein kinase activities with [gamma 32P]ATP as substrate and either endogenous or exogenous (casein or histone) protein acceptors both in the presence or absence of cyclic nucleotides. Total protein kinase activity of the nuclear fraction, with exogenous histone or casein as substrate, was elevated during the first week of carcinogen administration. Total cytoplasmic kinase activities exhibited a pattern of activity change with maxima at about 25 and 42-49 days after the onset of carcinogen administration. Cyclic AMP levels rose steadily to approximately a 4-fold elevation by day 49 in livers of animals receiving carcinogen with the increase beginning prior to the development of externally visible nodular hyperplastic lesions. The findings demonstrate consistent and reproducible patterns of change in protein kinase activities that accompany AAF-induced hepatocarcinogenesis in the rat and provides the basis for a more detailed investigation of specific kinases.

Flow kinetics of a nucleoside phosphatase common to endoplasmic reticulum, Golgi apparatus, and plasma membrane of rat liver

Nucleoside mono-, di- and triphosphatase activities of highly purified endoplasmic reticulum (ER), Golgi apparatus, and plasma membrane fractions of rat liver were compared. The highest rates of hydrolysis were always in ER or plasma membrane. Golgi apparatus activity was intermediate between those of ER and plasma membrane. This relationship was true for both freshly isolated fractions and salt-extracted membranes. Detergent solubilization of the membranes, polyacrylamide gel electrophoresis of the solubilized proteins, and localization of the enzyme activities on the gel revealed bands of enzyme activity which had identical mobilities in all three membrane fractions as well as other bands of activity that occurred only in ER and to a lesser degree in the Golgi apparatus. Antibodies raised against one of the phosphatase bands of plasma membrane which was common to all three membrane fractions cross-reacted with the corresponding phosphatase band in ER and Golgi apparatus. The anti-nucleoside phosphatase was utilized in combination with pulse-chase techniques to investigate the flow kinetics of transfer of newly synthesized enzyme among different cell compartments. Label first appeared in nucleoside phosphatase within the ER. Maximum specific activity was observed at about 5 min after injection of label and was followed by rapid loss of label. This was followed by appearance of label in Golgi apparatus 15 to 25 min after injection of label and by subsequent rapid loss of label. Plasma membranes were labeled last with no evidence of either rapid accumulation of label or of rapid turnover. Flow of nucleoside phosphatase from its site of synthesis and insertion into the membrane at the endoplasmic reticulum to the plasma membrane via the Golgi apparatus is indicated but in a manner whereby a significant fraction of the protein may be processed (removed?) from the membrane concomitant with the flow process.

Membrane differentiation in the Golgi apparatus of mammalian urinary bladder epithelium

The endomembrane system in superficial and intermediate epithelial cells of mammalian urinary bladder was studied by cytochemistry, thin-section and freeze-fracture electron microscopy to determine the sites where special forms of membrane differentiation first appear. Glutaraldehyde-resistant NADH-ferricyanide reductase, distinctive 11-12 nm intramembrane particles (IMP), and asymmetry of membrane leaflets served as markers of membrane maturation. The three markers were specifically associated with the maturing face of Golgi apparatus and were absent from the remainder of the endomembrane system. Activity of this enzyme was associated with the lateral regions of the maturing face, fusiform vesicles, and the plasmalemma. Asymmetric unit membrane (AUM) plaques were not observed in the Golgi apparatus per se but were present in immature fusiform vesicles that had not detached from the maturing face. When freeze-fracture replicas and thin sections were compared, randomly arranged 11-12 nm IMP first appeared in maturing face membranes that were adjacent to clusters of "free" polyribosomes in the Golgi apparatus region. The proximity of these polyribosomes suggests that they may be related to the coincident appearance of the 11-12 nm IMP in the maturing face membrane. Our observations support the hypothesis that membranes undergo differentiation during "flow" through compartments of the endomembrane system. The lateral regions of the maturing face of the Golgi apparatus appear to be a critical location for the morphogenesis of plasma membranes in urinary bladder.

Albumin secreted by rat liver bypasses Golgi apparatus cisternae

Albumin was isolated immunologically from various subcellular fractions from livers of adult male rats receiving an intraperitoneal injection of [3H]leucine to investigate the kinetics and pathway of subcellular transfer of newly synthesized albumin during secretion. At appropriate time intervals, livers were excised and fractionated into endoplasmic reticulum and Golgi apparatus. Golgi apparatus were further subfractionated into cisternae and secretory vesicles. In endoplasmic reticulum fractions labeled albumin appeared within 7.5 min of injection of isotope, followed by a rapid decline in specific activity. Albumin in Golgi apparatus was labeled and concentrated in secretory vesicles over 25 min. The radioactivity in albumin per mg total protein was highest in secretory vesicles and insignificant in the cisternal fraction. Labeled albumin was present in serum by 30 min and radioactivity in serum albumin reached a plateau within 60-90 min after injection of isotope. Results provide evidence for the migration of albumin from its site of synthesis on endoplasmic reticulum membrane-bound polyribosomes to its site of secretion into the circulation via the Golgi apparatus. The pathway of albumin transport to secretory vesicles is suggested to involve peripheral elements of the Golgi apparatus. Secretory vesicle formation and maturation required 20 to 30 min for completion, via a mechanism whereby the inner spaces of the central saccules may be bypassed.

Kinetics of appearance of lipoprotein particles in perisinusoidal cisternae of smooth endoplasmic reticulum of isolated rat livers perfused with free fatty acid

Vesicles containing single lipoprotein particles are found in the perisinusoidal cytoplasm of rat hepatocytes in the proximity of particle-containing rough-smooth endoplasmic reticulum transition elements. these lipoprotein particle-containing smooth endoplasmic reticulum elements provide an alternative to Golgi apparatus as a supplier of very low density lipoprotein particles to the circulation. To test this possibility, the kinetics of appearance of lipoprotein particles in these elements was determined for isolated rat livers first depleted of particles and then perfused with free fatty acid to restore particles. The results show that elements of the smooth endoplasmic reticulum in the perisinusoidal cytoplasm acquire lipoprotein particles in advance of elements of the conventional Golgi apparatus and that the particles contained within the perisinusoidal elements of smooth endoplasmic reticulum are not products of endocytotic uptake of particles from the circulation.

Endoplasmic Reticulum Formation during Germination of Wheat Seeds : A QUANTITATIVE ELECTRON MICROSCOPE STUDY

This study demonstrates germination-induced ultrastructural changes in wheat (Triticum aestivum L. cv Arthur) aleurone cells. Seeds imbided for 4 hours in water contained endoplasmic reticulum (ER) or ER-like membranes as vesicles or as short segments of membrane associated with the spherosomes on the periphery of aleurone grains. Aleurone cells incubated between 8 and 10 hours contained abundant ER membranes mainly associated with the nuclear envelope and, to a lesser extent, with the spherosomes surrounding the aleurone grain. The membranes located on the periphery of the nucleus occurred as regions of stacked cisternae. When aleurone cells were analyzed by morphometry, the increase in ER during incubation was found to be greater than 2-fold. During the same incubation period, other organelles did not change significantly. The early increase in ER was not affected by gibberellin incubation. Thus, the rapid proliferation of ER observed during the early stages of germination in aleurone cells of wheat is not likely to be controlled directly by gibberellin.

An alternative pathway for secretion of lipoprotein particles in rat liver

Vesicles containing single lipoprotein particles were observed in the perisinusoidal cytoplasm of rat hepatocytes in the proximity of particle-containing rough-smooth endoplasmic reticulum transition elements. Single particle vesicles were observed also in stages interpreted as fusion with the plasma membrane. The putative smooth endoplasmic reticulum-derived vesicles were clearly different from the multiparticle secretory vesicles derived from the Golgi apparatus. The findings suggest the operation of two pathways of lipoprotein secretion in rat liver. One follows the orthodox smooth endoplasmic reticulum Golgi apparatus secretory vesicle plasma membrane export route. The second seems to proceed via smooth endoplasmic reticulum secretory vesicle plasma membrane in a manner that bypasses the Golgi apparatus.

Inhibition of Golgi apparatus-mediated secretion by cinnamic acid derivatives

Two test systems have been utilized for screening potential inhibitors of Golgi apparatus function. In one system we measured formation of external polysaccharide droplets by the outer cap cells of maize roots. The droplets, direct products of the Golgi apparatus, were quantitated by visual estimates of droplet size compared to known standards. In the second system, we monitored secretory granule formation in the parotid gland of the rat, an activity also mediated by the Golgi apparatus. Parotid gland slices were stimulated with epinephrine to achieve 90% release of granules. The ability of the Golgi apparatus to package new secretory granules was then tested by incubating degranulated gland slices in the presence of putative inhibitors followed by restimulation with epinephrine. The amounts of alpha-amylase or (3H)leucine labeled proteins secreted into the medium were used as measures of secretory granule formation. The phenolic acid cinnamic acid and a related compound scopoletin were found to show substantial inhibition of secretion in both test systems.

Selective staining of dictyosome-like structures (DLS) from spermatocytes of the guinea pig using phosphotungstic acid at low pH

Dictyosome-like structures (DLS) occur abundantly in primary spermatocytes of the guinea pig. DLS superficially resemble dictyosomes of Golgi apparatus in that they consist of stacked cisternae and react similarly to some cytochemical markers. DLS saccules are also present in residual bodies and in the cytoplasmic droplet of the sperm, but the stacked configuration (or dictyosome form) is seldom present at these stages of development. A mixture of 1% phosphotungstic acid in 10% chromic acid selectively stains the DLS and DLS saccules of guinea pig germ cells. The thick cisternae of spermatid Golgi apparatus and the sperm plasma membrane also stain, but endoplasmic reticulum and the parts of the Golgi apparatus other than the thick cisternae do not stain. The specificity of the stain is retained in crude homogenates as well as in purified cell fractions and may be helpful in identification of DLS in cell fractionation studies. Additionally, the information obtained provides clues to the origin and fate of DLS in the developing mammalian germ cells.

In vitro promotion by auxins of divalent ion release from soybean membranes

Release of divalent ions from membrane pellets of soybean hypocotyls was promoted by the natural auxin, indole-3-acetic acid, and the synthetic auxin, 2,4-dichlorophenoxyacetic acid. The calcium release occurred at auxin concentrations as low as 1 nanomolar, and maximum release was observed at 1 micromolar. Hormone concentrations greater than 1 micromolar showed reduced effectiveness in releasing membrane-associated calcium. 2,3-Dichlorophenoxyacetic acid, a weak-auxin analog of 2,4-dichlorophenoxyacetic acid, did not promote calcium release. In some experiments, the analog actually promoted calcium association with the membranes. Red blood cells treated in a similar manner to soybean hypocotyl membranes did not release calcium in response to indole-3-acetic acid. The release phenomenon was hormone specific but not ion specific. Auxin released manganese from membranes in a manner similar to that of calcium. The calcium release, following auxin treatment, is accompanied by a decrease in membrane-associated sites for calcium binding.

Vitamin A effects on hepatic Golgi apparatus architecture

Effects of dietary vitamin A deficiency and excess on Golgi apparatus architecture were determined for livers of male CDF rats. In deficient animals, Golgi apparatus stacks were reduced in diameter and characterized by small, often highly fenestrated, saccules. Total membrane surface occupied by Golgi apparatus membranes also was less. Golgi apparatus of livers of animals receiving vitamin A in excess were more variable ranging from nearly normal appearance to much larger than normal. Saccules were larger and lacked the numerous fenestrations shown by Gol;gi apparatus of vitamin A deficient animals. Numbers and appearance of secretory vesicles and total number of small vesicular profiles were unaffected by vitamin A status of the animal. Unusual cisternal configurations were encountered much more frequent at the mature face of Golgi apparatus of animals fed vitamin A excess than in animals receiving adequate vitamin A and were absent from Golgi apparatus of animals receiving no vitamin A. Except for slightly swollen mitochondria and a loss of the "zone of exclusion" surrounding the nucleus, both observed in vitamin A excess, the Golgi apparatus response was the only ultrastructural alteration in liver that responded consistently to changes in vitamin A status.

Translocation of cytidine 5'-monophosphosialic acid across Golgi apparatus membranes

Golgi apparatus, isolated from rat liver, incorporate [14C]sialic acid from CMP[14C]sialic acid into endogenous glycolipid and glycoprotein acceptors. Incorporation of [14C]sialic acid into endogenous glycoprotein acceptors was stimulated an average of 3-fold by Triton X-100 at an optimal concentration of 0.05% and was inhibited at higher concentrations. Incorporation of [14C]sialic acid into endogenous glycolipid acceptors was not stimulated by detergent. The major glycolipid product was identified by thin-layer chromatography as the ganglioside GD3. SDS-polyacrylamide gel electrophoresis on the glycoprotein products demonstrated incorporation of [14C]sialic acid into 6--7 major bands. Neuraminidase studies determined that approximately 60% of the [14C]sialic acid incorporated into endogenous acceptors in the absence of detergent had a luminal orientation. Furthermore, electron microscopy studies showed that the isolated Golgi apparatus fraction consisted of intact membrane cisternae. Our results demonstrate that sialylation of cisternal acceptors located on the inside of the membrane occurs in the absence of detergent. They are consistent with carrier-mediated transport as a mechanism to allow CMPsialic acid to traverse the Golgi apparatus membrane and to be used to glycosylate endogenous glycoprotein and glycolipid acceptors.

Flow kinetics of mouse histocompatibility antigens

Major histocompatibility antigens of the mouse (H-2 antigens) are found on a variety of different cell types and constitute a class of integral membrane-bound glycoproteins involved in tissue graft rejection and immune surveillance. We used monospecific alloantibodies directed against mouse H-2 antigens and standard pulse-chase technique to investigate the flow kinetics of delivery of newly synthesized membrane constituents to the cell surface. [35S]Methionine was injected intraperitoneally, and, after a chase with unlabeled methionine, livers were excised and fractionated into endoplasmic reticulum, Golgi apparatus, and plasma membrane fractions. Label first appeared in H-2 antigens located within the endoplasmic reticulum. Maximum specific activity observed between 5-7 min after injection of label was followed by a rapid loss of label. H-2 antigens of Golgi apparatus also were labeled early. Peak specific activity observed 15-25 min after injection of label was again followed by rapid loss of label. H-2 antigens of the plasma membrane were labeled last and appeared to accumulate radioactivity with no evidence of rapid turnover. These results provide evidence for a precursor-product relationship between H-2 antigens located within the cell on membranes of endoplasmic reticulum and Golgi apparatus and those on the plasma membrane. Flow of individual membrane-bound glycoproteins from their sites of synthesis and insertion into the membrane of the endoplasmic reticulum to the plasma membrane through the Golgi apparatus is indicated.

Glycosylation reactions and tumor establishment: modulation by vitamin A

The ability of retinoids to prevent or alter the course of experimental tumorigenesis is well established. We have extended these observations to include effects on establishment of tumors and tumor metastases. A diet containing excess retinyl acetate fed to rats prior to injection of a metastatic line of transplantable hepatoma, prevented establishment of secondary tumor foci while 75% of the animals fed adequate retinyl acetate showed pulmonary metastases. Metastatic ability may be related to the ability to bind fibronectins, proteins that link cells to an underlying stroma. Findings suggest involvement of higher gangliosides in the attachment of cells to a fibronectin-collagen complex. Prior to metastasis, hepatoma lines become depleted in the putative fibronectin receptor gangliosides as an end result of a complex cascade of altered glycosyltransferase activities. After metastasis, fibronectin receptors are apparently restored in those secondary tumor foci that become established. Analyses suggest that excess vitamin A may prevent the reappearance of fibronectin receptor gangliosides so that secondary tumor foci do not establish.

Evidence for a plasma membrane redox system on intact ascites tumor cells with different metastatic capacity

A NADH-ferricyanide reductase of the external surface of intact mouse ascites tumor cells grown in culture was shown. The oxidation/reduction reaction was due to enzymatic rather than inorganic iron catalysis as demonstrated by the kinetics and specificity of the reaction. Activities of three markers for cytoplasmic contents were lacking with the intact tumor cells. The dehydrogenase activity was inhibited by p-chloromercuribenzoate, bathophenanthroline sulfonate, and the anticancer drug adriamycin. Sodium azide and potassium cyanide inhibited partially. The response to inhibitors resembled that of isolated plasma membranes rather than that of mitochondria. Concurrent with these findings, neither superoxide dismutase nor rotenone affected the redox activity. The findings provide evidence for the operation of a plasma membrane redox system at the surface of intact, living cells.

Glycolipids modulate glycosyl transfer to endogenous protein acceptors

Regulation by gangliosides of glycosylation of endogenous membrane glycoproteins is indicated from in vitro studies in which incorporation of radioactive sugars into endogenous protein acceptors was measured and from in vitro studies where transferase activities of membranes were correlated with ganglioside content during hepatic tumorigenesis. Galactosyl transfer from UDP galactose exhibited a complex response pattern and was stimulated by lactosyl ceramide and the ganglioside N-acetylgalactosaminyl-(N-acetylneuraminyl)-galactosylglucosylceramide (GM2) but was inhibited by higher gangliosides. Except for N-acetylneuraminylgalactosylglucosylceramide (GM3), which had no effect, inhibition was proportional to ganglioside complexity. Inhibition of glycosylation of the exogenous acceptor, ovomucoid, by ganglioside was slight by comparison. While marked structure-linked latency was observed with the high molecular weight exogenous acceptor, no latency was observed for incorporation into endogenous acceptors suggesting that the membranes were permeable to sugar nucleotides. Membrane disruption with detergents lessened rather than enhanced inhibition by gangliosides. Sialyl transfer from CMPsialic acid, on the other hand, was unaffected or stimulated by gangliosides. Stimulation by galactosyl-N-acetylgalactosaminyl-(N-acetylneuraminyl)-galactosylglucosylceramide (GM1) was proportional to concentration and reached 2-fold at 240 micrograms/mg protein. The results suggest that the ganglioside content of membrane may affect glycosylation of membrane glycoproteins.