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PubMed1975

Digitonin resistance of liver mitochondria isolated from copper-deficient and copper-loaded rats.

Liver mitochondria of copper-deficient and copper-loaded rats are with changed composition, ultrastructure and functions. The aim of the present work is to study the digitonin resistance of these mitochondria as an indicator for the changes in the membranes. Copper deficiency and copper loading are achieved by eliminating resp. by adding copper to the diet for four to six weeks. The copper content of the mitochondria in the control group of animals is 0.55 X 10(-9)M/mg protein. 0.20 X 10(-9) M/mg protein in the copper-deficient rats and 1.8 X 10(-9) M in the copper-loaded rats. Digitonin resistance is measured by the degree of increase in the activity of a matrix enzyme, malatedehydrogenase, upon destruction of the various mitochondrial membranes and to a different degree by ascending digitonin concentrations. The digitonin effect on the membranes is maximal for concentrations of 0.8 to 1.0 mg digitonin/mg protein. The digitonin resistance of the inner membranes increases in the case of copper deficiency, while coppor loading has the opposite effect on the resistance of the outer membranes. Moreover, coppor loading results in an increase in the general activity of the mitochondrial malatedehydrogenase as compared with the control mitochondria.

Animals↗
PubMed1991

Digitonin permeabilization does not affect mitochondrial function and allows the determination of the mitochondrial membrane potential of Trypanosoma cruzi in situ.

Digitonin can be used to permeabilize selectively the plasma membrane of Trypanosoma cruzi epimastigotes without significantly affecting the functional integrity of mitochondria. Addition of digitonin at concentrations close to 64 microM caused decrease in the rate of basal respiration of epimastigotes similar to that caused by oligomycin. A further addition of carbonyl cyanide p-trifluorophenylhydrazone (FCCP) brought respiration to the same rate observed prior to the inclusion of digitonin or oligomycin. This suggests that like oligomycin, digitonin is shifting respiration to a nonphosphorylating state probably by depleting the cells from adenine nucleotides due to permeabilization of the plasma membrane. The use of low concentrations of digitonin allowed the quantitative determination of the mitochondrial membrane potential of these cells in situ using safranine O. The response of epimastigotes mitochondrial membrane potential to phosphate, FCCP, valinomycin, nigericin, ADP, and Ca2+ indicates that these mitochondria behave similarly to vertebrate mitochondria regarding the properties of their electrochemical proton gradient. In addition, T. cruzi mitochondria are able to build up and retain a membrane potential of a value comparable to that of mammalian mitochondria. The trypanocidal drug crystal violet, as well as other cationic drugs such as dequalinium, induced a rapid dose-related collapse of the inner mitochondrial membrane potential.

Adenosine Diphosphate↗
PubMed1991

Role of protein kinase C in catecholamine secretion from digitonin-permeabilized bovine adrenal medullary cells.

The effects of staurosporine and K-252a, potent inhibitors of protein kinases, and 12-O-tetradecanoylphorbol-13-acetate (TPA) on catecholamine secretion and protein phosphorylation in digitonin-permeabilized bovine adrenal medullary cells were investigated. Staurosporine and K-252a (0.01-10 microM) did not cause large changes in catecholamine secretion evoked by Ca2+ in digitonin-permeabilized cells whereas these compounds strongly prevented TPA-induced enhancement of catecholamine secretion in a concentration-dependent manner. Incubation of digitonin-permeabilized cells with [gamma-32P]ATP resulted in 32Pi incorporation into a large number of proteins, detected as several major bands and darkened background in autoradiograms. Ca2+ and TPA increased phosphorylation of these proteins. Staurosporine and K-252a markedly inhibited Ca(2+)-induced and TPA-induced increases in protein phosphorylation as well as basal (0 Ca2+) protein phosphorylation in digitonin-permeabilized cells. Long term treatment (24 h) of adrenal medullary cells with 1 microM TPA markedly decreased total cellular protein kinase C activity to about 5.3% of control. Pretreatment of the cells with 1 microM TPA strongly inhibited the TPA-induced enhancement of catecholamine secretion whereas it did not cause large changes in total cellular catecholamine amounts, Ca(2+)-induced catecholamine secretion, and cAMP-induced enhancement of catecholamine secretion from digitonin-permeabilized cells. From these results we conclude that protein kinase C plays a modulatory role in catecholamine secretion rather than being essential for initiating catecholamine secretion.

Adrenal Medulla↗
PubMed1986

Protein phosphorylation and secretion in digitonin-permeabilized adrenal chromaffin cells. Effects of micromolar Ca2+, phorbol esters, and diacylglycerol.

The effects of phorbol esters, dioctanoylglycerol (DiC8), and micromolar Ca2+ on protein phosphorylation and catecholamine secretion in digitonin-treated chromaffin cells were investigated. [gamma-32P]ATP was used as a substrate for phosphorylation in the permeabilized cells. 12-O-Tetradecanoylphorbol-13-acetate (TPA) enhanced Ca2+-dependent catecholamine secretion from digitonin-permeabilized cells. The enhancement required MgATP. Only those phorbol esters which activate protein kinase C in vitro enhanced both catecholamine secretion and protein phosphorylation. DiC8, which activates protein kinase C in vitro and mimics phorbol ester effects in situ, also enhanced both catecholamine secretion and protein phosphorylation. Preincubation of intact cells with TPA or DiC8 was necessary for maximal effects on both catecholamine secretion and protein phosphorylation in subsequently digitonin-treated chromaffin cells. The TPA-induced enhancement of protein phosphorylation was almost entirely Ca2+-independent, whereas DiC8-induced enhancement of protein phosphorylation was mainly Ca2+-dependent. Micromolar Ca2+ alone also enhanced the phosphorylation of a large number of proteins. Most of the proteins phosphorylated in response to TPA or potentiated by DiC8 in combination with Ca2+ were also phosphorylated by micromolar Ca2+ in the absence of exogenous protein kinase C activators. In intact cells, 1,1-dimethyl-4-phenylpiperazinium (DMPP) induced Ca2+-dependent phosphorylation of at least 17 proteins which were detected by two-dimensional gel electrophoresis. All of the proteins phosphorylated upon incubation with 1,1-dimethyl-4-phenylpiperazinium were phosphorylated upon incubation with micromolar Ca2+ in digitonin-treated cells. These results demonstrate that TPA- or DiC8-enhanced Ca2+-dependent catecholamine secretion is associated with enhanced protein phosphorylation which is probably mediated by protein kinase C and that activation of protein kinase C modulates catecholamine secretion from digitonin-treated chromaffin cells.

Adenosine Triphosphate↗
PubMed1981

[Effect of digitonin on the activity of Na,K-ATPase from bovine brain].

The kinetic properties of intact and digitonin-treated Na,K-ATPase from bovine brain were studied. The temperature dependence curve for the rate of ATP hydrolysis under optimal conditions (upsilon 0) in the Arrhenius plots shows a break at 19-20 degrees. The temperature dependence curves for Km' and Km" have breaks at the same temperatures, while the Arrhenius plot for V is linear. The value of the Hill coefficient (nH) for ATP at 37 degrees is variable depending on ATP concentration, i. e. it is less than 1 at ATP concentrations below 50 mkM and is increased up to 3.2 at higher concentrations of the substrate. At high ATP concentrations the value of nH depends on temperature, falling down to 2.1 at 23 degrees and then down to 1 within the temperature range of 21-19 degrees. A further decrease in temperature does not significantly affect the nH value. Digitonin irreversibly inhibits Na, K-ATPase. ATP hydrolysis is more sensitive to the effect of the detergent than is nNPP hydrolysis, i. e. after complete inhibition of the ATPase about 40% of the phosphatase activity are retained. Treatment of Na,K-ATPase by digitonin results in elimination of the breaks in the Arrhenius plots for upsilon 0, Km' and Km", whereas the temperature dependence plot of V remains linear. Simultaneously digitonin eliminates the positive cooperativity of the enzyme for ATP. It is assumed that Na, K-ATPase from bovine brain is an oligomer of the (alpha beta) 4 type. Digitonin changes the type of interaction between the protomers within the oligomeric complex by changing the lipid environment of the enzyme or the type of protein -- lipid interactions.

Animals↗
PubMed1983

Calcium-evoked secretion from digitonin-permeabilized adrenal medullary chromaffin cells.

The effects of the steroid glycoside digitonin on cultured bovine adrenal chromaffin cells were studied as a way to gain access to the intracellular sites of calcium action in stimulus-secretion coupling. Chromaffin cells treated with digitonin secreted catecholamines by an exocytotic mechanism when exposed to free calcium concentrations of greater than 1 microM. At free calcium concentrations of less than 100 microM, an apparently saturable release of catecholamines was observed with a half-maximal effect (EC50) of 3-4 microM. Catecholamine release at these low calcium concentrations was limited to 10-15% of the cellular content, was partially dependent on the presence of MgATP, and was inhibited by magnesium (EC50 = 6 mM). At calcium concentrations between 0.1 and 10 mM, a second, apparently saturable component of catecholamine release from digitonin-treated chromaffin cells was observed (EC50 = 0.3 mM). This release at the higher calcium concentrations was unaffected by the addition of MgATP, magnesium, antipsychotic drugs known to bind and inactivate calmodulin, or calcium channel blockers, but was temperature-dependent and was inhibited by N-ethylmaleimide (EC50 = 140 microM). Strontium and manganese can also evoke catecholamine secretion from digitonin-treated chromaffin cells. The results suggest that digitonin will be a useful preparation in elucidating the molecular mechanisms of the secretory process.

Adenosine Triphosphate↗
PubMed1968

The action of digitonin on rat liver mitochondria. The effects on enzyme content.

1. The effects of repetitive treatment of rat liver mitochondria with digitonin were examined. The first treatment results in the removal of the outer membrane. Almost all the NADH-cytochrome c reductase (rotenone-insensitive) is lost whereas the major portions of the soluble and bound enzymes are retained. One exception appears to be the cytochromes, which undergo somewhat larger losses. The resulting inner-membrane complex carries out oxidative phosphorylation and P(i)-ATP exchange. 2. The properties of the inner-membrane complex are affected by the osmoticity of the medium. When it is suspended in water little protein is lost but there is a marked loss of phosphorylation. If after the suspension in water the particulate fraction is reisolated by centrifugation and treated with digitonin, or if the aqueous suspension is treated directly with digitonin and the particulate fraction then reisolated, the phosphorylation is largely restored. 3. Additional treatment of the inner mitochondrial complex with digitonin results in the formation of a particulate fraction that contains approx. 8% of the initial mitochondrial protein, no outer membrane, no soluble mitochondrial enzymes and is still capable of coupled oxidative phosphorylation and P(i)-ATP exchange. These effects cannot be reproduced by treatment with water. 4. The rat liver mitochondria and all of the resulting preparations obtained after digitonin treatment may be stored for long periods in dimethyl sulphoxide with little change of activity.

Animals↗
PubMed1990

Energetics of Ehrlich ascites mitochondria: membrane potential of isolated mitochondria and mitochondria within digitonin-permeabilized cells.

Ehrlich ascites tumour cells were treated with digitonin so that they became permeable for low-molecular-weight compounds but, at certain concentrations of digitonin, retained most of their cytoplasmic proteins. Respiration of mitochondria with exogenous substrates and their membrane potential could thus be measured in situ by means of oxygen electrode and tetraphenylphosphonium-sensitive electrode, respectively. The results were compared with data from similar measurements on mitochondria isolated from such digitonin-permeabilized cells. Isolated mitochondria and mitochondria in situ oxidized succinate at similar rates and developed membrane potential of comparable magnitude. Both preparations also exhibited an identical nonlinear relationship between resting state respiration (titrated with a respiratory inhibitor) and the membrane potential. In the cells permeabilized with low concentrations of digitonin (i.e., retaining most of cytoplasmic proteins) and suspended in medium containing NaCl and other major anions and cations at concentrations close to those in mammalian plasma, anaerobiosis did not produce a decrease in the mitochondrial membrane potential, which was collapsed only after a subsequent addition of oligomycin. In this medium, glucose had little effect on either respiration or the membrane potential.

Animals↗
PubMed1982

Radioimmunoassay of digitonin.

We describe a radioimmunoassay for digitonin which utilizes the ability of digotonin to compete with 125I-labeled digoxigenin for binding to anti-digoxin antiserum. As performed using the Gammaflo automated radioimmunoassay system and commercially available reagents, the assay can detect as little as 20 micrograms/ml (15 microM) digitonin. The assay is insensitive to interference by cholesterol or other cell membrane constituents and is useable above and below the critical micelle concentration of digitonin. It should be useful for the monitoring of digitonin concentrations in solubilized biochemical preparations.

Animals↗
PubMed1993

Modulation of cisplatin cytotoxicity by permeabilization of the plasma membrane by digitonin in vitro.

Killing of human ovarian carcinoma 2008 cells by cisplatin (DDP) is in direct proportion to the amount of drug entering the cell. DDP and its analogue [3H]dichloro(ethylenediamine)platinum[II] ([3H]-DEP) enter cells relatively slowly. We found that the uptake of [3H]DEP into 2008 cells could be increased by treating the cells briefly with the plasma membrane-selective detergent digitonin. A similar effect was observed in an 11-fold DDP-resistant subline of 2008 cells, designated 2008/C13*5.25. A measurable effect was produced by concentrations as low as 5 microM, and 40 microM digitonin increased [3H]DEP accumulation at 1 hr by 4.4 +/- 0.2- and 6.5 +/- 0.7-fold (means +/- SD) in 2008 and 2008/C13*5.25 cells, respectively. The effect was rapid, occurring within 1 min. Increased [3H]DEP uptake was accompanied by increased platination of DNA (8.5-fold in 2008 cells and 18.5-fold in 2008/C13*5.25 cells), and by enhanced killing of both the DDP-sensitive and -resistant cells that was shown to be synergistic by median effect analysis. The combination index at 50% cell kill was 0.64 +/- 0.14 (values < 1 indicate synergy). We conclude that a brief exposure to digitonin can increase [3H]DEP uptake in vitro, and can overcome the impaired [3H]DEP accumulation associated with acquired DDP resistance. DDP and digitonin interact synergistically to increase tumor cell kill in vitro.

Antineoplastic Agents↗
PubMed1985

Intracellular levels and distribution of Ca2+ in digitonin-permeabilized cells.

Digitonin and other saponins can be used to selectively permeabilize the plasma membrane of a wide variety of cells without significantly affecting the gross structure and function of Ca2+-sequestering organelles such as mitochondria and endoplasmic reticulum. These characteristics have allowed digitonin to be used in the determination of the intracellular levels and distribution of Ca2+, as well as the measurement of Ca2+ fluxes by organelles "in situ". Studies conducted with several different types of digitonin-permeabilized cells indicate that the endoplasmic reticulum functions as a high affinity and low-capacity intracellular Ca2+ buffer, whereas mitochondria operate as a relatively low affinity but high capacity Ca2+ buffering system. However, recent findings suggest that mitochondria have a comparable affinity for net Ca2+ uptake in the presence of physiological concentrations of polyamines. The use of permeabilized cells has also been important in the identification of the endoplasmic reticulum as a site at which the recently discovered second messenger inositol trisphosphate acts to bring about an increase in the cytosolic free Ca2+ concentration. Thus, the selective permeabilization of cells with digitonin and its analogues has been a powerful yet simple tool in the study of intracellular Ca2+ homeostasis.

Animals↗
PubMed1983

Structural changes of isolated hepatocytes during treatment with digitonin.

The structural changes accompanying digitonin-induced release of enzymes and metabolites from isolated hepatocytes have been studied by scanning and transmission electron microscopy. In the initial phase, characterized by total release of the cytosolic marker enzyme, lactate dehydrogenase, the plasma membrane was immediately damaged, rapidly followed by extensive damage to the endoplasmic reticulum. The shape of the cell, however, was maintained, and the mitochondria and nucleus remained tightly held together by the cytoskeleton. Mitochondria remained intact initially, whereas the cytosol became less electron dense and the nuclear chromatin was more dispersed. An intermediate phase was characterized by total release of adenylate kinase and most of the glucose-6-phosphatase, marker enzymes for the mitochondrial intermembrane space and the endoplasmic reticulum, respectively. The outer mitochondrial membrane was ruptured, but mitochondria maintained their normal matrix electron density. In the final phase, characterized by the beginning of citrate synthase release from the mitochondrial matrix space, the mitochondria became swollen, and only the nucleus, inner and outer mitochondrial membranes, and the cytoskeleton could be clearly distinguished. Although the plasma membrane could not be readily discerned in electron micrographs after the initial phase, the plasma membrane marker enzyme 5'-nucleotidase remained associated with digitonin-treated hepatocytes. Acetyl-CoA carboxylase was released much more slowly than lactate dehydrogenase, indicating some severe restriction on its release. The release of acetyl-CoA carboxylase closely paralleled the release of glucose-6-phosphatase. The controlled exposure of hepatocytes to digitonin, therefore, leads to the sequential release of soluble, compartmentalized cellular components and some membrane-bound components, but the mitochondrial membrane, cytoskeleton and the nucleoskeleton survive even long-term digitonin treatment.

Acetyl-CoA Carboxylase↗
PubMed1986

Human neutrophils permeabilized with digitonin respond with lysosomal enzyme release when exposed to micromolar levels of free calcium.

We have recently reported that human neutrophils can be permeabilized with the cholesterol complexing agent saponin and that these cells can be induced to secrete the granule enzyme lysozyme in response to micromolar levels of free calcium. We now report that digitonin can be used in place of saponin and that it has several advantages. Permeabilization of human neutrophils was accomplished with 10 micrograms/ml digitonin in a high potassium medium. Normally impermeant solutes such as [14C]sucrose and inulin [14C]carboxylic acid gained access to one half of the intracellular water space marked with [3H]H2O. Between 30 and 100% of the cytoplasmic enzyme, lactate dehydrogenase, leaked from the intracellular space. The permeabilization process and calcium-triggered granule secretion were critically dependent upon temperature, time and digitonin concentration. Permeabilized neutrophils secreted beta-glucuronidase, lysozyme and vitamin B-12 binding-protein, constituents of both azurophil and specific granules, when exposed to micromolar levels of free calcium. Release of specific granule constituents appeared to be more sensitive to free calcium than release from azurophil granules. Although the amount of permeabilization varied considerably with each batch of cells, release of these granule markers was a consistent finding. Release of granule markers was accompanied by resealing of the cells to high-molecular-weight (Mr greater than 5000) solutes. Electron microscopic evidence also suggested that granule and plasma membranes were intact following digitonin treatment and that fusion of these membranes occurred in response to calcium. These results suggest that elevation of intracellular free-calcium levels is a sufficient condition for lysosomal enzyme release.

Calcium Chloride↗
PubMed1975

The effect of digitonin of the stimulation by insulin of glucose uptake by isolated fat cells.

The effect of digitonin on glucose uptake by isolated fat cells in the presence and absence of insulin has been studied. At low concentrations of digitonin, the stimulation of glucose uptake by insulin was inhibited without severe cell damage as estimated by the leakage of lactate dehydrogenase from the cells. The inhibition of the insulin effect was not reversed by washing the cells or by the addition of cholesterol or lecithin-cholesterol liposomes to the incubation medium of the cells after treatment with digitonin. Cholesterol was shown to be present in the fat cells and it is suggested that the inhibition of the insulin effect is a consequence of the formation of digitonin-cholesterol complexes in the fat cell plasma membrane. Possible ways in which this may results in inhibition of the effect of insulin are discussed.

Adipose Tissue↗
PubMed1980

The cytoskeleton of digitonin-treated rat hepatocytes.

Treatment of isolated rat hepatocptes with low concentrations of digitonin increases the permeability of the plsma membrane to cytosolic proteins without causing release of organelles such as mitochondria into the surrounding medium. Electron microscopy showed that treatment of the cells with increasing concentations of digitonin results in a progressive loss in the continuity of the plasma membrane, while most other aspects of cellular morphology remain normal. Depletion of background staining material from the cytosol by digitonin treatment of the cells greatly enhances the visualization of the cytoskeleton. The use of this technique, together with immunofluorescent light microscopy, has verified the presence of an actin-containing filamentous network at the hepatocyte cortex as well as intermediate filaments distributed throughout the cell. Digitonin is thus useful both for selectively permeabilizing the plasma membrane and for intensifying the appearance of intracellular structures such as microfilaments that are normally difficult to observe in cells such as hepatocytes.

Actins↗
PubMedDate not supplied

The cholesterol-complexing agent digitonin modulates ligand binding of the bovine hippocampal serotonin 1A receptor.

The serotonin(1A) (5-HT(1A)) receptor is an important member of the superfamily of seven transmembrane domain G-protein-coupled receptors. We have examined the modulatory role of cholesterol on the ligand binding of the bovine hippocampal 5-HT(1A) receptor by cholesterol complexation in native membranes using digitonin. Complexation of cholesterol from bovine hippocampal membranes using digitonin results in a concentration-dependent reduction in specific binding of the agonist 8-OH-DPAT and antagonist p-MPPF to 5-HT(1A) receptors. The corresponding changes in membrane order were monitored by analysis of fluorescence polarization data of the membrane depth-specific probes, DPH and TMA-DPH. Taken together, our results point out the important role of membrane cholesterol in maintaining the function of the 5-HT(1A) receptor. An important aspect of these results is that non-availability of free cholesterol in the membrane due to complexation with digitonin rather than physical depletion is sufficient to significantly reduce the 5-HT(1A) receptor function. These results provide a comprehensive understanding of the effects of the sterol-complexing agent digitonin in particular, and the role of membrane cholesterol in general, on the 5-HT(1A) receptor function.

Animals↗
PubMed1987

Digitonin-permeabilized cells are exocytosis competent.

Release of norepinephrine from PC12 cells can be stimulated by free Ca2+ in micromolar concentrations after permeabilization with 10 micrograms/ml of digitonin. This release is time and temperature dependent, half-maximal at 0.3 microM Ca2+, and, after washing out of endogenous ATP, half-maximal at about 0.5 mM MgATP when exogenously added. Similar results were obtained with bovine adrenal chromaffin cells using the same protocol. Support for the idea that the mechanism of release from both permeabilized cell types is still exocytosis is demonstrated at the electron microscopic level by immunolabeling chromaffin granule membrane antigens that were introduced into the plasma membrane following stimulation. Electron micrographs furthermore demonstrate that chromaffin granules retain typical dense cores after permeabilization, indicating that leakiness of catecholamines from the granules was not a major factor. Pores, formed by digitonin in the plasma membranes, were utilized to introduce antibodies into such exocytosis-competent cells. Anti-actin and anti-chromaffin granule membrane antibodies show a staining pattern similar to conventionally fixed and stained preparations. Our results demonstrate that pores formed by digitonin do not impair the process of exocytosis although they are big enough to allow macromolecules to pass in both directions. The digitonin-permeabilized cell is therefore an ideal in vitro system with which to study the fusion process between chromaffin granules and the plasma membrane.

Adenosine Triphosphate↗
PubMed1981

Effect of digitonin on rat myometrium subcellular membrane fractions.

Isopycnic centrifugation experiments using sucrose density gradients showed that in digitonin-treated microsomes the distribution of the plasma membrane (PM) marker 5'-nucleotidase was shifted to higher densities. The treatment also caused similar but less pronounced changes in the distribution of protein, the putative endoplasmic reticulum (ER) marker NADPH-dependent cytochrome c reductase, and the inner mitochondrial marker cytochrome c oxidase. Similar experiments using more purified membrane fractions showed that the digitonin treatment led to a comparable increase in the densities of the fractions N1 and N2 previously described as subfractions of plasma membrane and to considerably less increase in the density of the fraction N3B which is enriched in the endoplasmic reticulum and the inner mitochondrial markers. Digitonin inhibited the ATP-dependent Ca uptake by the N1 fraction in a concentration-dependent manner (I50 = 0.3 mg/mL). Digitonin (0.5 mg/mL) inhibited the ATP-dependent azide-insensitive Ca uptake by all the fractions. The results support the hypothesis that (a) N1 and N2 are subfractions of plasma membrane, and (b) ATP-dependent azide-insensitive Ca uptake in rat myometrium is a property of plasma membranes.

5'-Nucleotidase↗