Extraction of mitochondrial protein-lipid complexes into organic solvents: inhibition of cytochrome oxidase electron transport by dicyclohexylcarbodiimide and triphenyltin chloride

Gene expression profiling of Bis(tri-n-butyltin)oxide (TBTO)-induced immunotoxicity in mice and rats

Bis(tri-n-butyltin)oxide (TBTO) is one of the organotin compounds that have been used as biocides and occur as persistent environmental pollutants. Human exposure to these compounds occurs through consumption of meat and fish products in which they accumulate. The most sensitive endpoint of TBTO exposure is immunotoxicity. TBTO causes thymus atrophy and thereby interferes with T-lymphocyte-mediated immune responses. Tributyltin compounds have been found to adversely affect a wide range of cellular components and processes in many species, organ systems, and cell types. Both inhibition of proliferation and induction of apoptosis have been observed in thymocytes. We conducted microarray experiments in mice and rats in order to investigate if the immunosuppressive actions of TBTO could be detected by gene expression profiling, and if so, to elucidate the mechanisms of action. Gene expression changes that were detected in mouse thymuses after exposure to a maximum tolerable dose of TBTO correlated to previously observed effects. Most notably, reduction of expression of cell surface determinants and T-cell receptor chains, suppression of cell proliferation, and a possible involvement of nuclear receptors in interference with lipid metabolism by TBTO were observed. The TBTO-induced thymus involution may therefore primarily be caused by inhibition of thymocyte proliferation. In contrast, in rats only limited effects of a lower dose of TBTO were found at the gene expression level in the thymus, even though thymus involution was observed. Here, most gene expression regulation by TBTO was detected in the liver. These preliminary results indicate that gene expression analysis is able to reveal effects of TBTO and to gain insight into its molecular mechanism of action. It may even be a suitable tool to investigate immunotoxicology in general. However, dose and inter-species differences are apparently clearly reflected in the gene expression profiles.

Prodigiosins uncouple lysosomal vacuolar-type ATPase through promotion of H+/Cl- symport

We reported previously [Kataoka, Muroi, Ohkuma, Waritani, Magae, Takatsuki, Kondo, Yamasaki and Nagai (1995) FEBS Lett. 359, 53-59] that prodigiosin 25-C (one of the red pigments of the prodigiosin group produced by micro-organisms like Streptomyces and Serratia) uncoupled vacuolar H+-ATPase, inhibited vacuolar acidification and affected glycoprotein processing. In the present study we show that prodigiosin, metacycloprodigiosin and prodigiosin 25-C, all raise intralysosomal pH through inhibition of lysosomal acidification driven by vacuolar-type (V-)ATPase without inhibiting ATP hydrolysis in a dose-dependent manner with IC50 values of 30-120 pmol/mg of protein. The inhibition against lysosomal acidification was quick and reversible, showing kinetics of simple non-competitive (for ATP) inhibition. However, the prodigiosins neither raised the internal pH of isolated lysosomes nor showed ionophoric activity against H+ or K+ at concentrations where they strongly inhibited lysosomal acidification. They required Cl- for their acidification inhibitory activity even when driven in the presence of K+ and valinomycin, suggesting that their target is not anion (chloride) channel(s). In fact, the prodigiosins inhibited acidification of proteoliposomes devoid of anion channels that were reconstituted from lysosomal vacuolar-type (V-)ATPase and Escherichia coli phospholipids. However, they did not inhibit the formation of an inside-positive membrane potential driven by lysosomal V-ATPase. Instead, they caused quick reversal of acidified pH driven by lysosomal V-ATPase and, in acidic buffer, produced quick acidification of lysosomal pH, both only in the presence of Cl-. In addition, they induced swelling of liposomes and erythrocytes in iso-osmotic ammonium salt of chloride but not of gluconate, suggesting the promotion of Cl- entry by prodigiosins. These results suggest that prodigiosins facilitate the symport of H+ with Cl- (or exchange of OH- with Cl-) through lysosomal membranes, resulting in uncoupling of vacuolar H+-ATPase.

Prodigiosins as a new group of H+/Cl- symporters that uncouple proton translocators

We reported previously (Kataoka, T., Muroi, M., Ohkuma, S., Waritani, T., Magae, J., Takatsuki, A., Kondo, S., Yamasaki, M., and Nagai, K. (1995) FEBS Lett. 359, 53-59) that prodigiosin 25-C uncoupled vacuolar H+-ATPase, inhibited vacuolar acidification, and affected glycoprotein processing. In the present study we show that prodigiosins (prodigiosin, metacycloprodigiosin, and prodigiosin 25-C) inhibit the acidification activity of H+-ATPase chloride dependently, but not membrane potential formation or ATP hydrolysis activity, and suggest that they promote H+/Cl- symport (or OH-/Cl- exchange, in its equivalence) across vesicular membranes. In fact, prodigiosins displayed H+/Cl- symport activity on liposomal membranes. First of all, they decreased the internal pH of liposomes depending on the external chloride, and raised it depending on the internal chloride when external buffer was free from chloride. Second, their effect was electroneutral and not seriously affected by the application of an inside positive membrane potential generated by K+ and valinomycin. Finally, they promoted the uptake of [36Cl] from external buffers with concomitant intraliposomal acidification when external pH was acidic relative to liposome interior. As prodigiosins hardly inhibit the catalytic activity (ATP hydrolysis) unlike well known OH-/Cl- exchangers (for example, tributyltin chloride), they should provide powerful tools for the study of molecular machinery and cellular activities involving transport of protons and/or chloride.

Thermostability of membrane systems in organic solvents

Two multisubunit enzymes of the inner mitochondrial membrane, cytochrome oxidase and the H+-ATPase may be transferred into highly apolar solvents as protein-lipid complexes. At 70 degrees C and an initial water concentration of 13 microliters per ml organic solvent (toluene), the half-life of the ATPase was approx. 11 h, whereas that of cytochrome oxidase was about 100 s. Thermostability of cytochrome oxidase could be increased more than 100-times by decreasing the water concentration to 3 microliters per ml toluene. At this latter concentration of water the half-life of the ATPase at 90, 80 and 70 degrees C was 5, 48 and 96 h, respectively.

Extraction of mitochondrial membrane proteins into organic solvents in a functional state

Protein-lipid complexes in organic solvents can be used as the starting material in the reassembly of functional planar and spherical bilayers (Montal, M., Darszon, A. and Schindler, H. (1981) Q. Rev. Biophys. 14, 1-79). The transfer of three enzymes of the inner mitochondrial membrane into organic solvents as protein-lipid complexes has been studied to understand better the extraction process. The enzymes studied were cytochrome c oxidase, ATPase and succinate dehydrogenase. These enzymes were transferred into hexane and diethyl ether in an active state, however, the activities extracted varied quantitatively, depending on the amount of protein of the starting preparation, the concentration of phospholipids and the cation employed. In all conditions cytochrome c oxidase was extracted with the highest yield and specific activity, and it was actually enriched in the organic extract. The values for succinate dehydrogenase and ATPase were lower, but their specific activities were similar to those of the starting material. This indicates that some membrane proteins are preferentially extracted into organic solvents in a functional state. The enzymes, as protein-lipid complexes, are fairly stable in organic solvents; in a month of storage at 4 degrees C in hexane some enzymes loose less than 50% of their activity.

Studies on the mechanism of action of triphenyltin on proton conduction by the H+-ATPase of mitochondria

A study is presented of the action of triphenyltin on the kinetics of the anaerobic relaxation of the proton gradient set up by respiration in various type of 'inside-out' inner membrane vesicles obtained by exposure of beef-heart mitochondria to ultrasonic energy. Triphenyltin is shown to act as a powerful inhibitor of the proton conductivity of the H+-ATPase. The inhibition persists after removal of the ATPase protein inhibitor, F1 and the oligomycin-sensitivity conferral protein (OSCP) from the particles. The inhibitory effect of triphenyltin is exerted, as in the case of oligomycin and N,N'-dicyclohexylcarbodiimide, on the F0 moiety of the ATPase complex. Comparison of the characteristics of the effect of triphenyltin on proton translocation in chloride and nitrate media shows that the inhibition of passive proton conductivity studied here is unrelated to the hydroxide/anion exchange induced by the organotin. Lack of additivity of the inhibition of H+ conduction by triphenyltin with that exerted by oligomycin and N,N'-dicyclohexylcarbodiimide and the kinetic pattern of the effect of triphenyltin show that the mechanism of action of the organotin is different from that of the other two inhibitors. The relevance of the results obtained with respect to the subunit location and chemical nature of the reaction site of triphenyltin in the H+-ATPase complex is discussed.

Extraction of mitochondrial protein-lipid complexes into organic solvents: an approach to study the interaction between the ATPase and the mitochondrial ATPase-inhibitor protein

Protein-lipid complexes were transferred directly from mitochondria and submitochondrial particles into hexane and ether. The protein-lipid residue left after solvent removal from these extracts was used to form liposomes which display low-temperature-resistant ATPase activity. Centrifugation experiments indicate that the ATPase activity is associated to the vesicles. Most of the F1-ATPases appear to be accessible to the external water phase of the liposomes. The ATPase activity of these particles was insensitive to dicyclohexylcarbodiimide and oligomycin. Incubation of these vesicles at room temperature activated (4--10-fold) the ATPase through a process that is partially sensitive to phenylmethylsulfonyl fluoride. The results with purified ATPase-inhibitor protein and (F1--ATPase)-inhibitor complex indicate that the activation process in the liposomes is due to the abolition of the inhibitory action of the inhibitor protein bound to a large fraction of the extracted ATPases. Liposomes prepared from hexane extracts obtained from submitochondrial particles having different levels of ATPase activity displayed an activation ratio which correlated with the number of ATPases that are inhibited by the inhibitor protein in the submitochondrial particles. The extraction of mitochondrial ATPase and its incorporation into liposomes followed by activity measurements may be used to judge the number of ATPases that in a given preparation contain the inhibitor protein in its inhibiting site.