3D Reconstruction of a Subcomplex of NADH-ubiquinone-oxidoreductase (Complex I) from Yarrowia lipolytica

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

The complete mitochondrial genome of yarrowia lipolytica

We here report the complete nucleotide sequence of the 47.9 kb mitochondrial (mt) genome from the obligate aerobic yeast Yarrowia lipolytica. It encodes, all on the same strand, seven subunits of NADH: ubiquinone oxidoreductase (ND1-6, ND4L), apocytochrome b (COB), three subunits of cytochrome oxidase (COX1, 2, 3), three subunits of ATP synthetase (ATP6, 8 and 9), small and large ribosomal RNAs and an incomplete set of tRNAs. The Y. lipolytica mt genome is very similar to the Hansenula wingei mt genome, as judged from blocks of conserved gene order and from sequence homology. The extra DNA in the Y. lipolytica mt genome consists of 17 group 1 introns and stretches of A+Trich sequence, interspersed with potentially transposable GC clusters. The usual mould mt genetic code is used. Interestingly, there is no tRNA able to read CGN (arginine) codons. CGN codons could not be found in exonic open reading frames, whereas they do occur in intronic open reading frames. However, several of the intronic open reading frames have accumulated mutations and must be regarded as pseudogenes. We propose that this may have been triggered by the presence of untranslatable CGN codons. This sequence is available under EMBL Accession No. AJ307410.

The structure of eukaryotic and prokaryotic complex I

The structures of the NADH dehydrogenases from Bos taurus and Aquifex aeolicus have been determined by 3D electron microscopy, and have been analyzed in comparison with the previously determined structure of Complex I from Yarrowia lipolytica. The results show a clearly preserved domain structure in the peripheral arm of complex I, which is similar in the bacterial and eukaryotic complex. The membrane arms of both eukaryotic complexes show a similar shape but also significant differences in distinctive domains. One of the major protuberances observed in Y. lipolytica complex I appears missing in the bovine complex, while a protuberance not found in Y. lipolytica connects in bovine complex I a domain of the peripheral arm to the membrane arm. The structural similarities of the peripheral arm agree with the common functional principle of all complex Is. The differences seen in the membrane arm may indicate differences in the regulatory mechanism of the enzyme in different species.

Regulatory demands on data quality for the environmental risk assessment of pharmaceuticals

The evaluation of the quality of data and their use within the review of environmental risk assessment of human as well as veterinary pharmaceuticals is described from a regulatory point of view. A definition and differentiation in three categories for the reliability of data are given. Existing criteria relating to international testing standards for categorising reliability are adopted for their use within the environmental risk assessment of pharmaceuticals. A systematic documentation of evaluating reliability for literature data as well as for experimental studies (effect and environmental fate studies) is proposed. The data quality criteria are defined in order to increase the transparency of the evaluation process in Germany and thus the quality of the environmental risk assessment of pharmaceuticals.

Phase I study of RAD001 (everolimus), cetuximab, and irinotecan as second-line therapy in metastatic colorectal cancer (mCRC)

e15115

Background: The PI3K/AKT/mTOR pathway is frequently dysregulated in colorectal cancer (Cancer Res 2005;65:11227). In a phase I study in patients with advanced solid tumors, everolimus an oral mTOR inhibitor demonstrated clinical benefit including a partial response in pts with colorectal cancer (J Clin Oncol 2008;26:1603–10; J Clin Oncol 2008; 26:1588–95). Methods: This open-label, multicenter phase I study uses a Bayesian logistic model to identify feasible doses of everolimus + irinotecan + cetuximab. Adult pts with mCRC progressing despite prior 5-FU/oxaliplatin (FOLFOX) or capecitabine/oxaliplatin (XELOX) plus bevacizumab (if standard practice) were treated using a sequential dose escalation scheme (Table). Dose decisions were driven by the probability of dose-limiting toxicity (DLT) in the first 2 cycles. Dose level decisions were based on maximizing the probability that end-of-cycle-2 DLT rate would be within the targeted toxicity interval (20% to <35%) and minimizing the risk of over-dosing (< 5% risk of unacceptable toxicity and < 25% risk of excessive/unacceptable toxicity). Results: 18 pts were treated from April ‘07 to August ‘08, 5 pts at dose level A1 and 13 pts at dose level B1. Two DLTs (G3 rash on cycle 2 day 1 lasting > 7 days and G3 mucositis on cycle 1 day 14 lasting > 7 days, 1 pt each) were reported in 4 evaluable pts at dose level A1. No DLTs were reported in 7 evaluable patients at dose level B1. Conclusions: At dose level B1 everolimus in combination with irinotecan and cetuximab was generally well tolerated. The study was stopped due to changes in clinical practice based on emerging data indicating that cetuximab has limited efficacy in mCRC patients with KRAS mutations and that efficacy data favors daily RAD001 over weekly dosing. Patients in this study were treated with cetuximab irrespective of KRAS status.

[Table: see text]

[Table: see text]

Expression of the chemokine receptor CXCR1 in human glomerular diseases

Leukocyte infiltration, a hallmark of renal diseases, is orchestrated in part by the actions of chemokines. The chemokine CXCL8/interleukin (IL)-8 is expressed during renal diseases and allograft rejection, whereas the corresponding receptor CXCR1 has not been described previously. Expression of CXCR1 was characterized in peripheral blood using multicolor fluorescence-activated cell sorter analysis (FACS). CXCR1 was localized in 81 formalin-fixed, paraffin-embedded renal specimens by immunohistochemistry using a monoclonal antibody against human CXCR1. Included were biopsies with crescentic glomerulonephritis (CGN, n = 22), immunoglobulin (Ig) A nephropathy (n = 15), membranoproliferative glomerulonephritis (MPGN, n = 17), lupus nephritis (n = 12), membranous nephropathy (n = 11), and non-involved parts of tumor nephrectomies (n = 4). Consecutive tissue sections of human tonsils, allograft explants, and renal biopsies were stained for CD15- and CD68-positive cells. Expression of CXCR1 and CXCL8/IL-8 mRNA was quantified by real-time reverse transcriptase-polymerse chain reaction of microdissected renal biopsies (n = 35) of the same disease entities. By FACS CXCR1 expression was found on polymorphonuclear CXCR1 expression by polymorphonuclear leukocytes (PMNs), natural killer cells, and a subpopulation of monocytes. By immunohistochemistry, CXCR1 expression was found on infiltrating inflammatory cells (predominantly PMNs), as well as on intrinsic renal cells (arterial smooth muscle cells, endothelial cells of peritubular capillaries). The distribution pattern of CXCR1 differed between disease entities. The highest numbers of glomerular CXCR1-positive cells were present in biopsies with MPGN, followed by lupus nephritis, and CGN. CXCR1 might be involved in the recruitment of PMNs to the glomerular tuft, which could be targeted by CXCR1-blocking agents.

The three-dimensional structure of complex I from Yarrowia lipolytica: a highly dynamic enzyme

The structure of complex I from Yarrowia lipolytica was determined by three-dimensional electron microscopy. A random conical data set was collected from deep stain embedded particles. More than 14000 image pairs were analyzed. Through extensive classification combined with three-dimensional reconstruction, it was possible for the first time to show a much more detailed substructure of the complex. The peripheral arm is subdivided in at least six domains. The membrane arm shows two major protrusions on its matrix facing side and exhibits a channel like feature on the side facing the cytoplasm. Structures resembling a tether connecting the subunits near the catalytic center with the protrusions of the membrane arm provide a second connection between matrix and membrane domain.

Cluster N1 of complex I from Yarrowia lipolytica studied by pulsed EPR spectroscopy

After reduction with nicotinamide adenine dinucleotide (NADH), NADH:ubiquinone oxidoreductase (complex I) of the strictly aerobic yeast Yarrowia lipolytica shows clear signals from five different paramagnetic iron-sulfur (FeS) clusters (N1-N5) which can be detected using electron paramagnetic resonance (EPR) spectroscopy. The ligand environment and the assignment of several FeS clusters to specific binding motifs found in several subunits of the complex are still under debate. In order to characterize the hyperfine interaction of the surrounding nuclei with FeS cluster N1, one- and two-dimensional electron spin echo envelope modulation experiments were performed at a temperature of 30 K. At this temperature only cluster N1 contributes to the overall signal in a pulsed EPR experiment. The hyperfine and quadrupole tensors of a nitrogen nucleus and the isotropic and dipolar hyperfine couplings of two sets of protons could be determined by numerical simulation of the one- and two-dimensional spectra. The values obtained are in perfect agreement with a ferredoxin-like binding structure by four cysteine amino acid residues and allow the assignment of the nitrogen couplings to a backbone nitrogen nucleus and the proton couplings to the beta-protons of the bound cysteine residues.

Structure-function relationships in mitochondrial complex I of the strictly aerobic yeast Yarrowia lipolytica

The obligate aerobic yeast Yarrowia lipolytica has been established as a powerful model system for the analysis of mitochondrial complex I. Using a combination of genomic and proteomic approaches, a total of 37 subunits was identified. Several of the accessory subunits are predicted to be STMD (single transmembrane domain) proteins. Site-directed mutagenesis of Y. lipolytica complex I has provided strong evidence that a significant part of the ubiquinone reducing catalytic core resides in the 49 kDa and PSST subunits and can be modelled using X-ray structures of distantly related enzymes, i.e. water-soluble [NiFe] hydrogenases from Desulfovibrio spp. Iron-sulphur cluster N2, which is related to the hydrogenase proximal cluster, is directly involved in quinone reduction. Mutagenesis of His226 and Arg141 of the 49 kDa subunit provided detailed insight into the structure-function relationships around cluster N2. Overall, our findings suggest that proton pumping by complex I employs long-range conformational interactions and ubiquinone intermediates play a critical role in this mechanism.

[The gap1 operon of the cyanobacterium Synechococcus PCC 7942 carries a gene encoding glycogen phosphorylase and is induced under anaerobic conditions]

The cloning and sequencing of the gap1 operon, which encodes the glycolytic NAD-specific glyceraldehyde-3-phosphate dehydrogenase in the cyanobacterium Synechococcus PCC 7942, showed that the gap1 gene is closely linked to the glgP gene encoding glycogen phosphorylase (an enzyme that catalyzes the first step of glycogen degradation). Northern blotting experiments showed that the gap1 and glgP genes are co-expressed and organized in a bicistronic operon, whose expression is enhanced under anaerobic conditions. The nucleotide sequence of the operon has been submitted to GenBank under accession number AF428099.

External alternative NADH:ubiquinone oxidoreductase redirected to the internal face of the mitochondrial inner membrane rescues complex I deficiency in Yarrowia lipolytica

Alternative NADH:ubiquinone oxidoreductases are single subunit enzymes capable of transferring electrons from NADH to ubiquinone without contributing to the proton gradient across the respiratory membrane. The obligately aerobic yeast Yarrowia lipolytica has only one such enzyme, encoded by the NDH2 gene and located on the external face of the mitochondrial inner membrane. In sharp contrast to ndh2 deletions, deficiencies in nuclear genes for central subunits of proton pumping NADH:ubiquinone oxidoreductases (complex I) are lethal. We have redirected NDH2 to the internal face of the mitochondrial inner membrane by N-terminally attaching the mitochondrial targeting sequence of NUAM, the largest subunit of complex I. Lethality of complex I mutations was rescued by the internal, but not the external version of alternative NADH:ubiquinone oxidoreductase. Internal NDH2 also permitted growth in the presence of complex I inhibitors such as 2-decyl-4-quinazolinyl amine (DQA). Functional expression of NDH2 on both sides of the mitochondrial inner membrane indicates that alternative NADH:ubiquinone oxidoreductase requires no additional components for catalytic activity. Our findings also demonstrate that shuttle mechanisms for the transfer of redox equivalents from the matrix to the cytosolic side of the mitochondrial inner membrane are insufficient in Y. lipolytica.

Three molecules of ubiquinone bind specifically to mitochondrial cytochrome bc1 complex

Bifurcated electron flow to high potential "Rieske" iron-sulfur cluster and low potential heme b(L) is crucial for respiratory energy conservation by the cytochrome bc(1) complex. The chemistry of ubiquinol oxidation has to ensure the thermodynamically unfavorable electron transfer to heme b(L). To resolve a central controversy about the number of ubiquinol molecules involved in this reaction, we used high resolution magic-angle-spinning nuclear magnetic resonance experiments to show that two out of three n-decyl-ubiquinones bind at the ubiquinol oxidation center of the complex. This substantiates a proposed mechanism in which a charge transfer between a ubiquinol/ubiquinone pair explains the bifurcation of electron flow.

A central functional role for the 49-kDa subunit within the catalytic core of mitochondrial complex I

We have analyzed a series of eleven mutations in the 49-kDa protein of mitochondrial complex I (NADH:ubiquinone oxidoreductase) from Yarrowia lipolytica to identify functionally important domains in this central subunit. The mutations were selected based on sequence homology with the large subunit of [NiFe] hydrogenases. None of the mutations affected assembly of complex I, all decreased or abolished ubiquinone reductase activity. Several mutants exhibited decreased sensitivities toward ubiquinone-analogous inhibitors. Unexpectedly, seven mutations affected the properties of iron-sulfur cluster N2, a prosthetic group not located in the 49-kDa subunit. In three of these mutants cluster N2 was not detectable by electron-paramagnetic resonance spectroscopy. The fact that the small subunit of hydrogenase is homologous to the PSST subunit of complex I proposed to host cluster N2 offers a straightforward explanation for the observed, unforeseen effects on this iron-sulfur cluster. We propose that the fold around the hydrogen reactive site of [NiFe] hydrogenase is conserved in the 49-kDa subunit of complex I and has become part of the inhibitor and ubiquinone binding region. We discuss that the fourth ligand of iron-sulfur cluster N2 missing in the PSST subunit may be provided by the 49-kDa subunit.

Exploring the catalytic core of complex I by Yarrowia lipolytica yeast genetics

We have developed Yarrowia lipolytica as a model system to study mitochondrial complex I that combines the application of fast and convenient yeast genetics with efficient structural and functional analysis of its very stable complex I isolated by his-tag affinity purification with high yield. Guided by a structural model based on homologies between complex I and [NiFe] hydrogenases mutational analysis revealed that the 49 kDa subunit plays a central functional role in complex I. We propose that critical parts of the catalytic core of complex I have evolved from the hydrogen reactive site of [NiFe] hydrogenases and that iron-sulfur cluster N2 resides at the interface between the 49 kDa and PSST subunits. These findings are in full agreement with the "semiquinone switch" mechanism according to which coupling of electron and proton transfer in complex I is achieved by a single integrated pump comprising cluster N2, the binding site for substrate ubiquinone, and a tightly bound quinone or quinoid group.

Efficient large scale purification of his-tagged proton translocating NADH:ubiquinone oxidoreductase (complex I) from the strictly aerobic yeast Yarrowia lipolytica

Proton translocating NADH:ubiquinone oxidoreductase (complex I) is the largest membrane bound multiprotein complex of the respiratory chain and the only one for which no molecular structure is available so far. Thus, information on the mechanism of this central enzyme of aerobic energy metabolism is still very limited. As a new approach to analyze complex I, we have recently established the strictly aerobic yeast Yarrowia lipolytica as a model system that offers a complete set of convenient genetic tools and contains a complex I that is stable after isolation. For crystallization of complex I and to obtain its molecular structure it is a prerequisite to prepare large amounts of highly pure enzyme. Here we present the construction of his-tagged complex I that for the first time allows efficient affinity purification. Our protocol recovers almost 40% of complex I present in Yarrowia mitochondrial membranes. Overall, 40-80 mg highly pure and homogeneous complex I can be obtained from 10 l of an overnight Y. lipolytica culture. After reconstitution into asolectin proteoliposomes, the purified enzyme exhibits full NADH:ubiquinone oxidoreductase activity, is fully sensitive to inhibition by quinone analogue inhibitors and capable of generating a proton-motive force.

Application of the obligate aerobic yeast Yarrowia lipolytica as a eucaryotic model to analyse Leigh syndrome mutations in the complex I core subunits PSST and TYKY

We have used the obligate aerobic yeast Yarrowia lipolytica to reconstruct and analyse three missense mutations in the nuclear coded subunits homologous to bovine TYKY and PSST of mitochondrial complex I (proton translocating NADH:ubiquinone oxidoreductase) that have been shown to cause Leigh syndrome (MIM 25600), a severe progressive neurodegenerative disorder. While homozygosity for a V122M substitution in NDUFS7 (PSST) has been found in two siblings with neuropathologically proven Leigh syndrome (R. Triepels et al., Ann. Neurol. 45 (1999) 787), heterozygosity for a P79L and a R102H substitution in NDUFS8 (TYKY) has been found in another patient (J. Loeffen et al., Am. J. Hum. Genet. 63 (1998) 1598). Mitochondrial membranes from Y. lipolytica strains carrying any of the three point mutations exhibited similar complex I defects, with V(max) being reduced by about 50%. This suggests that complex I mutations that clinically present as Leigh syndrome may share common characteristics. In addition changes in the K(m) for n-decyl-ubiquinone and I(50) for hydrophobic complex I inhibitors were observed, which provides further evidence that not only the hydrophobic, mitochondrially coded subunits, but also some of the nuclear coded subunits of complex I are involved in its reaction with ubiquinone.

Function of conserved acidic residues in the PSST homologue of complex I (NADH:ubiquinone oxidoreductase) from Yarrowia lipolytica

Proton-translocating NADH:ubiquinone oxidoreductase (complex I) is the largest and least understood enzyme of the respiratory chain. Complex I from bovine mitochondria consists of more than forty different polypeptides. Subunit PSST has been suggested to carry iron-sulfur center N-2 and has more recently been shown to be involved in inhibitor binding. Due to its pH-dependent midpoint potential, N-2 has been proposed to play a central role both in ubiquinone reduction and proton pumping. To obtain more insight into the functional role of PSST, we have analyzed site-directed mutants of conserved acidic residues in the PSST homologous subunit of the obligate aerobic yeast Yarrowia lipolytica. Mutations D136N and E140Q provided functional evidence that conserved acidic residues in PSST play a central role in the proton translocating mechanism of complex I and also in the interaction with the substrate ubiquinone. When Glu(89), the residue that has been suggested to be the fourth ligand of iron-sulfur center N-2 was changed to glutamine, alanine, or cysteine, the EPR spectrum revealed an unchanged amount of this redox center but was shifted and broadened in the g(z) region. This indicates that Glu(89) is not a ligand of N-2. The results are discussedin the light of structural similarities to the homologous [NiFe] hydrogenases.

Biophysical and structural characterization of proton-translocating NADH-dehydrogenase (complex I) from the strictly aerobic yeast Yarrowia lipolytica

Mitochondrial proton-translocating NADH-dehydrogenase (complex I) is one of the largest and most complicated membrane bound protein complexes. Despite its central role in eukaryotic oxidative phosphorylation and its involvement in a broad range of human disorders, little is known about its structure and function. Therefore, we have started to use the powerful genetic tools available for the strictly aerobic yeast Yarrowia lipolytica to study this respiratory chain enzyme. To establish Y. lipolytica as a model system for complex I, we purified and characterized the multisubunit enzyme from Y lipolytica and sequenced the nuclear genes coding for the seven central subunits of its peripheral part. Complex I from Y lipolytica is quite stable and could be isolated in a highly pure and monodisperse state. One binuclear and four tetranuclear iron-sulfur clusters, including N5, which was previously known only from mammalian mitochondria, were detected by EPR spectroscopy. Initial structural analysis by single particle electron microscopy in negative stain and ice shows complex I from Y. lipolytica as an L-shaped particle that does not exhibit a thin stalk between the peripheral and the membrane parts that has been observed in other systems.

Binding of detergents and inhibitors to bovine complex I - a novel purification procedure for bovine complex I retaining full inhibitor sensitivity

Mitochondrial complex I exhibits some peculiar and poorly understood features regarding the effects of detergents on activity and sensitivity to hydrophobic inhibitors that are not seen with other membrane complexes using ubiquinone as a substrate. Therefore, we investigated the interaction of complex I from bovine heart mitochondria with different types of detergents by monitoring activity, degree of inhibition and inhibitor binding in the presence of increasing concentrations of detergent. It is shown that apart from their nature as solubilizing and delipidating agents the polyoxyethylene-ether detergents Triton X-100, Brij-35 and Thesit act as specific inhibitors of complex I and compete with classical complex I inhibitors for a common binding domain. These findings were used to develop a novel large-scale chromatographic procedure for isolation of inhibitor-sensitive NADH:ubiquinone oxidoreductase (complex I) from bovine heart mitochondria. The enzyme was purified by selective solubilization in Triton X-100 and subsequent hydroxylapatite, ion-exchange and gel-exclusion chromatography. By switching detergents from Triton X-100 to dodecylmaltoside after hydroxylapatite chromatography the procedure yields highly pure, monodisperse and fully inhibitor-sensitive enzyme.

Anesthesiology and intensive care during epilepsy surgery in children and adolescents

Thus far there are no known studies of the management of anesthesiology and intensive care in cases of implantation of subdural plates for the purpose of determining the epileptogenic areal and of subsequent epilepsy surgery. Such operative measures are still considered too risky, especially in the case of small children. The authors report on cases of 45 children and adolescents who underwent this kind of surgery between December 1992 and December 1997 in Gilead hospital. In order to judge the anesthesiological risk the children were divided into three age groups: A, 1-5 years (n = 12); B, 6-11 years (n = 14); C, 12-18 years (n = 19). In none of these groups were there complications which in retrospect would have shed a negative light on the operation.

Autoimmune intestinal pathology induced by hsp60-specific CD8 T cells

Due to their ubiquitous distribution and high degree of structural similarity, heat shock proteins (hsp) are potential target antigens in autoimmune diseases. Here, we describe induction of intestinal inflammation following transfer of hsp60-reactive CD8 T cells into mice. Inflammatory reactions were MHC class I dependent and developed primarily in the small intestine. IFN gamma and TNF alpha, as well as gut-derived hsp60, were elevated at sites of T cell infiltration. Intestinal lesions were drastically reduced in mice lacking receptors for TNF alpha. Pathology also developed in germ-free mice, indicating recognition of host-derived hsp60 by CD8 T cells. This report demonstrates that CD8 T cells with defined antigen specificity cause intestinal inflammation, emphasizing a link between infection and autoimmune disease.

A single external enzyme confers alternative NADH:ubiquinone oxidoreductase activity in Yarrowia lipolytica

NADH:ubiquinone oxidoreductases catalyse the first step within the diverse pathways of mitochondrial NADH oxidation. In addition to the energy-conserving form commonly called complex I, fungi and plants contain much simpler alternative NADH:ubiquinone oxido-reductases that catalyze the same reaction but do not translocate protons across the inner mitochondrial membrane. Little is known about the distribution and function of these enzymes. We have identified YLNDH2 as the only gene encoding an alternative NADH:ubiquinone oxidoreductase (NDH2) in the obligate aerobic yeast Yarrowia lipolytica. Cells carrying a deletion of YLNDH2 were fully viable; full inhibition by piericidin A indicated that complex I activity was the sole NADH:ubiquinone oxidoreductase activity left in the deletion strains. Studies with intact mitochondria revealed that NDH2 in Y. lipolytica is oriented towards the external face of the mitochondrial inner membrane. This is in contrast to the situation seen in Saccharomyces cerevisiae, Neurospora crassa and in green plants, where internal alternative NADH:ubiquinone oxidoreductases have been reported. Phylogenetic analysis of known NADH:ubiquinone oxidoreductases suggests that during evolution conversion of an ancestral external alternative NADH:ubiquinone oxidoreductase to an internal enzyme may have paved the way for the loss of complex I in fermenting yeasts like S. cerevisiae.

Control of ubiquinol oxidation at center P (Qo) of the cytochrome bc1 complex

The unique bifurcated oxidation of ubiquinol at center P (Qo) of the cytochrome bc1 complex is the reaction within the Q-cycle reaction scheme that is most critical for the link between electron transfer and vectorial proton translocation. While there is a general consensus about the overall reaction at center P, the nature of the intermediates and the way the reaction is controlled to ensure obligatory bifurcation is still controversial. By reducing the reaction to its essential steps, a kinetic net rate model is developed in which the activation barrier is associated with the deprotonation of ubiquinol, but the steady state rate is kinetically controlled by the occupancy of the ubiquinol anion and the semiquinone state. This concept is used to interpret experimental data and is discussed in terms of various mechanistic models that are under discussion. It is outlined how other aspects of the center P mechanism like the proposed "prosthetic" ubiquinone and the moving domain of the "Rieske" protein could be incorporated in the kinetic framework.

Activation of G12/G13 results in shape change and Rho/Rho-kinase-mediated myosin light chain phosphorylation in mouse platelets

Platelets respond to various stimuli with rapid changes in shape followed by aggregation and secretion of their granule contents. Platelets lacking the alpha-subunit of the heterotrimeric G protein Gq do not aggregate and degranulate but still undergo shape change after activation through thromboxane-A2 (TXA2) or thrombin receptors. In contrast to thrombin, the TXA2 mimetic U46619 led to the selective activation of G12 and G13 in Galphaq-deficient platelets indicating that these G proteins mediate TXA2 receptor-induced shape change. TXA2 receptor-mediated activation of G12/G13 resulted in tyrosine phosphorylation of pp72(syk) and stimulation of pp60(c-src) as well as in phosphorylation of myosin light chain (MLC) in Galphaq-deficient platelets. Both MLC phosphorylation and shape change induced through G12/G13 in the absence of Galphaq were inhibited by the C3 exoenzyme from Clostridium botulinum, by the Rho-kinase inhibitor Y-27632 and by cAMP-analogue Sp-5,6-DCl-cBIMPS. These data indicate that G12/G13 couple receptors to tyrosine kinases as well as to the Rho/Rho-kinase-mediated regulation of MLC phosphorylation. We provide evidence that G12/G13-mediated Rho/Rho-kinase-dependent regulation of MLC phosphorylation participates in receptor-induced platelet shape change.

Three classes of inhibitors share a common binding domain in mitochondrial complex I (NADH:ubiquinone oxidoreductase)

We have developed two independent methods to measure equilibrium binding of inhibitors to membrane-bound and partially purified NADH:ubiquinone oxidoreductase (complex I) to characterize the binding sites for the great variety of hydrophobic compounds acting on this large and complicated enzyme. Taking advantage of a partial quench of fluorescence upon binding of the fenazaquin-type inhibitor 2-decyl-4-quinazolinyl amine to complex I in bovine submitochondrial particles, we determined a Kd of 17 +/- 3 nM and one binding site per complex I. Equilibrium binding studies with [3H]dihydrorotenone and the aminopyrimidine [3H]AE F119209 (4(cis-4-[3H]isopropyl cyclohexylamino)-5-chloro-6-ethyl pyrimidine) using partially purified complex I from Musca domestica exhibited little unspecific binding and allowed reliable determination of dissociation constants. Competition experiments consistently demonstrated that all tested hydrophobic inhibitors of complex I share a common binding domain with partially overlapping sites. Although the rotenone site overlaps with both the piericidin A and the capsaicin site, the latter two sites do not overlap. This is in contrast to the interpretation of enzyme kinetics that have previously been used to define three classes of complex I inhibitors. The existence of only one large inhibitor binding pocket in the hydrophobic part of complex I is discussed in the light of possible mechanisms of proton translocation.

Proton translocation in the respiratory chain involving ubiquinone--a hypothetical semiquinone switch mechanism for complex I

The protonmotive Q-cycle is the generally accepted reaction scheme of the cytochrome bc1 complex of the respiratory chain. It employs the redox-dependent protonation and deprotonation of ubiquinone (coenzyme Q10) to translocate protons across the inner mitochondrial or bacterial plasma membrane. The requirements for the operation of a similar mechanism in proton translocating NADH:ubiquinone oxidoreductase (complex I) and the specific roles of the 'Rieske' iron-sulfur center in the cytochrome bc1 complex and iron-sulfur center N-2 in complex I are discussed. Recent results suggest that there is only one ubiquinone-reactive site in complex I which seems to exclude a classical Q-cycle type mechanism. Therefore, a "semiquinone switch" mechanism is proposed involving one tightly bound and one substrate quinone. It is based on the same principles as a Q-cycle, but is a localized rather than a ligand conduction type mechanism.

Using two factor structures of the Mental Adjustment to Cancer (MAC) scale for assessing adaptation to breast cancer

The validity and reliability of two factor structures of the Mental Adjustment to Cancer (MAC) scale for assessing coping style was assessed by examining the relationship between the subscales, psychological distress, and quality of life (QOL) among Stage II and Stage IV breast cancer patients in four phases. First, MANOVAs assessed differences in coping, distress, and QOL across disease stages. Second, for each MAC factor structure, canonical correlation analyses assessed the relationship between coping styles, distress, and QOL, for each disease-stage group separately. Third, structural equation modeling (SEM) assessed the relationship among coping styles, distress, and QOL for all participants. Finally, the internal consistency of both MAC factor structures was assessed using Chronbach's alpha. The results were as follows: (1) significant differences across disease stages were found for coping styles using either the Watson or the Schwartz MAC subscales, but there were no differences in levels of distress or QOL; (2) for both MAC factor structures, coping style was found to be highly related to emotional distress and QOL, however, the strengths of the correlations between individual coping styles and distress/QOL indicators varied across disease stages; (3) SEM indicated that coping style was significantly related to distress and QOL when stage of disease was not considered, and that coping style and indicators of distress/QOL are separate, but highly correlated factors, as opposed to a single latent construct; and, (4) the Watson MAC subscales showed slightly better internal consistency than the Schwartz MAC subscales. Taken together, these findings highlight: (i) the validity of both MAC factor structures for clinical and research use with American breast cancer patients; (ii) the role of coping style as a mediator between disease stage and psychological distress and QOL; and, (iii) the need for refinement of certain Watson and Schwartz subscales.

The chemistry and mechanics of ubihydroquinone oxidation at center P (Qo) of the cytochrome bc1 complex

The emerging X-ray structures of the cytochrome bc1 complexes from bovine and chicken heart mitochondria support the protonmotive Q-cycle as the overall electron- and proton-pathway within the cytochrome bc1 complex. The energy conserving reaction within this reaction scheme is the unique bifurcation of electron flow into a high potential and a low potential pathway occurring at the ubihydroquinone-oxidation center (center P or Qo). This step is prerequisite for the 'recycling' of every second electron across the membrane onto the ubiquinone-reduction center, which results in vectorial proton translocation. It has been shown that during steady-state the step controlling this reaction is the first deprotonation of ubihydroquinone and not, as proposed earlier, the formation of a highly unstable semiquinone species. Ubiquinone has not yet been detected at the ubihydroquinone-oxidation center of the protein structures now available, but the pocket seems spacious enough to accommodate two ubiquinone molecules. This is in line with recent enzymological studies, which have shown that not only two ubiquinones, but also two inhibitor molecules can bind to center P. The most striking result from the structures is that the hydrophilic domain of the 'Rieske' protein can be found in two different positions which seem to allow electron transfer between the iron-sulfur cluster and either ubiquinone binding at center P or heme c1. This provides strong support for the 'catalytic switch' model proposed earlier based on detailed analysis of inhibitor binding to cytochrome bc1 complex in different redox states.

Identification and characterization of a novel 9.2-kDa membrane sector-associated protein of vacuolar proton-ATPase from chromaffin granules

Vacuolar proton-translocating ATPase (holoATPase and free membrane sector) was isolated from bovine chromaffin granules by blue native polyacrylamide gel electrophoresis. A 5-fold excess of membrane sector over holoenzyme was determined in isolated chromaffin granule membranes. M9.2, a novel extremely hydrophobic 9.2-kDa protein comprising 80 amino acids, was detected in the membrane sector. It shows sequence and structural similarity to Vma21p, a yeast protein required for assembly of vacuolar ATPase. A second membrane sector-associated protein (M8-9) was identified and characterized by amino-terminal protein sequencing.

Effects of mutation of residue I67 on redox-linked protonation processes in yeast cytochrome c oxidase

We describe effects of a mutation, Ile-67-->Asn, in subunit I of yeast cytochrome c oxidase on redox-linked protonation processes within the protein. The mutation lowers the midpoint potential of haem a and weakens its pH dependency, but has little effect on the potential of haem a3. The residue is close to a conserved glutamate (Glu-243) in the crystal structure. We propose that protonation of Glu-243 is redox-linked to haem a, that Asn-167 perturbs its pK and that redox-linked protonation in this location is essential for the catalytic reactions of the binuclear centre. These proposals are discussed in terms of a 'glutamate trap' mechanism for proton translocation in the haem/copper oxidases.

A structural model of the relationships among stage of disease, age, coping, and psychological adjustment in women with breast cancer

The present study used structural equation modeling to examine the relationships among disease stage (i.e. Stage II versus Stage IV), age, coping style, and psychological adjustment in 100 women diagnosed with breast cancer. Five separate models were examined: a full model, a mediational model, a demographic-disease model, a coping style model, and a regression model The analyses revealed that the present data best fit the mediational model in which age and stage of disease were not directly associated with psychological adjustment but, instead, were mediated by coping style (chi 2(25) = 45.776, AASR = 0.05, CFI = 0.94). The mediational model accounted for 56% of the variance in psychological adjustment. In particular, the model showed that younger women and women with an earlier disease stage used greater levels of the coping strategy characterized as a fighting spirit and lower levels of the coping strategies characterized as hopelessness/helplessness, anxious preoccupation, and fatalism which, in turn, were related to better psychological adjustment. Overall, these findings may offer an explanation for the conflicting findings regarding the relationship between age, stage of disease, and psychological adjustment to breast cancer by illustrating that coping strategies may be an essential mediating factor; in turn, a mediating model of psychological adaptation may offer useful information for clinicians as they implement interventions designed to improve patients coping efforts.

Action of insulin on the surface morphology of hepatocytes: role of phosphatidylinositol 3-kinase in insulin-induced shape change of microvilli

In previous studies we have shown that the insulin-responding glucose transporter isoform of 3T3-L1 adipocytes, GluT4, is almost completely located on microvilli. Furthermore, insulin caused the integration of these microvilli into the plasma membrane, suggesting that insulin-induced stimulation of glucose uptake may be due to the destruction of the cytoskeletal diffusion barrier formed by the actin filament bundle of the microvillar shaft regions [Lange et al. (1990) FEBS Lett. 261, 459-463; Lange et al. (1990) FEBS Lett. 276, 39-41]. Similar shape changes in microvilli were observed when the transport rates of adipocytes were modulated by glucose feeding or starvation. Here we demonstrate that the action of insulin on the surface morphology of hepatocytes is identical to that on 3T3L1 adipocytes; small and narrow microvilli on the surface of unstimulated hepatocytes were rapidly shortened and dilated on top of large domed surface areas. The aspect and mechanism of this effect are closely related to "membrane ruffling" induced by insulin and other growth factors. Pretreatment of hepatocytes with the PI 3-kinase inhibitor wortmannin (100 nM), which completely prevents transport stimulation by insulin in adipocytes and other cell types, also inhibited insulin-induced shape changes in microvilli on the hepatocyte surface. In contrast, vasopressin-induced microvillar shape changes in hepatocytes [Lange et al. (1997) Exp. Cell Res. 234, 486-497] were insensitive to wortmannin pretreatment. These findings indicate that PI 3-kinase products are necessary for stimulation of submembrane microfilament dynamics and that cytoskeletal reorganization is critically involved in insulin stimulation of transport processes. The mechanism of the insulin-induced cytoskeletal reorganization can be explained on the basis of the recent finding of Lu et al. [Biochemistry 35(1996) 14027-14034] that PI 3-kinase products exhibit much higher affinity for the profilin-actin complex than the primary products, PIP and PIP2. Thus, activated PI 3-kinase may direct a flux of profilin-actin complexes to the membrane locations of activated insulin receptors, where, due to the release of actin monomers after binding of profilactin to PI(3,4)P2 and PI(3,4,5)P3, massive actin polymerization is initiated. As a consequence, PI 3-kinase activation initiates a vectorial reorganization of the cellular actin system to membrane sites neighboring activated insulin receptors, giving rise to local membrane stress as visualized by extensive surface deformations and shortening of microvilli. In addition, extensive high-affinity binding of F-actin-barbed endcapping proteins enhances the cytoplasmic concentration of rapidly polymerizing filament ends. Consequently, the actin monomer concentration is lowered and the (cytoplasmic) pointed ends of the microvillar shaft bundle depolymerize and become shorter. The observations presented strengthen the previously postulated diffusion-barrier concept of glucose- and ion-uptake regulation and provide a mechanistic basis for explaining the action of insulin and other growth factors on transport processes across the plasma membrane.

Binding of MOA-stilbene to the mitochondrial cytochrome bc1 complex is affected by the protonation state of a redox-Bohr group of the 'Rieske' iron-sulfur protein

MOA-stilbene is a specific inhibitor of the ubihydroquinone oxidation center (center P or o) of cytochrome bc1 complex. Binding of this inhibitor does not require the 'Rieske' iron-sulfur protein, but is affected by the redox-state of the cytochrome bc1 complex. We have analyzed the pH dependence of the apparent dissociation constant for MOA-stilbene. A 2.5 fold change in affinity between pH 6.0 and 9.5 was observed for oxidized bovine cytochrome bc1 complex. The pH profile could be simulated by assuming a single protonable group with pKA = 7.7. This pKA was not observed after partial or complete reduction of the enzyme or after removal of the iron-sulfur protein. We conclude that this protonable group was identical to the redox-Bohr group with the same pKA that has been reported to be associated with the 'Rieske' iron-sulfur cluster. Fully reduced cytochrome bc1 complex exhibited an additional binding site for MOA-stilbene. As this second binding site was abolished by the center P inhibitor stigmatellin, but not by antimycin, an inhibitor of ubiquinone reduction at center N, we conclude that it is also located at center P.

Role of deprotonation events in ubihydroquinone:cytochrome c oxidoreductase from bovine heart and yeast mitochondria

The pH dependence of bovine and yeast cytochrome bc1 complex catalyzing electron transfer from ubi- and plastohydroquinone to cytochrome c have been analyzed. The pH dependence of the steady-state rate was found to be governed by two protonable groups, one of which (pK approximately 6.6) has to be deprotonated while the other (pK approximately 9.2) has to be protonated to allow catalysis. Using ubideuteroquinone instead of ubihydroquinone as a substrate resulted in 1.4- and 1.7-fold lower steady-state rates for the bovine and yeast enzymes, respectively. The activation energy at pH 8.0 was 33 kJ/mol for the bovine and 44 kJ/mol for the yeast enzyme and exhibited a linear decrease between pH 5.4 and 9.2. For ubihydroquinone the slope was very close to a value of -5.7 kJ/mol expected if the activation energy depended on a single deprotonation event. When plastohydroquinone was used instead, the slope more than doubled, indicating that a second deprotonation contributed to the activation barrier with this nonphysiological substrate. In contrast to previous kinetic models for the cytochrome bc1 complex, which propose that the activation barrier is associated with the formation of ubisemiquinone at the ubihydroquinone oxidation center, our results strongly suggest that the best approximation of the transition state is the singly deprotonated form of ubihydroquinone. This supports the recently proposed proton-gated charge transfer mechanism, which has control of catalysis by the first deprotonation of ubihydroquinone as one of its key features [Brandt, U. (1996) FEBS Lett. 387, 1-6]. All results reported here can be rationalized in a straightforward way based on other aspects of the same hypothesis.

Structural and functional analysis of deficient mutants in subunit I of cytochrome c oxidase from Saccharomyces cerevisiae

Four point mutations in subunit I of cytochrome c oxidase from Saccharomyces cerevisiae that had been selected for respiratory incompetence but still contained spectrally detectable haem aa3 were analysed. The isolated mutant enzymes exhibited minor band shifts in their optical spectra and contained all eleven subunits. However, steady state activities were only a few percent compared to wild type enzyme. Using a comprehensive experimental approach, we first checked the integrity of the enzyme preparations and then identified the specific functional defect. The results are discussed using information from the recently solved structures of cytochrome c oxidase at 2.8 A. Mutation 167N is positioned between haem a and a conserved glutamate residue (E243). It caused a distortion of the EPR signal of haem a and shifted its midpoint potential by 54 mV to the negative. The high-resolution structure suggests that the primary reason for the low activity of the mutant enzyme could be that asparagine in position 67 might form a stable hydrogen bond to E243, which is part of a proposed proton channel. Cytochrome c oxidase isolated from mutant T316K did not meet our criteria for homogeneity and was therefore omitted from further analysis. Mutants G352V and V380M exhibited an impairment of electron transfer from haem a to a3 and ligand binding to the binuclear centre was affected. In mutant V380M also the midpoint potential of CuB was shifted by 65 mV to the positive. The results indicated for these two mutants changes primarily associated with the binuclear centre, possibly associated with an interference in the routes and/or sites of protonation which are required for stable formation of the catalytic intermediates. This interpretation is discussed in the light of the high resolution structure.

A new concept for risk assessment of the hazards of non-genotoxic chemicals--electronmicroscopic studies of the cell surface. Evidence for the action of lipophilic chemicals on the Ca2+ signaling system

Recently, we presented evidence for the localization of components of the cellular Ca2+ signaling pathway in microvilli. On stimulation of this pathway, microvilli undergo characteristic morphological changes which can be detected by scanning electron microscopy (SEM) of the cell surface. Here we show that both receptor-mediated (vasopressin) and unspecific stimulation of the Ca2+ signaling system by the lipophilic tumor promoters thapsigargin (TG) and phorbolmyristateacetate (PMA) are accompanied by the same type of morphological changes of the cell surface. Since stimulated cell proliferation accelerates tumor development and sustained elevation of the intracellular Ca2+ concentrations is a precondition for stimulated cell proliferation, activated Ca2+ signaling is one possible mechanism of non-genomic tumor promotion. Using isolated rat hepatocytes we show that all tested lipophilic chemicals with known tumor promoter action, caused characteristic microvillar shape changes. On the other hand, lipophilic solvents that were used as differentiating agents in cell cultures such as dimethylsulfoxide (DMSO) and dimethylformamide also, failed to change the microvillar shapes. Instead DMSO stabilized the original appearance of microvilli. The used technique provides a convenient method for the evaluation of non-genomic carcinogenicity of chemicals prior to their industrial application.

Pterulinic acid and pterulone, two novel inhibitors of NADH:ubiquinone oxidoreductase (complex I) produced by a Pterula species. I. Production, isolation and biological activities

Pterulinic acid (1) and pterulone (2), two novel halogenated antibiotics, were isolated from fermentations of Pterula sp. 82168. Both compounds exhibited significant antifungal and weak or no cytotoxic activities. 1 and 2 are effective inhibitors of eucaryotic respiration. The target of the antibiotics resides within the mitochondrial NADH:ubiquinone oxidoreductase (complex I).

Proton-translocation by membrane-bound NADH:ubiquinone-oxidoreductase (complex I) through redox-gated ligand conduction

For the catalytic mechanism of proton-translocating NADH-dehydrogenase (complex I, EC 1.6.99.3) a number of hypothetical models have been proposed over the last three decades. These models are discussed in the light of recent substantial progress on the structure and function of this very complicated multiprotein complex. Only the high-potential iron-sulfur center N-2 and ubiquinone seem to contribute to the proton-translocating machinery of complex I: Based on the pH dependent midpoint potential of iron-sulfur cluster N-2 and the physical properties of ubiquinone intermediates a novel mechanism is proposed. The model builds on a series of defined chemical reactions taking place at three different ubiquinone-binding sites. Therefore, some aspects of this redox-gated ligand conduction mechanism are reminiscent to the proton-motive Q-cycle. However, its central feature is the abstraction of a proton from ubihydroquinone by a redox-Bohr group associated with iron-sulfur cluster N-2. Thus, in the proposed mechanism proton translocation is driven by a direct linkage between redox dependent protonation of iron-sulfur cluster N-2 and the redox chemistry of ubiquinone.

Evolutionary origin of cryptomonad microalgae: two novel chloroplast/cytosol-specific GAPDH genes as potential markers of ancestral endosymbiont and host cell components

Cryptomonads are complex microalgae which share characteristics of chromophytes (chlorophyll c, extra pair of membranes surrounding the plastids) and rhodophytes (phycobiliproteins). Unlike chromophytes, however, they contain a small nucleus-like organelle, the nucleomorph, in the periplastidial space between the inner and outer plastid membrane pairs. These cellular characteristics led to the suggestion that cryptomonads may have originated via a eukaryote-eukaryote endosymbiosis between a phagotrophic host cell and a unicellular red alga, a hypothesis supported by rRNA phylogenies. Here we characterized cDNAs of the nuclear genes encoding chloroplast and cytosolic glyceraldehyde-3-phosphate dehydrogenases (GAPDH) from the two cryptomonads Pyrenomonas salina and Guillardia theta. Our results suggest that in cryptomonads the classic Calvin cycle GAPDH enzyme of cyanobacterial origin, GapAB, is absent and functionally replaced by a photosynthetic GapC enzyme of proteobacterial descent, GapC1. The derived GapC1 precursor contains a typical signal/transit peptide of complex structure and sequence signatures diagnostic for dual cosubstrate specificity with NADP and NAD. In addition to this novel GapC1 gene a cytosol-specific GapC2 gene of glycolytic function has been found in both cryptomonads showing conspicuous sequence similarities to animal GAPDH. The present findings support the hypothesis that the host cell component of cryptomonads may be derived from a phototrophic rather than a organotrophic cell which lost its primary plastid after receiving a secondary one. Hence, cellular compartments of endosymbiotic origin may have been lost or replaced several times in eukaryote cell evolution, while the corresponding endosymbiotic genes (e.g., GapC1) were retained, thereby increasing the chimeric potential of the nuclear genome.

Detection of the ATP-dependent nonmitochondrial calcium store in a cell surface-derived vesicle fraction from isolated rat hepatocytes

In preceding studies, the IP3-sensitive Ca2+ store of the hamster insulinoma cell line, HIT, was detected in cell surface protrusions such as microvilli and related membrane structures [Lange, K., and Brandt, U. (1993) FEBS Lett. 320, 183-188; and (1993) FEBS Lett. 325, 205-209]. In this study, these experiments were extended on rat hepatocytes. We used the previously described shearing technique for isolating cell surface-derived vesicle fractions from freshly isolated and 48-h-cultured rat hepatocytes. As shown by Western blot analysis, these vesicles contained the hepatocyte-specific glucose transporter, GluT2, and actin, which are both typical microvillar components. Scanning electron microscopy revealed that a spherical vesicle population of uniform size (about 1 microm in diameter) originates from the hepatocyte microvilli. This vesicle fraction exhibited ATP-dependent and thapsigargin-sensitive Ca2+ storage activity with properties identical to those of the known microsomal systems and of HIT cell surface-derived vesicles, except that the ATP-dependent Ca2+ pool was insensitive to IP3. Like HIT surface vesicles, hepatocyte surface vesicles rapidly took up ATP via a 4,4'-diisocyanostilbene-2,2'-disulfonic acid (DIDS)-sensitive anion pathway. Inhibition of ATP influx into the vesicles by DIDS also completely inhibited ATP-dependent Ca2+ storage. Moreover, determination of efflux kinetics of Ca2+ from passively (in the absence of ATP) loaded vesicles revealed a La(3+)-sensitive but IP3-independent Ca2+ pathway which rapidly equilibrated intravesicular free Ca2+ with the external medium. Permeabilization of the vesicles with saponin (0.005%) opened an additional efflux pathway for Ca2+ which is not La(3+)-sensitive. However, saponin treatment of vesicles preloaded with Ca2+ in the presence of ATP did not affect the thapsigargin-sensitive vesicular Ca2+ store but only released a small portion (about 20%) of the vesicular Ca2+ that is not part of the thapsigargin-sensitive Ca2+ pool. Also, the size of the saponin-releasable Ca2+ pool was not affected by depletion of the thapsigargin-sensitive Ca2+ store. These findings indicate that hepatocyte surface vesicles are readily permeable for Ca2+ and ATP via cation and anion pathways. Consequently, Ca2+ storage into these vesicles does not occur by concentrative Ca2+ pumping but rather appears to be due to an internal, ATP-dependent mechanism of Ca2+ sequestration. The presented data are in accord with the previously reported colocalization of the ATP-dependent Ca2+ store and its functionally coupled, store-regulated Ca2+ influx pathway in special cell surface organelles, the microvilli.

Calcium storage and release properties of F-actin: evidence for the involvement of F-actin in cellular calcium signaling

Preceding studies have shown that the bulk of the ATP-dependent, inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ store of hamster insulinoma (HIT) cells is located in microvilli on the cell surface. Similar results were obtained with isolated rat hepatocytes. Moreover, in vesicles of microvillar origin, passive fluxes of Ca2+, ATP, and IP3 occur through cation and anion channels, respectively, suggesting that Ca2+ storage is due to ATP-dependent Ca2+ binding to an intravesicular component. Here we demonstrate that F-actin may be a possible candidate for this function. ATP-actin monomers bind Ca2+ with high affinity (Kd = 2-8 nM) to their divalent cation binding sites. Polymerization of actin monomers decreases the rate constant for divalent cation exchange at this binding site by more than 3 orders of magnitude rendering bound cations nearly unavailable. F-actin-bound Ca2+ can be released by depolymerization and dissociation from Ca(2+)-ADP-actin monomers (Kd = 375 nM). We now provide additional evidence for the possible involvement of actin in Ca2+ storage. (1) Preincubation of surface-derived Ca(2+)-storing vesicles from HIT cells with the F-actin stabilizer, phalloidin, strongly inhibited ATP-dependent Ca2+ uptake, reducing the IP3-sensitive Ca2+ pool by 70%. Phalloidin, when added after the loading process, affected neither the amount of stored Ca2+ nor IP3 action on the store. (2) F-actin polymerized in the presence of Mg2+ in nominally Ca(2+)-free buffer still contained about half of the high affinity sites occupied with Ca2+ (Mg/Ca-F-actin). (3) Using the fura-2 technique, we found that in the presence of ATP, Mg/Ca-F-actin incorporated free Ca2+ at a relatively low rate. Short pulses of ultrasound (3-10 s) strongly accelerated Ca2+ uptake, decreasing free Ca2+ from 500 nM to below 100 nM. (4) In the presence of physiological levels of Mg2+ (0.5 mM), sonication liberated large amounts of Ca2+ from Mg/Ca-F-actin. (5) Ca-F-actin released bound Ca2+ at a very slow rate. Short ultrasonic pulses rapidly elevated free Ca2+ from about 50 nM up to 500 nM. (6) Small amounts of profilin, an actin-binding protein, released Ca2+ both from Ca- and Mg/Ca-F-actin and also inhibited uptake of Ca2+ into Mg/Ca-F-actin. (7) Phalloidin completely inhibited Ca-uptake into Mg/Ca-F-actin even during ultrasonic treatment. These findings suggest that Ca2+ storage may occur by addition of Ca-ATP-actin monomers to reactive ends of the polymer and emptying of this store by profilin-stimulated release of Ca-ADP-actin. Thus, receptor-operated Ca2+ signaling, initiated by phospholipase C activation, may proceed via the well-known phosphatidylinositol phosphate-regulated profilin/gelsolin pathway of actin reorganization/depolymerization. The importance of the proposed microvillar Ca2+ signaling system for living cells remains to be established.

Energy conservation by bifurcated electron-transfer in the cytochrome-bc1 complex

The overall electron- and proton-pathways within the cytochrome-bc1 complex are described by a widely accepted mechanism known as the protonmotive Q-cycle. Within this reaction scheme, the unique bifurcation of electron flow into a high potential and a low potential pathway occurring at the ubihydroquinone-oxidation center is the energy conserving reaction. It is this reaction, which results in vectorial proton translocation, as it allows the 'recycling' of every second electron across the membrane onto the ubiquinone-reduction center. However, the Q-cycle reaction scheme does not address the detailed chemistry of this central step. Based on a structural model of the ubihydroquinone-oxidation pocket and the assumption that the reaction involves two ubiquinone molecules in a stacked configuration, here I propose a detailed chemical model for the reactions occurring during steady-state catalysis. In this proton-gated charge-transfer mechanism the reaction is controlled by the deprotonation of the substrate ubihydroquinone and not, as proposed earlier, by the formation of a highly unstable semiquinone species.

2-Nitrosofluorene and N-hydroxy-2-aminofluorene react with the ubiquinone-reduction center (center N) of the mitochondrial cytochrome bc1 complex

We determined the sites of artificial electron transfer onto 2-nitrosofluorene (NOF), a metabolite of carcinogenic 2- acetylaminofluorene in mitochondria and isolated cytochrome bc1 complex. NOF-induced O2 consumption in mitochondria was sensitive to antimycin A, but insensitive to myxothiazol. In the isolated cytochrome bc1 complex, NOF induced rapid MOA-stilbene-insensitive reoxidation of cytochrome b, whereas in the presence of antimycin A, reoxidation was very slow. The corresponding hydroxylamine, N-hydroxy-2-aminofluorene (N-OH-AF), reduced cytochrome b specifically through center N of the cytochrome bc1 complex. We conclude that NOF and N-OH-AF bind to center N of the cytochrome bc1 complex and act as electron acceptor and donor, respectively. The N-OH-AF/NOF interconversion is considered to be involved in the cytotoxicity of 2-acetylaminofluorene in vivo.