M Degli Esposti

Incorporation of ubiquinone homologs into lipid vesicles and mitochondrial membranes

Abstract We have investigated the incorporation of ubiquinone homologs from an aqueous medium into intact and ubiquinone-depleted mitochondria and into monolamellar lipid vesicles formed from either pure lecithin or mixed phospholipids. The state of ubiquinones in the aqueous medium was found to be nonmonomeric by the results of EPR spin label studies showing immobilization of nitroxide stearate derivatives in the presence of ubiquinones. The uv spectra of ubiquinone homologs in media of different polarities and in phospholipid bilayers suggest that the incorporated ubiquinones are completely immersed in a hydrophobic medium. The incorporation of ubiquinones into lipid vesicles and mitochondrial membranes was assayed by first removing the nonincorporated quinones by pentane washings and then by measuring the absorbance of the incorporated quinones after extraction with methanol-light petroleum. In a concentration range of added ubiquinones up to 0.3 m m , the incorporation into lipid vesicles and mitochondrial membranes is linear, allowing the calculation of apparent partition coefficients for the ubiquinone homologs between water and the membranes. The partition coefficients largely increased by cosonication of ubiquinones with phospholipids. The incorporation into mitochondrial membranes is about threefold greater than into lipid vesicles. These studies provide a basis for the interpretation of the effects of exogenous ubiquinones in model systems and in membrane-linked activities.

A model for the molecular organization of cytochrome β‐561 in chromaffin granule membranes

The CD spectrum of reduced cytochrome (cyt.) β‐561 in chromaffin granule membranes resembles that of mitochondrial cyt. β1 and indicates possible heme‐heme interaction in the protein. Based on spectroscopic data and analysis of the amino acid sequence, a model of cyt. β‐561 is suggested, in which the protein carries two transmembrane‐localized hemes, each coordinated by two histidines. The model accounts for the presence of two different forms of cyt. β‐561 in chromaffin granule membranes and provides a mechanism of transmembrane electron transfer by this hemoprotein.

Quantization of a class of piecewise affine transformations on the torus

We present a unified framework for the quantization of a family of discrete dynamical systems of varying degrees of “chaoticity”. The systems to be quantized are piecewise affine maps on the two-torus, viewed as phase space, and include the automorphisms, translations and skew translations. We then treat some discontinuous transformations such as the Baker map and the sawtooth-like maps. Our approach extends some ideas from geometric quantization and it is both conceptually and calculationally simple.

Thermodynamical limit for correlated Gaussian random energy models

Abstract: Let {EΣ(N)}ΣΣN be a family of |ΣN|=2N centered unit Gaussian random variables defined by the covariance matrix CN of elements cN(Σ,τ):=Av(EΣ(N)Eτ(N)) and the corresponding random Hamiltonian. Then the quenched thermodynamical limit exists if, for every decomposition N=N1+N2, and all pairs (Σ,τ)ΣN×ΣN: where πk(Σ),k=1,2 are the projections of ΣΣN into ΣNk. The condition is explicitly verified for the Sherrington-Kirkpatrick, the even p-spin, the Derrida REM and the Derrida-Gardner GREM models.

Quantum Variance and Ergodicity for the Baker's Map

We prove an Egorov theorem, or quantum-classical correspondence, for the quantised bakers map, valid up to the Ehrenfest time. This yields a logarithmic upper bound for the decay of the quantum variance, and, as a corollary, a quantum ergodic theorem for this map.

The cytochrome b of the sea urchin Paracentrotus lividus is naturally resistant to myxothiazol and mucidin

The ubiquinol:cytochrome c reductase activity of Paracentrotus lividus mitochondria is relatively insensitive to the specific inhibitors myxothiazol and mucidin. The I 50 of myxothiazol and mucidin are three and two orders of magnitude higher, respectively, in P. lividus than in bovine heart mitochondria. The natural resistance of the P. lividus reductase to these inhibitors can be correlated with a single amino replacement, an alanine for a glycine at position 143, in the sequence of cytochrome b. This position is located in a conserved region of the molecule, believed to be important in the oxidation of ubiquinol by the reductase.

The quantum perturbed cat map and symmetry

We investigate a class of quantum symmetries of the perturbed cat map which exist only for a subset of possible values of Plancks constant. The effect of these symmetries is to change the spectral statistics along this positive-density subset. The symmetries are shown to be related to some simple classical symmetries of the map.

Interaction between ubiquinone and ATPase in mitochondrial membranes.

The extraction of ubiquinone from mitochondrial membranes produces alterations of ATPase activity including a reversible loss of oligomycin sensitivity which is restored by long-chain Q-homologs. Short-chain ubiquinones like Q3 produce a loss of oligomycin and dicyclohexylcarbodiimide (DCCD) sensitivity in submitochondrial particles. The effect shows uncompetitive or noncompetitive kinetics with respect to oligomycin or DCCD respectively. Long-chain ubiquinones have a competitive effect with Q3, thus restoring oligomycin sensitivity; they behave, however, in about the same way as Q3 in lowering the DCCD sensitivity in submitochondrial particles. On the basis of these observations we suggest that ubiquinone may be a physiological modulator of ATPase activity in the mitochondrial membrane.

Statistics of energy levels and zero temperature dynamics for deterministic spin models with glassy behaviour

We consider the zero-temperature dynamics for the infinite-range, non translation invariant one-dimensional spin model introduced by Marinari, Parisi and Ritort to generate glassy behaviour out of a deterministic interaction. It is argued that there can be a large number of metastable (i.e., one-flip stable) states with very small overlap with the ground state but very close in energy to it, and that their total number increases exponentially with the size of the system.We consider the zero-temperature dynamics for the infinite-range, non translation invariant one-dimensional spin model introduced by Marinari, Parisi and Ritort to generate glassy behaviour out of a deterministic interaction. It is argued that there can be a large number of metastable (i.e., one-flip stable) states with very small overlap with the ground state but very close in energy to it, and that their total number increases exponentially with the size of the system.

Role of Ubiquinone Diffusion in Mitochondrial Electron Transfer

It is presently recognized that the transfer of electrons from substrates to molecular oxygen in mitochondria involves integral membrane protein complexes containing redox groups of increasing redox potentials (Tzagoloff, 1982). According to the original intuition of Green (1966), it is widely accepted that electron transfer between complexes is ensured by mobile components acting as cosubstrates, vz. ubiquinone (Q) and cytochrome c (cyt.c). In particular, ubiquinone holds a central position in the chain, being reduced by a large collection of flavoprotein dehydrogenases and reoxidized by the ubiquinol-cyt.c reductase complex (bql complex or Complex III)(Lenaz et al., 1985 b): such position is likely to mark a strategic point for a regulatory role in electron transfer.