Existence of phospholipid bilayer structure in the inner membrane of mitochondria

Influence of tissue, diet, and enzymatic remodeling on cardiolipin fatty acyl profile

Cardiolipin is a specialized phospholipid found primarily in the inner mitochondrial membrane. Because of its unique dimeric structure, cardiolipin plays an important role in mitochondrial function, stability, and membrane fluidity. As such, cardiolipin is subject to a high degree of remodeling by phospholipases, acyltransferases, and transacylases that create a fatty acyl profile that tends to be highly tissue-specific. Despite this overarching regulation, the molecular species of cardiolipin produced are also influenced by dietary lipid composition. A number of studies have characterized the tissue-specific profile of cardiolipin species and have investigated the specific nature of cardiolipin remodeling, including the role of both enzymes and diet. The aim of this review is to highlight tissue specific differences in cardiolipin composition and, collectively, the enzymatic and dietary factors that contribute to these differences. Consequences of aberrant cardiolipin fatty acyl remodeling are also discussed.

Long-chain fatty acids act as protonophoric uncouplers of oxidative phosphorylation in rat liver mitochondria

The effect of long-chain fatty acids (LCFA) on respiration and transmembrane potential (delta psi) in the resting state, and the rate of delta psi dissipation [d delta psi/dt)i) was investigated with oligomycin-inhibited rat liver mitochondria using succinate (plus rotenone) as substrate. The results obtained were compared with those of classical protonophores such as 2,4-dinitrophenol (DNP) and 4,5,6,7-tetrachloro-2-trifluoromethylbenzimidazole (TTFB). The effects of oleate or palmitate and that of DNP or TTFB on respiration and delta psi can be described by a common force-flow relationship. These facts all in all are not compatible with a decoupler-type uncoupling mechanism of LCFA; still, they indicate that the latter are protonophores. Moreover, the oleate-induced increase in the rate of delta psi dissipation closely correlates with that in respiration, suggesting that the uncoupling activity and the protonophoric activity of LCFA are interrelated. Carboxyatractyloside (CAT) exerted only a small inhibitory effect on oleate-induced respiration and delta psi dissipation, indicating that the adenine nucleotide translocase contributes to the uncoupling effect of LCFA to a minor extent only. Proton transport through the lipid region of the membrane as mediated by permeation of the protonated and deprotonated forms of LCFA is interpreted as the main process of the uncoupling of LCFA.

The kinetic mechanism of action of an uncoupler of oxidative phosphorylation

The chemiosmotic hypothesis predicts that the mechanism by which weak acids uncouple oxidative phosphorylation in mitochondria is identical to the mechanism by which they transport hydrogen ions across artificial bilayer membranes. We report here the results of a kinetic study of uncoupler-mediated hydrogen ion transport across bilayer membranes. We made electrical relaxation measurements on black lipid membranes exposed to the substituted benzimidazole 5,6-dichloro-2-trifluoromethylbenzimidazole. The simplest model consistent with our experimental data allowed us to deduce values for adsorption coefficients and rate constants. Our major conclusions are that the back diffusion of the neutral species is the rate limiting step for the steady state transport of hydrogen ions, that both the neutral and charged forms of the uncoupler adsorb strongly to the interfaces, and that the reactions at the membrane-solution interfaces occur sufficiently rapidly for equilibrium to be maintained. Independent measurements of the adsorption coefficients of both the neutral and anionic forms of the weak acid and also of the permeability of the membrane to the neutral form agreed well with the values deduced from the kinetic study.

Biophysics with nitroxyl radicals

The present status of the spin labeling method as applied to Biophysics is examined. After an outline of the chemical and physical properties of NO radicals, the analysis of linear and non-linear ESR spectra of spin labels and the information it yields is described. The possibilities of the method are critically discussed in the light of recent experiments.

Pressure effect on the membrane action of a nerve-blocking spin label

A reversible nerve-blocking spin label, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) was used to study the nature of anesthetic-binding sites in nerve membranes as a function of pressure. The nerve-blocking effect of TEMPO is enhanced under pressure. At atmospheric pressure, TEMPO blocks nerve conduction by solubilizing in the apolar region of the nerve membrane. However, the nerve-conduction-block by TEMPO at 150 atm of helium was related to the binding of TEMPO to a pressure-induced high-affinity polar site in the nerve membrane. The new TEMPO-binding site could not be detected in lipid model membranes and, thus, the involvement of membrane protein in the new site was inferred. Pressure may induce a nerve membrane conformation change in the presence of TEMPO. The observation that under different pressure, a single anesthetic, i.e., TEMPO, was capable of blocking nerve conduction by binding to two different sites within the nerve membrane, supports the view that there are multiple anesthetic receptor sites, which differ in chemical composition and location within the nerve membrane. These sites, when occupied by different classes of anesthetics, produce the general phenomenon of nerve-conduction block. The enhancement of nerve-conduction block by pressure may be due to the increased concentration of TEMPO in the new site in the nerve membrane under pressure.

Orientation and motion of amphiphilic spin labels in hexagonal lipid phases

The acyl chain of spin-labeled fatty acids intercalates between the lipid hydrocarbon chains in hexagonal and micellar phases with the carboxyl group anchored at the lipid water interface. The spectra are characteristic of anisotropic motion and cannot be distinguished from the spectra of these probes in lamellar dispersions. In the hexagonal and micellar phases the molecular motion of the spin label increases as it is moved further away from the carboxyl group, similar to the behavior in the lamellar phase (Jost et al. (1971) J. Mol. Biol. 59, 77-98). The similarity in packing of the acyl chains in the hexagonal and lamellar phases suggests that localized regions of hexagonal phase are compatible with a bilayer matrix.