Flip-Flop Is the Rate-Limiting Step for Transport of Free Fatty Acids across Lipid Vesicle Membranes

Fatty acid uptake in Trypanosoma brucei: Host resources and possible mechanisms

Trypanosoma brucei spp. causes African Sleeping Sickness in humans and nagana, a wasting disease, in cattle. As T. brucei goes through its life cycle in its mammalian and insect vector hosts, it is exposed to distinct environments that differ in their nutrient resources. One such nutrient resource is fatty acids, which T. brucei uses to build complex lipids or as a potential carbon source for oxidative metabolism. Of note, fatty acids are the membrane anchoring moiety of the glycosylphosphatidylinositol (GPI)-anchors of the major surface proteins, Variant Surface Glycoprotein (VSG) and the Procyclins, which are implicated in parasite survival in the host. While T. brucei can synthesize fatty acids de novo, it also readily acquires fatty acids from its surroundings. The relative contribution of parasite-derived vs. host-derived fatty acids to T. brucei growth and survival is not known, nor have the molecular mechanisms of fatty acid uptake been defined. To facilitate experimental inquiry into these important aspects of T. brucei biology, we addressed two questions in this review: (1) What is known about the availability of fatty acids in different host tissues where T. brucei can live? (2) What is known about the molecular mechanisms mediating fatty acid uptake in T. brucei? Finally, based on existing biochemical and genomic data, we suggest a model for T. brucei fatty acid uptake that proposes two major routes of fatty acid uptake: diffusion across membranes followed by intracellular trapping, and endocytosis of host lipoproteins.

Phospholipid components of the synthetic pulmonary surfactant CHF5633 probed by fluorescence spectroscopy

CHF5633 (Chiesi Farmaceutici, Italy) is a synthetic pulmonary surfactant currently under clinical development for the treatment of Respiratory Distress Syndrome in premature infants. The product is composed of phospholipids in liposomal organization, together with two peptide analogues of human surfactant proteins B and C. Phospholipids in liposomes can undergo oxidation of unsaturated lipids and hydrolysis, with formation of fatty acids and lysolipids, both affecting the physico-chemical properties of the formulation. We exploited two fluorescence probes, Prodan and ADIFAB, to evaluate the stability of the phospholipid components of CHF5633. While Prodan enters the phospholipid bilayer and probes the polarity of this environment, ADIFAB binds free fatty acids in the aqueous phase, allowing to determine their concentration. Changes of Prodan fluorescence emission indicated an increase in the polarity of the phospholipid bilayer as a function of time. This behavior is coupled with an increase in fatty acids concentration in the aqueous phase, as determined by ADIFAB, and an increase in lysolipids concentration, as determined by HPLC-MS. Prodan and ADIFAB resulted efficient probes to monitor phospholipids hydrolysis in liposomes, reporting an increased stability of CHF5633 at pH values higher than 6.5.

Proton transfer dynamics dictate quinone speciation at lipid-modified electrodes

Proton-coupled electron transfer (PCET) reactions are ubiquitous in biochemistry and alternative energy schemes. Natural enzymes utilize quinones in proton transfer chains and energy conversion processes. Here, we utilize a bio-inspired hybrid bilayer membrane system to control the reaction mechanism of a quinone molecule covalently bound to an electrode surface. In particular, by impeding proton access to the quinone moiety, we change the reaction pathway from a PCET process to a pure electron transfer step. We further alter the reaction pathway to a stepwise PCET process by controlling the proton flux through the use of an alkyl proton carrier incorporated in the lipid membrane. By modulating proton availability, we control the quinone reaction pathway without changing the molecular structure of the redox species. This work provides unique insight into PCET reactions and a novel electrochemical platform for interrogating them.

Proton transfer dynamics control the mechanism of O2 reduction by a non-precious metal electrocatalyst

Many chemical and biological processes involve the transfer of both protons and electrons. The complex mechanistic details of these proton-coupled electron transfer (PCET) reactions require independent control of both electron and proton transfer. In this report, we make use of lipid-modified electrodes to modulate proton transport to a Cu-based catalyst that facilitates the O2 reduction reaction (ORR), a PCET process important in fuel cells and O2 reduction enzymes. By quantitatively controlling the kinetics of proton transport to the catalyst, we demonstrate that undesired side products such as H2O2 and O2(-) arise from a mismatch between proton and electron transfer rates. Whereas fast proton kinetics induce H2O2 formation and sluggish proton flux produces O2(-), proton transfer rates commensurate with O-O bond breaking rates ensure that only the desired H2O product forms. This fundamental insight aids in the development of a comprehensive framework for understanding the ORR and PCET processes in general.

Factors influencing the eicosanoids synthesis in vivo

External factors activate a sequence of reactions involving the reception, transduction, and transmission of signals to effector cells. There are two main phases of the body's reaction to harmful factors: the first aims to neutralize the harmful factor, while in the second the inflammatory process is reduced in size and resolved. Secondary messengers such as eicosanoids are active in both phases. The discovery of lipoxins and epi-lipoxins demonstrated that not all arachidonic acid (AA) derivatives have proinflammatory activity. It was also revealed that metabolites of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) such as resolvins, protectins, and maresins also take part in the resolution of inflammation. Knowledge of the above properties has stimulated several clinical trials on the influence of EPA and DHA supplementation on various diseases. However, the equivocal results of those trials prevent the formulation of guidelines on EPA and DHA supplementation. Prescription drugs are among the substances with the strongest influence on the profile and quantity of the synthesized eicosanoids. The lack of knowledge about their influence on the conversion of EPA and DHA into eicosanoids may lead to erroneous conclusions from clinical trials.

Flip-flop of oleic acid in a phospholipid membrane: rate and mechanism

Flip-flop of protonated oleic acid molecules dissolved at two different concentrations in membranes made of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine is studied with the aid of molecular dynamics simulations at a time scale of several microseconds. Direct, single-molecule flip-flop events are observed at this time scale, and the flip-flop rate is estimated at 0.2-0.3 μs(-1). As oleic acid molecules move toward the center of the bilayer during flip-flop, they undergo gradual, correlated translational, and rotational motion. Rare, double-flipping events of two hydrogen-bonded oleic acid molecules are also observed. A two-dimensional free energy surface is obtained for the translational and rotational degree of freedom of the oleic acid molecule, and the minimum energy path on this surface is determined. A barrier to flip-flop of ~4.2 kcal/mol is found at the center of the bilayer. A two-dimensional diffusion model is found to provide a good description of the flip-flop process. The fast flip-flop rate lends support to the proposal that fatty acids permeate membranes without assistance of transport proteins. It also suggests that desorption rather than flip-flop is the rate-limiting step in fatty acid transport through membranes. The relation of flip-flop rates to the evolution of ancestral cellular systems is discussed.

Fatty acid (FFA) transport in cardiomyocytes revealed by imaging unbound FFA is mediated by an FFA pump modulated by the CD36 protein

Free fatty acid (FFA) transport across the cardiomyocyte plasma membrane is essential to proper cardiac function, but the role of membrane proteins and FFA metabolism in FFA transport remains unclear. Metabolism is thought to maintain intracellular FFA at low levels, providing the driving force for FFA transport, but intracellular FFA levels have not been measured directly. We report the first measurements of the intracellular unbound FFA concentrations (FFA(i)) in cardiomyocytes. The fluorescent indicator of FFA, ADIFAB (acrylodan-labeled rat intestinal fatty acid-binding protein), was microinjected into isolated cardiomyocytes from wild type (WT) and FAT/CD36 null C57B1/6 mice. Quantitative imaging of ADIFAB fluorescence revealed the time courses of FFA influx and efflux. For WT mice, rate constants for efflux (∼0.02 s(-1)) were twice influx, and steady state FFA(i) were more than 3-fold larger than extracellular unbound FFA (FFA(o)). The concentration gradient and the initial rate of FFA influx saturated with increasing FFA(o). Similar characteristics were observed for oleate, palmitate, and arachidonate. FAT/CD36 null cells revealed similar characteristics, except that efflux was 2-3-fold slower than WT cells. Rate constants determined with intracellular ADIFAB were confirmed by measurements of intracellular pH. FFA uptake by suspensions of cardiomyocytes determined by monitoring FFA(o) using extracellular ADIFAB confirmed the influx rate constants determined from FFA(i) measurements and demonstrated that rates of FFA transport and etomoxir-sensitive metabolism are regulated independently. We conclude that FFA influx in cardiac myocytes is mediated by a membrane pump whose transport rate constants may be modulated by FAT/CD36.