An infrared investigation of palmitoyl-coenzyme A and palmitoylcarnitine interaction with perdeuterated-chain phospholipid bilayers

Carnitine and acylcarnitines: pharmacokinetic, pharmacological and clinical aspects

L-Carnitine (levocarnitine) is a naturally occurring compound found in all mammalian species. The most important biological function of L-carnitine is in the transport of fatty acids into the mitochondria for subsequent β-oxidation, a process which results in the esterification of L-carnitine to form acylcarnitine derivatives. As such, the endogenous carnitine pool is comprised of L-carnitine and various short-, medium- and long-chain acylcarnitines. The physiological importance of L-carnitine and its obligatory role in the mitochondrial metabolism of fatty acids has been clearly established; however, more recently, additional functions of the carnitine system have been described, including the removal of excess acyl groups from the body and the modulation of intracellular coenzyme A (CoA) homeostasis. In light of this, acylcarnitines cannot simply be considered by-products of the enzymatic carnitine transfer system, but provide indirect evidence of altered mitochondrial metabolism. Consequently, examination of the contribution of L-carnitine and acylcarnitines to the endogenous carnitine pool (i.e. carnitine pool composition) is critical in order to adequately characterize metabolic status. The concentrations of L-carnitine and its esters are maintained within relatively narrow limits for normal biological functioning in their pivotal roles in fatty acid oxidation and maintenance of free CoA availability. The homeostasis of carnitine is multifaceted with concentrations achieved and maintained by a combination of oral absorption, de novo biosynthesis, carrier-mediated distribution into tissues and extensive, but saturable, renal tubular reabsorption. Various disorders of carnitine insufficiency have been described but ultimately all result in impaired entry of fatty acids into the mitochondria and consequently disturbed lipid oxidation. Given the sensitivity of acylcarnitine concentrations and the relative carnitine pool composition in reflecting the intramitochondrial acyl-CoA to free CoA ratio (and, hence, any disturbances in mitochondrial metabolism), the relative contribution of L-carnitine and acylcarnitines within the total carnitine pool is therefore considered critical in the identification of mitochondria dysfunction. Although there is considerable research in the literature focused on disorders of carnitine insufficiency, relatively few have examined relative carnitine pool composition in these conditions; consequently, the complexity of these disorders may not be fully understood. Similarly, although important studies have been conducted establishing the pharmacokinetics of exogenous carnitine and short-chain carnitine esters in healthy volunteers, few studies have examined carnitine pharmacokinetics in patient groups. Furthermore, the impact of L-carnitine administration on the kinetics of acylcarnitines has not been established. Given the importance of L-carnitine as well as acylcarnitines in maintaining normal mitochondrial function, this review seeks to examine previous research associated with the homeostasis and pharmacokinetics of L-carnitine and its esters, and highlight potential areas of future research.

Sphingosine-1-phosphate as an amphipathic metabolite: its properties in aqueous and membrane environments

Sphingosine-1-phosphate (S1P) is currently considered to be an important signaling molecule in cell metabolism. We studied a number of relevant biophysical properties of S1P, using mainly Langmuir balance, differential scanning calorimetry, (31)P-NMR, and infrared (IR) spectroscopy. We found that, at variance with other, structurally related sphingolipids that are very hydrophobic, S1P may occur in either an aqueous dispersion or a bilayer environment. S1P behaves in aqueous media as a soluble amphiphile, with a critical micelle concentration of approximately 12 muM. Micelles give rise to larger aggregates (in the micrometer size range) at and above a 1 mM concentration. The aggregates display a thermotropic transition at approximately 60 degrees C, presumably due to the formation of smaller structures at the higher temperatures. S1P can also be studied in mixtures with phospholipids. Studies with dielaidoylphosphatidylethanolamine (DEPE) or deuterated dipalmitoylphosphatidylcholine (DPPC) show that S1P modifies the gel-fluid transition of the glycerophospholipids, shifting it to lower temperatures and decreasing the transition enthalpy. Low (<10 mol %) concentrations of S1P also have a clear effect on the lamellar-to-inverted hexagonal transition of DEPE, i.e., they increase the transition temperature and stabilize the lamellar versus the inverted hexagonal phase. IR spectroscopy of natural S1P mixed with deuterated DPPC allows the independent observation of transitions in each molecule, and demonstrates the existence of molecular interactions between S1P and the phospholipid at the polar headgroup level that lead to increased hydration of the carbonyl group. The combination of calorimetric, IR, and NMR data allowed the construction of a temperature-composition diagram ("partial phase diagram") to facilitate a comparative study of the properties of S1P and other related lipids (ceramide and sphingosine) in membranes. In conclusion, two important differences between S1P and ceramide are that S1P stabilizes the lipid bilayer structure, and physiologically relevant concentrations of S1P can exist dispersed in the cytosol.

Studies on the Critical Micellar Concentration and Phase Transitions of Stearoylcarnitine

The critical micellar concentration (CMC) of stearoylcarnitine was determined at different pH values at room temperature by fluorescence spectroscopy, monitoring the spectral changes of 8-anilinonaphthalene-1-sulfonate (ANS). The CMC was found to vary with pH, increasing from about 10 μM at pH 3.0 to ca. 25 μM at pH 7.0, but decreasing slightly with further increase in pH to approximately 19 μM at pH 10.0. Differential scanning calorimetry (DSC) shows that stearoylcarnitine dispersed in water at low concentration undergoes a broad thermotropic phase transition at 44.5°C, with a transition enthalpy of 15.0 kcal/mol. The transition temperature (Tt) shifts to ca. 50.5°C in the presence of 1 mM EDTA or when the concentration is increased significantly. The turbidity of aqueous dispersions of stearoylcarnitine was found to be considerably high at low temperatures, which decreases quite abruptly over a short temperature range, indicating that a transition occurs from a phase of large aggregates to one of much smaller aggregates, most likely micelles. The phase transition temperature was determined as 29.1°C at pH 3.0, which increased with increasing pH up to a value of 55.3°C at pH 8.6 and remains nearly constant thereafter up to pH 11.2. The pH dependence of CMC and Ttsuggest that the pKa of the carboxyl group of long chain acylcarnitines shifts to higher temperatures upon aggregation (micelles or bilayer membranes).

Redox-Coupled Dynamics and Folding in Cytochrome c

Cytochrome c functions as an electron carrier in the mitochondrial electron-transport chain using the Fe(II)-Fe(III) redox couple of a covalently attached heme prosthetic group, and it has served as a paradigm for both biological redox activity and protein folding. On the basis of a wide variety of biophysical techniques, it has been suggested that the protein is more flexible in the oxidized state than in the reduced state, which has led to speculation that it is the dynamics of the protein that has been evolved to control the cofactor's redox properties. To test this hypothesis, we incorporated carbon-deuterium bonds throughout cytochrome c and characterized their absorption frequencies and line widths using IR spectroscopy. The absorption frequencies of several residues on the proximal side of the heme show redox-dependent changes, but none show changes in line width, implying that the flexibility of the oxidized and reduced proteins is not different. However, the spectra demonstrate that folded protein is in equilibrium with a surprisingly large amount of locally unfolded protein, which increases with oxidation for residues localized to the proximal side of the heme. The data suggest that while the oxidized protein is not more flexible than the reduced protein, it is more locally unfolded. Local unfolding of cytochrome c might be one mechanism whereby the protein evolved to control electron transfer.

Detecting protein-protein interactions by isotope-edited infrared spectroscopy: a numerical approach

We present a theoretical and numerical analysis of the vibrational coupling between isotope-edited amino acids in protein dimers. Depending on the presence and magnitude of coupling between 13Calpha=O peptide bond oscillators, characteristic level splittings of vibrational eigenstates are predicted. For the example of the Gramicidin A ion channel polypeptide, we observe typical IR fingerprints for the head-to-head and the antiparallel double-helical conformation of the dimer. We suggest that these findings can be used to clearly identify the structure of polypeptide aggregates using a particularly simple isotope substitution pattern.

Interactions of acyl carnitines with model membranes: a (13)C-NMR study

Esterification of fatty acids with the small polar molecule carnitine is a required step for the regulated flow of fatty acids into mitochondrial inner matrix. We have studied the interactions of acyl carnitines (ACs) with model membranes [egg yolk phosphatidylcholine (PC) vesicles] by (13)C-nuclear magnetic resonance (NMR) spectroscopy. Using AC with (13)C-enrichment of the carbonyl carbon of the acyl chain, we detected NMR signals from AC on the inside and outside leaflets of the bilayer of small unilamellar vesicles prepared by cosonication of PC and AC. However, when AC was added to the outside of pre-formed PC vesicles, only the signal for AC bound to the outer leaflet was observed, even after hours at equilibrium. The extremely slow transmembrane diffusion ("flip-flop") is consistent with the zwitterionic nature of the carnitine head group and the known requirement of transport proteins for movement of ACs through the mitochondrial membrane. The partitioning of ACs (8-18 carbons) between water and PC vesicles was studied by monitoring the [(13)C]carbonyl chemical shift of ACs as a function of pH and concentration of vesicles. Significant partitioning into the water phase was detected for ACs with chain lengths of 12 carbons or less. The effect of ACs on the integrity of the bilayer was examined in vesicles with up to 25 mol% myristoyl carnitine; no gross disruption of the bilayer was observed. We hypothesize that the effects of high levels of long-chain AC (as found in ischemia or in certain diseases) on cell membranes result from molecular effects on membrane functions rather than from gross disruption of the lipid bilayer.

Spectroscopic techniques in the study of membrane solubilization, reconstitution and permeabilization by detergents

This review focuses on the use of spectroscopic techniques for the study of membrane solubilization, reconstitution, and permeabilization by detergents. Turbidity and light scattering, visible and infrared spectroscopic methods, fluorescence, nuclear magnetic resonance, electron spin resonance and X-ray diffraction are examined from the point of view of their applicability to the above detergent-mediated phenomena. A short introduction is provided about each of the techniques, and references are given for further study.

Ceramides in phospholipid membranes: effects on bilayer stability and transition to nonlamellar phases

The effects of ceramides of natural origin on the gel-fluid and lamellar-inverted hexagonal phase transitions of phospholipids (mainly dielaidoylphosphatidylethanolamine) have been studied by differential scanning calorimetry, with additional support from infrared and 31P nuclear magnetic resonance (NMR) spectroscopy. In the lamellar phase, ceramides do not mix ideally with phospholipids, giving rise to the coexistence of domains that undergo the gel-fluid transition at different temperatures. The combination of differential scanning calorimetry and infrared spectroscopy, together with the use of deuterated lipids, allows the demonstration of independent melting temperatures for phospholipid and ceramide in the mixtures. In the lamellar-hexagonal phase transitions, ceramides (up to 15 mol %) decrease the transition temperature, without significantly modifying the transition enthalpy, thus facilitating the inverted hexagonal phase formation. 31P-NMR indicates the coexistence, within a certain range of temperatures, of lamellar and hexagonal phases, or hexagonal phase precursors. Ceramides from egg or from bovine brain are very similar in their effects on the lamellar-hexagonal transition. They are also comparable to diacylglycerides in this respect, although ceramides are less potent. These results are relevant in the interpretation of certain forms of interfacial enzyme activation and in the regulation and dynamics of the bilayer structure of cell membranes.

Role of long-chain fatty acyl-CoA esters in the regulation of metabolism and in cell signalling

The intracellular concentration of free unbound acyl-CoA esters is tightly controlled by feedback inhibition of the acyl-CoA synthetase and is buffered by specific acyl-CoA binding proteins. Excessive increases in the concentration are expected to be prevented by conversion into acylcarnitines or by hydrolysis by acyl-CoA hydrolases. Under normal physiological conditions the free cytosolic concentration of acyl-CoA esters will be in the low nanomolar range, and it is unlikely to exceed 200 nM under the most extreme conditions. The fact that acetyl-CoA carboxylase is active during fatty acid synthesis (Ki for acyl-CoA is 5 nM) indicates strongly that the free cytosolic acyl-CoA concentration is below 5 nM under these conditions. Only a limited number of the reported experiments on the effects of acyl-CoA on cellular functions and enzymes have been carried out at low physiological concentrations in the presence of the appropriate acyl-CoA-buffering binding proteins. Re-evaluation of many of the reported effects is therefore urgently required. However, the observations that the ryanodine-senstitive Ca2+-release channel is regulated by long-chain acyl-CoA esters in the presence of a molar excess of acyl-CoA binding protein and that acetyl-CoA carboxylase, the AMP kinase kinase and the Escherichia coli transcription factor FadR are affected by low nanomolar concentrations of acyl-CoA indicate that long-chain acyl-CoA esters can act as regulatory molecules in vivo. This view is further supported by the observation that fatty acids do not repress expression of acetyl-CoA carboxylase or Delta9-desaturase in yeast deficient in acyl-CoA synthetase.

Palmitoylcarnitine, a surface-active metabolite

Palmitoylcarnitine is a well-known intermediate in mitochondrial fatty acid oxidation. Less known are its properties as a surfactant, with a capacity to solubilize biological membranes similar to that of many synthetic detergents used in the biochemical laboratory. Some of the physico-chemical properties of palmitoylcarnitine may help to explain the need for coenzyme A-carnitine-coenzyme A acyl exchange during mitochondrial fatty acid import. The amphiphilic character of palmitoylcarnitine may also explain its proposed involvement in the pathogenesis of myocardial ischemia.

Effect of long-chain acyl-CoAs and acylcarnitines on gel-fluid and lamellar-hexagonal phospholipid phase transitions

The effect of a series of long-chain acyl-CoAs and acylcarnitines has been tested by differential scanning calorimetry on the gel-fluid transition of saturated phosphatidylcholines and of dietai-doylphosphatidylethanolamine (DEPE), and on the lamellar-Hexagonal transition of DEPE. Both series of acylderivatives have similar effects (the acylcarnitines being more potent): a decrease in the gel-fluid transition enthalpy and an increase in the gel-fluid transition width. Mixtures of dipalmitoylphosphatidylcholine with palmitoyl-CoA or palmitoylcarnitine (i.e. when all three hydrocarbon chains are 16C in length) display a peculiar behaviour, in that the main endotherm remains unchanged until a high proportion of palmitoylderivative is present, then it collapses suddenly. The disappearance of the gel-fluid main endotherm in the presence of palmitoylcarnitine is due to the fragmentation of the bilayer below the cooperative unit size of the phospholipid, while the same effect is caused by palmitoyl-CoA through the interaction of the coenzyme A polar moiety with the lipid-water interface, the overall bilayer structure being maintained. The effect of both series of compounds on the lamellar to inverted hexagonal phase transition of DEPE is also similar: they both stabilize the lamellar phase, increase the transition temperature and smear out the transition endotherm. Their behaviour may be rationalized considering that they are compounds with a bulky polar head, relative to their single hydrophobic chain, that would favour a positive curvature in the monolayer, while the inverted hexagonal phase requires a negative curvature.