Lipid-protein interactions of reconstituted membrane-associated adenosinetriphosphatases. Use of a gel-filtration procedure to examine phospholipid-activity relationships

Current problems and future avenues in proteoliposome research

Membrane proteins (MPs) are the gatekeepers between different biological compartments separated by lipid bilayers. Being receptors, channels, transporters, or primary pumps, they fulfill a wide variety of cellular functions and their importance is reflected in the increasing number of drugs that target MPs. Functional studies of MPs within a native cellular context, however, is difficult due to the innate complexity of the densely packed membranes. Over the past decades, detergent-based extraction and purification of MPs and their reconstitution into lipid mimetic systems has been a very powerful tool to simplify the experimental system. In this review, we focus on proteoliposomes that have become an indispensable experimental system for enzymes with a vectorial function, including many of the here described energy transducing MPs. We first address long standing questions on the difficulty of successful reconstitution and controlled orientation of MPs into liposomes. A special emphasis is given on coreconstitution of several MPs into the same bilayer. Second, we discuss recent progress in the development of fluorescent dyes that offer sensitive detection with high temporal resolution. Finally, we briefly cover the use of giant unilamellar vesicles for the investigation of complex enzymatic cascades, a very promising experimental tool considering our increasing knowledge of the interplay of different cellular components.

Disruption of oxidative phosphorylation and synaptic Na(+), K(+)-ATPase activity by pristanic acid in cerebellum of young rats

AIMS

Peroxisomal biogenesis disorders (PBD) are inherited disorders clinically manifested by neurological symptoms and brain abnormalities, in which the cerebellum is usually involved. Biochemically, patients affected by these neurodegenerative diseases accumulate branched-chain fatty acids, including pristanic acid (Prist) in the brain and other tissues.

MAIN METHODS

In the present investigation we studied the in vitro influence of Prist, at doses found in PBD, on oxidative phosphorylation, by measuring the activities of the respiratory chain complexes I-IV and ATP production, as well as on creatine kinase and synaptic Na(+), K(+)-ATPase activities in rat cerebellum.

KEY FINDINGS

Prist significantly decreased complexes I-III (65%), II (40%) and especially II-III (90%) activities, without altering the activities of complex IV of the respiratory chain and creatine kinase. Furthermore, ATP formation and synaptic Na(+), K(+)-ATPase activity were markedly inhibited (80-90%) by Prist. We also observed that this fatty acid altered mitochondrial and synaptic membrane fluidity that may have contributed to its inhibitory effects on the activities of the respiratory chain complexes and Na(+), K(+)-ATPase.

SIGNIFICANCE

Considering the importance of oxidative phosphorylation for mitochondrial homeostasis and of Na(+), K(+)-ATPase for the maintenance of cell membrane potential, the present data indicate that Prist compromises brain bioenergetics and neurotransmission in cerebellum. We postulate that these pathomechanisms may contribute to the cerebellar alterations observed in patients affected by PBD in which Prist is accumulated.

Marked inhibition of Na+, K(+)- ATPase activity and the respiratory chain by phytanic acid in cerebellum from young rats: possible underlying mechanisms of cerebellar ataxia in Refsum disease

Refsum disease is an autosomal recessive disorder of peroxisomal metabolism biochemically characterized by highly elevated concentrations of phytanic acid (Phyt) in a variety of tissues including the cerebellum. Reduction of plasma Phyt levels by dietary restriction intake ameliorates ataxia, a common clinical manifestation of this disorder, suggesting a neurotoxic role for this branched-chain fatty acid. Therefore, considering that the underlying mechanisms of cerebellum damage in Refsum disease are poorly known, in the present study we tested the effects of Phyt on important parameters of bioenergetics, such as the activities of the respiratory chain complexes I to IV, creatine kinase and Na(+), K(+)- ATPase in cerebellum preparations from young rats. The activities of complexes I, II, I-III and II-III and Na(+), K(+)- ATPase were markedly inhibited (65-85%) in a dose-dependent manner by Phyt. In contrast, creatine kinase and complex IV activities were not altered by this fatty acid. Therefore, it is presumed that impairment of the electron flow through the respiratory chain and inhibition of Na(+), K(+)- ATPase that is crucial for synaptic function may be involved in the pathophysiology of the cerebellar abnormalities manifested as ataxia in Refsum disease and in other peroxisomal disorders in which brain Phyt accumulates.

Long-chain 3-hydroxy fatty acids accumulating in long-chain 3-hydroxyacyl-CoA dehydrogenase and mitochondrial trifunctional protein deficiencies uncouple oxidative phosphorylation in heart mitochondria

Cardiomyopathy is a common clinical feature of some inherited disorders of mitochondrial fatty acid β-oxidation including mitochondrial trifunctional protein (MTP) and isolated long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) deficiencies. Since individuals affected by these disorders present tissue accumulation of various fatty acids, including long-chain 3-hydroxy fatty acids, in the present study we investigated the effect of 3-hydroxydecanoic (3 HDCA), 3-hydroxydodecanoic (3 HDDA), 3-hydroxytetradecanoic (3 HTA) and 3-hydroxypalmitic (3 HPA) acids on mitochondrial oxidative metabolism, estimated by oximetry, NAD(P)H content, hydrogen peroxide production, membrane potential (ΔΨ) and swelling in rat heart mitochondrial preparations. We observed that 3 HTA and 3 HPA increased resting respiration and diminished the respiratory control and ADP/O ratios using glutamate/malate or succinate as substrates. Furthermore, 3 HDDA, 3 HTA and 3 HPA decreased ΔΨ, the matrix NAD(P)H pool and hydrogen peroxide production. These data indicate that these fatty acids behave as uncouplers of oxidative phosphorylation. We also verified that 3 HTA-induced uncoupling-effect was not mediated by the adenine nucleotide translocator and that this fatty acid induced the mitochondrial permeability transition pore opening in calcium-loaded organelles since cyclosporin A prevented the reduction of mitochondrial ΔΨ and swelling provoked by 3 HTA. The present data indicate that major 3-hydroxylated fatty acids accumulating in MTP and LCHAD deficiencies behave as strong uncouplers of oxidative phosphorylation potentially impairing heart energy homeostasis.

Lipid bilayer composition affects transmembrane protein orientation and function

Sperm membranes change in structure and composition upon ejaculation to undergo capacitation, a molecular transformation which enables spermatozoa to undergo the acrosome reaction and be capable of fertilization. Changes to the membrane environment including lipid composition, specifically lipid microdomains, may be responsible for enabling capacitation. To study the effect of lipid environment on proteins, liposomes were created using lipids extracted from bull sperm membranes, with or without a protein (Na⁺ K⁺-ATPase or α-amylase). Protein incorporation, function, and orientation were determined. Fluorescence resonance energy transfer (FRET) confirmed protein inclusion in the lipid bilayer, and protein function was confirmed using a colourometric assay of phosphate production from ATP cleavage. In the native lipid liposomes, ATPase was oriented with the β subunit facing the outer leaflet, while changing the lipid composition to 50% native lipids and 50% exogenous lipids significantly altered this orientation of Na⁺ K⁺-ATPase within the membranes.

Neurochemical evidence that lysine inhibits synaptic Na+,K+-ATPase activity and provokes oxidative damage in striatum of young rats in vivo

Lysine (Lys) accumulation in tissues and biological fluids is the biochemical hallmark of patients affected by familial hyperlysinemia (FH) and other inherited metabolic disorders. In the present study we investigated the effects of acute administration of Lys on relevant parameters of energy metabolism and oxidative stress in striatum of young rats. We verified that Lys in vivo intrastriatal injection did not change the citric acid cycle function and creatine kinase activity, but, in contrast, significantly inhibited synaptic Na(+),K(+)-ATPase activity in striatum prepared 2 and 12 h after injection. Moreover, Lys induced lipid peroxidation and diminished the concentrations of glutathione 2 h after injection. These effects were prevented by the antioxidant scavengers melatonin and the combination of α-tocopherol and ascorbic acid. Lys also inhibited glutathione peroxidase activity 12 h after injection. Therefore it is assumed that inhibition of synaptic Na(+),K(+)-ATPase and oxidative damage caused by brain Lys accumulation may possibly contribute to the neurological manifestations of FH and other neurometabolic conditions with high concentrations of this amino acid.

cis-4-decenoic acid provokes mitochondrial bioenergetic dysfunction in rat brain

AIMS

In the present work we investigated the in vitro effect of cis-4-decenoic acid, the pathognomonic metabolite of medium-chain acyl-CoA dehydrogenase deficiency, on various parameters of bioenergetic homeostasis in rat brain mitochondria.

MAIN METHODS

Respiratory parameters determined by oxygen consumption were evaluated, as well as membrane potential, NAD(P)H content, swelling and cytochrome c release in mitochondrial preparations from rat brain, using glutamate plus malate or succinate as substrates. The activities of citric acid cycle enzymes were also assessed.

KEY FINDINGS

cis-4-decenoic acid markedly increased state 4 respiration, whereas state 3 respiration and the respiratory control ratio were decreased. The ADP/O ratio, the mitochondrial membrane potential, the matrix NAD(P)H levels and aconitase activity were also diminished by cis-4-decenoic acid. These data indicate that this fatty acid acts as an uncoupler of oxidative phosphorylation and as a metabolic inhibitor. cis-4-decenoic acid also provoked a marked mitochondrial swelling when either KCl or sucrose was used in the incubation medium and also induced cytochrome c release from mitochondria, suggesting a non-selective permeabilization of the inner mitochondrial membrane.

SIGNIFICANCE

It is therefore presumed that impairment of mitochondrial homeostasis provoked by cis-4-decenoic acid may be involved in the brain dysfunction observed in medium-chain acyl-CoA dehydrogenase deficient patients.

Disturbance of mitochondrial energy homeostasis caused by the metabolites accumulating in LCHAD and MTP deficiencies in rat brain

AIMS

We investigated the in vitro effects of 3-hydroxydodecanoic (3HDA), 3-hydroxytetradecanoic (3HTA) and 3-hydroxypalmitic (3HPA) acids, which accumulate in tissues of patients affected by mitochondrial trifunctional protein (MTP) and isolated long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) deficiencies, on various parameters of energy homeostasis in mitochondrial preparations from brain of young rats.

MAIN METHODS

We measured the respiratory parameters state 4, state 3, respiratory control ratio (RCR) and ADP/O ratio by the rate of oxygen consumption, as well as the mitochondrial membrane potential and the matrix NAD(P)H levels in the presence of the fatty acids.

KEY FINDINGS

We found that 3HDA, 3HTA and 3HPA markedly increased state 4 respiration and diminished the RCR using glutamate plus malate or succinate as substrates. 3HTA and 3HPA also diminished the mitochondrial membrane potential and the matrix NAD(P)H levels. In addition, 3HTA decreased state 3 respiration using glutamate/malate, but not pyruvate/malate or succinate as substrates. Our data indicate that the long-chain 3-hydroxy fatty acids that accumulate in LCHAD/MTP deficiencies act as uncouplers of oxidative phosphorylation, while 3HTA also behaves as a metabolic inhibitor.

SIGNIFICANCE

It is presumed that impairment of brain energy homeostasis caused by these endogenous accumulating compounds may contribute at least in part to the neuropathology of LCHAD/MTP deficiencies.

Close encounters of the oily kind: regulation of transporters by lipids

Neurotransmitter transporters are membrane proteins that serve as key regulators of extracellular neurotransmitter concentrations and have been long viewed as important targets for drug development by the pharmaceutical industry. Although many cellular signaling systems are known to modulate transport activity, much less is known about how transporters communicate with and are regulated by the various components of the lipid sea in which they reside. Variations in lipid content clearly affect the activity of a variety of transport systems, and with advances in techniques for lipid analysis and a clearer vision of carrier structure, this area of research appears poised for major advances.

Evidence that the major metabolites accumulating in medium-chain acyl-CoA dehydrogenase deficiency disturb mitochondrial energy homeostasis in rat brain

Medium-chain acyl-CoA dehydrogenase deficiency (MCADD) is an inherited metabolic disorder of fatty acid oxidation in which the affected patients predominantly present high levels of octanoic (OA) and decanoic (DA) acids and their glycine and carnitine by-products in tissues and body fluids. It is clinically characterized by episodic encephalopathic crises with coma and seizures, as well as by progressive neurological involvement, whose pathophysiology is poorly known. In the present work, we investigated the in vitro effects of OA and DA on various parameters of energy homeostasis in mitochondrial preparations from brain of young rats. We found that OA and DA markedly increased state 4 respiration and diminished state 3 respiration as well as the respiratory control ratio, the mitochondrial membrane potential and the matrix NAD(P)H levels. In addition, DA-elicited increase in oxygen consumption in state 4 respiration was partially prevented by atractyloside, indicating the involvement of the adenine nucleotide translocator. OA and DA also reduced ADP/O ratio, CCCP-stimulated respiration and the activities of respiratory chain complexes. The data indicate that the major accumulating fatty acids in MCADD act as uncouplers of oxidative phosphorylation and as metabolic inhibitors. Furthermore, DA, but not OA, provoked a marked mitochondrial swelling and cytochrome c release from mitochondria, reflecting a permeabilization of the inner mitochondrial membrane. Taken together, these data suggest that OA and DA impair brain mitochondrial energy homeostasis that could underlie at least in part the neuropathology of MCADD.

Membrane basis for fish oil effects on the heart: linking natural hibernators to prevention of human sudden cardiac death

The concept that diet-induced changes in membrane lipids could modify heart function partly arose from observations that membrane composition and physical properties were closely associated with the capacity of the heart to respond appropriately to torpor and hibernation. Observations of natural hibernators further revealed that behavior of key membrane-bound enzymes could be influenced through the lipid composition of the cell membrane, either by changing the surrounding fatty acids through reconstitution into a foreign lipid milieu of different composition, or by alteration through diet. Myocardial responsiveness to beta-adrenoceptor stimulation, including initiation of spontaneous dysrhythmic contractions, was altered by both hibernation and dietary modulation of membrane fatty acids, suggesting modified vulnerability to cardiac arrhythmia. Subsequent studies using whole-animal models recognized that vulnerability to ventricular fibrillation decreased as the polyunsaturated: saturated fat (P:S) ratio of the diet increased. However, dietary fish oils, which typically contain at least 30% saturated fatty acids and only 30% long-chain n-3 (omega-3) polyunsaturated fatty acids (PUFA), exhibit antiarrhythmic effects that exceed the predicted influence of the P:S ratio, suggesting properties unique to the long-chain n-3 PUFA. Large-scale clinical trials and epidemiology have confirmed the arrhythmia prevention observed in vitro in myocytes, papillary muscles, and isolated hearts and in whole-animal models of sudden cardiac death. Some progress has been made towards a biologically plausible mechanism. These developments highlight nature's ability to provide guidance for the most unexpected applications.

Na,K-ATPase reconstituted in liposomes: effects of lipid composition on hydrolytic activity and enzyme orientation

In this paper, the reconstitution of Na,K-ATPase in liposomes (formed by single or mixed phospholipids and cholesterol) was investigated and the enzyme orientation was determined on kinetic basis using only specific inhibitors of ATP hydrolysis. A condition of foremost importance for enzyme reconstitution is the achievement of complete solubilization of the lipid in the initial stage of the cosolubilization process for the subsequent formation of the liposomes and/or proteoliposomes. PC-liposomes showed that increasing the fatty acid chain length increases the percentage of Na,K-ATPase incorporated. The average diameter of the proteoliposomes also increases in proportion, reaching a maximum with phospholipids with 16 carbon chains, resulting in 75.1% protein reconstitution and 319.4 nm diameter size, respectively. Binary lipid systems with PC and PE were efficient for incorporation of Na,K-ATPase, depending on the lipid:protein ratio used, varying from 15 to 80% recovery of total ATPase activity. The best results for Na,K-ATPase reconstitution using PC and PE mixture were obtained using a lipid:lipid ratio 1:1 (w/w) and lipid:protein 1:3 (w/w). Integrity studies using calcein release mediated by detergent or alamethicin, in association with inhibition of ATPase activity (ouabain and vanadate) showed that the enzyme is oriented inside-out in DPPC:DPPE proteoliposomes. In these vesicular systems, the enzyme is reconstituted with about 78.9% ATPase activity recovery and 89% protein incorporation, with an average diameter of 140 nm. Systems constituted by DPPC:DPPE, DPPC:DLOPE or DLOPC:DLOPE showed approximately 80, 71 and 70% of recovery of total ATPase activity, but no homogeneity in the distribution of Na,K-ATPase orientation. Reconstitution of Na,K-ATPase in DPPC:DPPE:cholesterol or DPPC:DLOPE:cholesterol systems (55% of cholesterol) showed recovery of about 86 and 82%, respectively, of its total ATPase activity. The results point to an important effect of the lipid acyl chain length and lipid-protein ratio in relation to the composition of the lipid matrix to finely tune the structural asymmetry and the amount of enzyme that can be incorporated a lipid bilayer vesicle while preserving membrane permeability.

Evidence that antioxidants prevent the inhibition of Na+,K(+)-ATPase activity induced by octanoic acid in rat cerebral cortex in vitro

The objective of the present study was to investigate the in vitro effects of octanoic acid, which accumulates in medium-chain acyl-CoA dehydrogenase (MCAD) deficiency and in Reye syndrome, on key enzyme activities of energy metabolism in the cerebral cortex of young rats. The activities of the respiratory chain complexes I-IV, creatine kinase, and Na+,K(+)-ATPase were evaluated. Octanoic acid did not alter the electron transport chain and creatine kinase activities, but, in contrast, significantly inhibited Na+,K(+)-ATPase activity both in synaptic plasma membranes and in homogenates prepared from cerebral cortex. Furthermore, decanoic acid, which is also increased in MCAD deficiency, and oleic acid strongly reduced Na+,K(+)-ATPase activity, whereas palmitic acid had no effect. We also examined the effects of incubating glutathione and trolox (alpha-tocopherol) alone or with octanoic acid on Na+,K(+)-ATPase activity. Tested compounds did not affect Na+,K(+)-ATPase activity by itself, but prevented the inhibitory effect of octanoic acid. These results suggest that inhibition of Na+,K(+)-ATPase activity by octanoic acid is possibly mediated by oxidation of essential groups of the enzyme. Considering that Na+,K(+)-ATPase is critical for normal brain function, it is feasible that the significant inhibition of this enzyme activity by octanoate and also by decanoate may be related to the neurological dysfunction found in patients affected by MCAD deficiency and Reye syndrome.

Modulation of Na,K-ATPase and Na-ATPase activity by phospholipids and cholesterol. I. Steady-state kinetics

The effects of phospholipid acyl chain length (n(c)), degree of acyl chain saturation, and cholesterol on Na,K-ATPase reconstituted into liposomes of defined lipid composition are described. The optimal acyl chain length of monounsaturated phosphatidylcholine in the absence of cholesterol was found to be 22 but decreased to 18 in the presence of 40 mol % cholesterol. This indicates that the hydrophobic matching of the lipid bilayer and the transmembrane hydrophobic core of the membrane protein is a crucial parameter in supporting optimal Na,K-ATPase activity. In addition, the increased bilayer order induced by both cholesterol and saturated phospholipids could be important for the conformational mobility of the Na,K-ATPase changing the distribution of conformations. Lipid fluidity was important for several parameters of reconstitution, e.g., the amount of protein inserted and the orientation in the liposomes. The temperature dependence of the Na,K-ATPase as well of the Na-ATPase reactions depends both on phospholipid acyl chain length and on cholesterol. Cholesterol increased significantly both the enthalpy of activation and entropy of activation for Na,K-ATPase activity and Na-ATPase activity of Na,K-ATPase reconstituted with monounsaturated phospholipids. In the presence of cholesterol the free energy of activation was minimum at a lipid acyl chain length of 18, the same that supported maximum turnover. In the case of ATPase reconstituted without cholesterol, the minimum free energy of activation and the maximum turnover both shifted to longer acyl chain lengths of about 22.

Binding of ethylenediamine to phosphatidylserine is inhibitory to Na+/K(+)-ATPase

Covalent linkage of ethylenediamine with the Na+/K(+)-ATPase complex from rabbit kidney outer medulla by the use of the water-soluble carbodiimide, N-ethyl,N'-(3-dimethylaminopropyl)carbodiimide, resulted in a 73% reaction with phosphatidylserine and only 27% with carboxylic groups in the proteic component of the enzyme. Condensation products from the reaction between phosphatidylserine and ethylenediamine, N-(O-phosphatidylseryl)ethylenediamine, N,N'-bis(O-phosphatidylseryl)ethylenediamine and its intermediary product O-phosphatidyl-[N,N'-bis(seryl)]ethylenediamine, were synthesised. Symmetrically substituted ethylenediamine was the most likely condensation product of ethylenediamine with endogenous phosphatidylserine. The synthesised lipids were incorporated in proteoliposomes containing Na+/K(+)-ATPase and only the addition of the phospholipid phosphatidylcholine. The ratio of phospholipid to protein was 52 (w/w). These proteoliposomes were perforated by the addition of 0.5% cholate and both the Na(+)-dependent phosphorylation level and its dependence on Na+, Mg2+ and ATP were measured. Phosphatidylcholine alone increased the half-maximal activation concentration for Na+ ([Na+]0.5) from 0.2 to 1-2 mM, for Mg2+ from 0.1 to 0.8 microM and for ATP from 0.02 to 0.3 microM. The Ki for K+ (in the absence of Na+) was unaffected: 12.8 microM vs. 12.5 microM in the non-reconstituted system. Replacing 10 mol% of phosphatidylcholine by phosphatidylethanolamine: or phosphatidylserine had no significant effect on [Na+]0.5: 1.1 and 0.7 mM, respectively. Replacing 5 mol% phosphatidylcholine by the bis(phosphatidylseryl) substituent of ethylenediamine further increased [Na+]0.5 to 13.7 mM, while half-maximal activation concentrations for Mg2+ and ATP were unaltered. The mono-phosphatidylseryl derivatives of ethylenediamine, each 5 mol%, also increased [Na+]0.5, but to a lesser extent (3.2-3.8 mM). In addition to their competitive effects, the phosphatidylseryl-substituted ethylenediamine compounds exerted a slowly-increasing non-competitive inhibition, not only in phosphorylation, but also in overall ATPase activity, which was reduced, although not abolished, by exogenous protein (bovine serum albumin). A detergent-like action in the usual sense is unlikely since liposomes containing these lipids remained intact. These studies prove that phospholipids are not only required for optimal activity of this transport enzyme, but in excess or in compositions deviating from the normal, may also be inhibitory.

Modulation of membrane protein function by bilayer lipids

In many instances, the composition of fatty acyl groups of membrane phospholipids can be modified to achieve a range of fatty acyl unsaturation without any detectable change in bulk membrane fluidity. At the same time, the function of membrane proteins may be considerably altered, raising questions concerning the property of the lipids that brings about this altered protein function. There is some evidence that the lipids may be laterally distributed in a heterogeneous manner throughout the membrane, and changes in this distribution could be responsible for the effects on proteins. There is also increasing evidence for specific interactions between individual molecular species and membrane proteins that may also modulate membrane protein function.

Current concepts in membrane protein reconstitution

The reconstitution of integral proteins into artificial lipid vesicles is largely prompted by the complexity of most biological membranes and the inherent difficulty of studying individual components in situ. Ideally, therefore, the reconstituted system should consist of a single protein in a lipid matrix which mimics the native membrane in all but its diversity. While such an approach allows individual components of a complex system to be studied in isolation it should also be sufficiently versatile to permit the generation of increasingly sophisticated multicomponent models. From the considerable number of reconstitution techniques which have been developed I have tried in this review to identify those characteristics of a particular system which maximise both the information it can provide and its versatility.

Effects of the ATP, ADP and inorganic phosphate on the transport rate of the Na+,K+-pump

(Na+ + K+)-ATPase from kidney outer medulla was incorporated into artificial dioleoylphosphatidylcholine vesicles. In the reconstituted system the pump can be activated by adding ATP to the external medium. ATP-driven potassium extrusion by the Na+,K+-pump was studied using a voltage-sensitive dye in the presence of valinomycin. ADP strongly reduced the turnover rate of the pump with a concentration for half-maximal inhibition of cD,1/2 = 0.1 mM. cD,1/2 was found to be virtually independent of ATP concentration, indicating that the inhibition is non-competitive with respect to ATP. The non-competitive inhibition by ADP can be explained on the basis of the Post-Albers reaction cycle of the Na+,K+-pump, assuming that the main action of ADP is the reversal of the phosphorylation step. A similar 'product inhibition' was observed with inorganic phosphate, but at much higher concentrations (cP,1/2 = 14 mM).

(Na+ + K+)-ATPase in artificial lipid vesicles: influence of lipid structure on pumping rate

(Na+ + K+)-ATPase from kidney outer medulla was incorporated into tightly-sealed, single-shelled lipid vesicles by a detergent-dialysis procedure. The rate of ATP-driven potassium extrusion from vesicles formed from different phosphatidylcholines (PC) was measured optically, using a voltage-sensitive dye in the presence of valinomycin. High transport rates were observed for di(18:1)PC, di(20:1)PC and di(22:1)PC, whereas vesicles formed from di(14:1)PC and di(16:1)PC were virtually inactive. The variation of pumping activity with lipid structure mainly results from differences in the amount of enzyme incorporated with the correct orientation into the vesicle membrane, and to a lesser extent from lipid-dependent variations of the intrinsic turnover rate of the enzyme. The activation energy of ion transport decreases in the order di(16:1)PC, di(18:1)PC, di(20:1)PC approximately equal to di(22:1)PC.

Optical study of active ion transport in lipid vesicles containing reconstituted Na,K-ATPase

A fluorescence method is described for the measurement of ATP-driven ion fluxes in lipid vesicles containing purified Na,K-ATPase. The membrane voltage of enzyme containing vesicles was measured by using a voltage-sensitive indocyanine dye. By addition of valinomycin the vesicle membrane is made selectively permeable to K+ so that the membrane voltage approaches the Nernst potential for K+. With constant external K+ concentration, the time course of internal K+ concentration can be continuously measured as change of the fluorescence signal after activation of the pump. The optical method has a higher time resolution than tracer-flux experiments and allows an accurate determination of initial flux rates. From the temperature dependence of active K+ transport its activation energy was determined to be 115 kJ/mol. ATP-stimulated electrogenic pumping can be measured as fast fluorescence change when the membrane conductance is low (i.e., at low or zero valinomycin concentration). In accordance with expectation, the amplitude of the fast signal change increases with decreasing passive ion permeability of the vesicle membrane. The resolution of the charge movement is so high that a few pump turnovers can be easily detected.

Response of rat heart membranes and associated ion-transporting ATPases to dietary lipid

The effects of different dietary fat intake on the lipid composition and enzyme behaviour of sarcolemmal (Na+ + K+)ATPase and sarcoplasmic reticulum Ca2+-ATPase from rat heart were investigated. Rat diets were supplemented with either sunflower seed oil (unsatd./satd. 5.6) or sheep kidney fat (unsatd./satd. 0.8). Significant changes in the phospholipid fatty acid composition were observed in both membranes after 9 weeks dietary lipid treatment. For both membranes, the total saturated/unsaturated fatty acid levels were unaffected by the dietary lipid treatment, however the proportions of the major unsaturated fatty acids were altered. Animals fed the sunflower seed oil diet exhibited an increase in n-6 fatty acids, including linoleic (18:2(n-6] and arachidonic (20:4(n-6] while the sheep kidney fat dietary rats were higher in n-3 fatty acids, principally docosahexaenoic (22:6), with the net result being a higher n-6/n-3 ratio in the sunflower seed oil group compared to sheep kidney fat dietary animals. Fluorescence polarization indicated that the fluidity of sarcoplasmic reticular membrane was greater than that of sarcolemmal membrane, with a dietary lipid-induced decrease in fluidity being observed in the sarcoplasmic reticular membrane from sheep kidney fat dietary animals. Despite these significant changes in membrane composition and physical properties, neither the specific activity nor the temperature-activity relationship (Arrhenius profile) of the associated ATPases were altered. These results suggest that with regard to the parameters measured in this study, the two ion-transporting ATPases are not modulated by changes which occur in the membrane lipid composition as a result of the diet.

Modulation of cardiac glycoside inhibition of (Na+ + K+)-ATPase by membrane lipids. Difference between species

The role of lipids in the modulation of the ouabain-sensitivity of membrane (Na+ + K+)-ATPase from different species has been studied using a reconstitution procedure which promotes lipid exchange during detergent depletion by Sephadex chromatography. Hybrid reconstitution of delipidated (Na+ + K+)-ATPase preparations from bovine brain into the lipids obtained from crab nerve enzyme preparations significantly reduces the sensitivity of the brain enzyme to inhibition by ouabain. Conversely, reconstitution of crab nerve enzyme into the lipids from bovine brain enzyme preparations increases the sensitivity of the crab enzyme to ouabain inhibition. These opposing effects demonstrate the role of membrane lipids in modulating the enzyme-inhibition relationship in preparations from these different species.