Lipid-protein interactions and the function of the Ca2+-ATPase of sarcoplasmic reticulum

Nonthermal plasma-activated water: A comprehensive review of this new tool for enhanced food safety and quality

Nonthermal plasma (NTP) is an advanced technology that has gained extensive attention because of its capacity for decontaminating food from both biological and chemical sources. Plasma-activated water (PAW), a product of NTP's reaction with water containing a rich diversity of highly reactive oxygen species (ROS) and reactive nitrogen species (RNS), is now being considered as the primary reactive chemical component in food decontamination. Despite exciting developments in this field recently, at present there is no comprehensive review specifically focusing on the comprehensive effects of PAW on food safety and quality. Although PAW applications in biological decontamination have been extensively evaluated, a complete analysis of the most recent developments in PAW technology (e.g., PAW combined with other treatments, and PAW applications in chemical degradation and as curing agents) is nevertheless lacking. Therefore, this review focuses on PAW applications for enhanced food safety (both biological and chemical safeties) according to the latest studies. Further, the subsequent effects on food quality (chemical, physical, and sensory properties) are discussed in detail. In addition, several recent trends of PAW developments, such as curing agents, thawing media, preservation of aquatic products, and the synergistic effects of PAW in combination with other traditional treatments, are also presented. Finally, this review outlines several limitations presented by PAW treatment, suggesting several future research directions and challenges that may hinder the translation of these technologies into real-life applications.

Memory Improvement Effect of Ethanol Garlic (A. sativum) Extract in Streptozotocin-Nicotinamide Induced Diabetic Wistar Rats Is Mediated through Increasing of Hippocampal Sodium-Potassium ATPase, Glutamine Synthetase, and Calcium ATPase Activities

Studies suggest that garlic (A. sativum) improves memory dependent on the hippocampus. However, the effect of ethanol garlic extract on hippocampus Na⁺/K⁺ ATPase, Ca²⁺ ATPase, and glutamine synthetase (GS) activities as possible mechanisms in memory improvement in diabetic Wistar rats has not been reported. Twenty-four male Wistar rats weighing 200-250 g were divided into three groups with 8 rats each. Group (A), normal control rats, and Group (B), diabetic rats, received 1 ml of normal saline; diabetic rats in Group (C) received 1000 mg/kg of garlic extract orally for 21 days. Hyperglycemia was induced by a single intraperitoneal injection of streptozotocin 60 mg/kg followed by 120 mg/kg nicotinamide while extraction of garlic was done by cold maceration method. Memory was tested in all groups. After that, the rats were sacrificed, the brain was removed, and the hippocampi were carefully excised and then homogenized. Activities of Na⁺/K⁺ ATPase, calcium ATPase, and GS were analyzed from the homogenate. Results showed improvement in memory and a significant increase (P < 0.05) in hippocampus Na⁺/K⁺ ATPase, Ca²⁺ ATPase, and GS activities in diabetic rats treated with garlic extract. In conclusion, the increased activity of hippocampus Na⁺/K⁺ ATPase, calcium ATPase, and glutamine synthetase may account for the memory improvement.

High affinity (3)H-phenylalanine uptake by brush border membrane vesicles from whole larvae of Aedes aegypti (AaBBMVw)

Brush border membrane vesicles from whole Aedes aegypti larvae (AaBBMVw) are confirmed to be valid preparations for membrane transport studies. The Abdul-Rauf and Ellar method was used to isolate AaBBMVw that were frozen, stored for several months, transported to a distant site, thawed and used to study Na(+)-coupled, (3)H-labeled, phenylalanine (Phe) uptake. The affinity for all components of the uptake was very high with half maximal values in the sub-micromolar range. By contrast a K(0.5)(Phe) of 0.2mM and a K(0.5)(Na) of 26 mM were calculated from Phe-induced electrical currents in Xenopus oocytes that were heterologously expressing the Anopheles gambiae symporter (co-transporter), AgNAT8, in a buffer with 98 mM Na(+). What accounts for the >1000-fold discrepancy in affinity for substrates between the BBMV and oocyte experiments? Is it because Ae. aegypti were used to isolate BBMVw whereas An. gambiae were used to transfect oocytes? More likely, it is because BBMVw were exposed to [Na(+)] in the micromolar range with the transporter(s) being surrounded by native lipids. By contrast, the oocyte measurements were made at [Na(+)] 100,000 times higher with AgNAT8 surrounded by foreign frog lipids. The results show that AaBBMVw are osmotically sealed; the time-course has a Na(+)-induced overshoot, the pH optimum is ∼7 and the K(0.5) values for Phe and Na(+) are very low. The transport is virtually unchanged when Na(+) is replaced by K(+) or Li(+) but decreased by Rb(+). This approach to resolving discrepancies between electrical data on solute transporters such as AgNAT8 that are over-expressed in oocytes and flux data on corresponding transporters that are highly expressed in native membrane vesicles, may serve as a model for similar studies that add membrane biochemistry to molecular biology in efforts to identify targets for the development of new methods to control disease-vector mosquitoes.

H(+) V-ATPase-energized transporters in brush border membrane vesicles from whole larvae of Aedes aegypti

Brush border membrane vesicles (BBMVs) from Whole larvae of Aedes aegypti (AeBBMVWs) contain an H(+) V-ATPase (V), a Na(+)/H(+) antiporter, NHA1 (A) and a Na(+)-coupled, nutrient amino acid transporter, NAT8 (N), VAN for short. All V-ATPase subunits are present in the Ae. aegypti genome and in the vesicles. AgNAT8 was cloned from Anopheles gambiae, localized in BBMs and characterized in Xenopus laevis oocytes. AgNHA1 was cloned and localized in BBMs but characterization in oocytes was compromised by an endogenous cation conductance. AeBBMVWs complement Xenopus oocytes for characterizing membrane proteins, can be energized by voltage from the V-ATPase and are in their natural lipid environment. BBMVs from caterpillars were used in radio-labeled solute uptake experiments but approximately 10,000 mosquito larvae are needed to equal 10 caterpillars. By contrast, functional AeBBMVWs can be prepared from 10,000 whole larvae in 4h. Na(+)-coupled (3H)phenylalanine uptake mediated by AeNAT8 in AeBBMVs can be compared to the Phe-induced inward Na(+) currents mediated by AgNAT8 in oocytes. Western blots and light micrographs of samples taken during AeBBMVW isolation are labeled with antibodies against all of the VAN components. The use of AeBBMVWs to study coupling between electrogenic V-ATPases and the electrophoretic transporters is discussed.

Purification of heterologous sarcoplasmic reticulum Ca2+-ATPase Serca1a allowing phosphoenzyme and Ca2+-affinity measurements

We describe here a protocol to prepare milligrams of active and stable heterologous sarcoplasmic reticulum Ca(2+)-ATPase (Serca1a). Serca1a was tagged with 6 histidines at its C-terminal end and overexpressed using the baculovirus-Sf9 system. In a first trial, Serca1a accounted for 24% of membrane proteins, 95% of which were inactive. Glucose in the culture medium reduced the production of Serca1a to 3 to 5% of membrane proteins and all Serca1a was active. Seventy-five percent of active Serca1a was solubilized by C(12)E(8) in the presence of phosphatidylcholine under conditions avoiding denaturation. Purification by Ni(2+)-nitrilo-triacetic acid affinity chromatography was tried, but only 3% of active Serca1a remained bound to the column, as if the His-tag were not accessible. Yields of 43% were reached by purification on reactive red 120 columns when eluting with 2 M NaCl. The purity was about 25% and Serca1a was stable for at least 1 week at 0 degrees C. Typically, 500 ml of culture medium produced 3 mg of active Serca1a and 1 mg of purified active Serca1a allowing measurements of phosphoenzyme (2 nmol/mg) or Ca(2+) affinity (2 microM at pH 7).

Changes in microsomal membrane phospholipids and fatty acids and in activities of membrane-bound enzyme in diabetic rat heart

Diabetes mellitus is associated with alterations in lipid metabolism and cardiac dysfunction despite an absence of coronary arteriosclerotic changes. To investigate mechanisms of cardiac dysfunction in diabetic cardiomyopathy, we studied the relation between activities of membrane-bound enzymes and surrounding phospholipids in rats with diabetes induced with a single intravenous injection of streptozotocin (65 mg/kg). We found that total phospholipid content of sarcoplasmic reticulum membrane increased significantly 8 weeks after treatment with streptozotocin owing to increases in phosphatidylcholine and phosphatidylethanolamine, a decrease in arachidonic acid, and an increase in docosahexaenoic acid in the early stage of diabetes. Sarcolemmal Na+/K(+)-ATPase activity and the number of receptors decreased in isolated cardiomyocytes of diabetic rats 8 weeks after streptozotocin administration. The Ca2+ uptake of both sarcoplasmic reticulum and mitochondria decreased simultaneously in permeabilized, isolated cardiomyocytes from diabetic rats. The depression of membrane-bound enzyme activities was correlated with alterations in phospholipids, which are closely related to the microenvironment of membrane-bound enzymes and influence intracellular Ca2+ metabolism. Because these changes in phospholipids and fatty acids were reversible with insulin therapy, they are diabetes-specific and might be a cause of cardiac dysfunction in diabetes.

Preservation of the native structure and function of Ca2+-ATPase from sarcoplasmic reticulum: solubilization and reconstitution by new short-chain phospholipid detergent 1,2-diheptanoyl-sn-phosphatidylcholine

The properties of Ca2+-ATPase purified and reconstituted from rabbit skeletal sarcoplasmic reticulum (SR) has been studied in comparison with the preparations obtained by the commonly used detergent poly(oxyethylene)8-lauryl ether (C12E8) and the bile salt detergents cholate and deoxycholate. 1,2-Diheptanoyl-sn-phosphatidylcholine (DHPC) has been shown to be excellent for solubilizing a wide variety of membrane proteins [Kessi, Poiree, Wehrli, Bachofen, Semenza and Hauser (1994) Biochemistry 33, 10825-10836]. The DHPC method consistently gave higher yields of purified Ca2+-ATPase with a greater specific activity than the methods with C12E8, cholate, or deoxycholate. DHPC and C12E8 were superior to cholate and deoxycholate in active enzyme yields and specific activity. DHPC-solubilized Ca2+-ATPase purified on a density gradient retained the E1Ca-E1(*)Ca conformational transition, whereas the enzyme from the C12E8 purification did not retain this transition. The coupling of Ca2+ transported to ATP hydrolysed in the DHPC-purified enzyme was maximal and matched the values obtained with native SR, whereas the coupling was much lower for the C12E8-purified enzyme. The specific activity of Ca2+-ATPase reconstituted into dioleoylphosphatidylcholine vesicles with DHPC was up to 2-fold greater than that achieved with C12E8, and is comparable to that measured in the native SR. Finally, the dissociation of Ca2+-ATPase into monomers by DHPC preserved the ATPase activity, whereas similar dissociation by C12E8 gave only one-sixth the activity of that obtained with DHPC. These studies show that the Ca2+-ATPase solubilized, purified and reconstituted with DHPC is superior to that obtained with C12E8 in significant ways, making it a preparation suitable for detailed studies on the mechanism of ion transport and the role of protein-lipid interactions in the function of membrane proteins.

The relationship between phospholipid content and Ca2+-ATPase activity in the sarcoplasmic reticulum

The relationship between the phospholipid composition of sarcoplasmic reticulum and the activity of the Ca2+, Mg2+-stimulated ATPase was analyzed by digestion of membrane phospholipids with phospholipase C and A2 enzymes of diverse specificity and by detergent extraction. Phospholipase C of Clostridium perfringens and Clostridium welchii, that hydrolyze preferentially phosphatidylcholine (PC), inhibited the Ca2+-ATPase activity parallel with the depletion of phosphatidylcholine from the membrane. Phospholipase C of Bacillus cereus hydrolyzed in addition to PC, phosphatidylethanolamine (PE) and phosphatidylserine (PS), causing complete inhibition of Ca2+-stimulated ATPase activity. Digestion of sarcoplasmic reticulum with the phospholipase A2 of snake or bee venom produced similar effects. The phosphatidylinositol (PI)-specific phospholipases of B. cereus and Bacillus thuringiensis caused less than 10% inhibition of the Ca2+-ATPase, accompanied by the hydrolysis of more than 70% of the phosphatidylinositol content of the membrane, without significant change in PC, PE and PS content. The inhibition of ATPase activity by the C type phospholipases was nearly completely reversed by octaethyleneglycol dodecyl ether (C12E8). These experiments suggest that the full phospholipid content of native sarcoplasmic reticulum (congruent to 100 mol phospholipid per mol Ca2+-ATPase), is required for ATPase activity and there is no indication that PE, PS, and PI play a specific role in ATP hydrolysis. Extraction of sarcoplasmic reticulum phospholipids by detergents such as deoxycholate, cholate and C12E8 also caused proportional inhibition of ATPase activity with the decrease in phospholipid content; the parallel extraction of PC, PE and PI left the phospholipid composition largely unchanged during delipidation. These observations do not support the requirement for a 'lipid annulus' of congruent to 30 phospholipid molecules/Ca2+-ATPase as proposed by Hesketh et al. ((1976) Biochemistry 15, 4145-4151) or the specific interaction of phosphatidylethanolamine with the ATPase molecule proposed by Bick et al. ((1991) Arch. Biochem. Biophys. 286, 346-352).

Effect of the biochemical state of the Ca-ATPase protein of scallop sarcoplasmic reticulum on its interaction with trans-parinaric acid

The polyene fluorescent probe trans-parinaric acid (tPA) was used to compare lipid-protein interactions in the scallop fragmented sarcoplasmic reticulum (FSR) between biochemical states where the Ca-ATPase molecules were arranged differently in the membrane and had different tertiary conformations. The state of the bulk lipid phase was examined over the temperature range -3 to +32 degrees C by exciting the tPA directly at 320 nm. The state of the system close to the Ca-ATPase protein was followed over the same temperature range by indirectly exciting the tPA through resonance energy transfer from the Ca-ATPase protein, with approximately one twenty-fifth the quantum yield of the directly excited probe. Raising the tPA/lipid ratio in the membrane to high levels (approx. 1:9), caused the quantum yield of indirectly excited tPA to reach a maximum, which may reflect saturation of the annular lipid phase with the probe, or contribution to the fluorescence from indirectly excited tPA bound directly to the protein. In the presence of 0.1 M KCl, a thermal perturbation was observed at approx. 7 degrees C using indirect excitation when the Ca(2+)-binding sites on the Ca-ATPase were occupied, and the subunits were disorganized. This transition was not detected in the presence of 0.1 M KCl and EGTA, when the Ca(2+)-binding sites were empty, and the Ca-ATPase subunits were organized in dimeric arrays. The transition seen with the E1(Ca2+)2 form of the membrane may involve an event at the protein/lipid interface, or a change in the environment of tPA bound to the Ca-ATPase. The temperature at which the perturbation occurs is close to that of a discontinuity in the Arrhenius plot of the Ca-ATPase enzyme activity determined in the presence of 0.1 M KCl (Kalabokis, V.N. and Hardwicke, P.M.D. (1988) J. Biol. Chem. 263, 15184-15188). No perturbation was observed in the bulk properties of the lipid component of the membrane in either the E1(Ca2+)2 or E2 states.

Influence of omega-3 fatty acid treatment on cardiac phospholipid composition and coronary flow of streptozocin-diabetic rats

Cardiac effects of omega-3 fatty acid treatment were studied in streptozocin (STZ)-induced (55 mg/kg intravenously [IV]) diabetic male Wistar rats. Nondiabetic control and STZ-diabetic animals were treated with Promega (0.5 mL/kg/d; Warner-Lambert, Morris Plains, NJ) for a period of 4 weeks beginning 2 weeks after either vehicle or STZ injection. Plasma glucose, triglyceride, and cholesterol concentrations were significantly (P < .05) elevated in diabetic animals; omega-3 fatty acid treatment did not significantly affect these parameters. An isolated working heart preparation was used to determine aortic and coronary flow rates in control, diabetic, treated control, and treated diabetic animals. Aortic and coronary flow rates of untreated STZ-diabetic rats were significantly (P < .05) lower than those of controls over a range of left atrial filling pressures (7.5 to 20 cm water). Both aortic and coronary flow rates of omega-3 fatty acid-treated diabetic animals were significantly (P < .05) increased above those of untreated diabetic rats. Aortic and coronary flow rates of treated diabetic rats paralleled those of control animals; omega-3 fatty acid treatment did not affect aortic or coronary flow rates of control animals. Cardiac phosphatidylcholine (PC) and phosphatidylethanolamine (PE) and sarcoplasmic reticulum (SR) total phospholipid were isolated and the acyl composition was determined. Stearic acid and C22:4, n-6 were significantly reduced in cardiac PE of diabetic animals. Relative to PE acyl species of untreated nondiabetic controls, treated diabetic PE had increased eicosapentaenoic acid (EPA) and decosahexaenoic acid (DHA) and reduced C22:4, n-6 levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of melittin on lipid-protein interactions in sarcoplasmic reticulum membranes

To investigate the physical mechanism by which melittin inhibits Ca-adenosine triphosphatase (ATPase) activity in sarcoplasmic reticulum (SR) membranes, we have used electron paramagnetic resonance spectroscopy to probe the effect of melittin on lipid-protein interactions in SR. Previous studies have shown that melittin substantially restricts the rotational mobility of the Ca-ATPase but only slightly decreases the average lipid hydrocarbon chain fluidity in SR. Therefore, in the present study, we ask whether melittin has a preferential effect on Ca-ATPase boundary lipids, i.e., the annular shell of motionally restricted lipid that surrounds the protein. Paramagnetic derivatives of stearic acid and phosphatidylcholine, spin-labeled at C-14, were incorporated into SR membranes. The electronic paramagnetic resonance spectra of these probes contained two components, corresponding to motionally restricted and motionally fluid lipids, that were analyzed by spectral subtraction. The addition of increasing amounts of melittin, to the level of 10 mol melittin/mol Ca-ATPase, progressively increased the fraction of restricted lipids and increased the hyperfine splitting of both components in the composite spectra, indicating that melittin decreases the hydrocarbon chain rotational mobility for both the fluid and restricted populations of lipids. No further effects were observed above a level of 10 mol melittin/mol Ca-ATPase. In the spectra from control and melittin-containing samples, the fraction of restricted lipids decreased significantly with increasing temperature. The effect of melittin was similar to that of decreased temperature, i.e., each spectrum obtained in the presence of melittin (10:1) was nearly identical to the spectrum obtained without melittin at a temperature approximately 5 degrees C lower. The results suggest that the principal effect of melittin on SR membranes is to induce protein aggregation and this in turn, augmented by direct binding of melittin to the lipid, is responsible for the observed decreases in lipid mobility. Protein aggregation is concluded to be the main cause of inactivation of the Ca-ATPase by melittin, with possible modulation also by the decrease in mobility of the boundary layer lipids.

The effect of high pressure on the conformation, interactions and activity of the Ca(2+)-ATPase of sarcoplasmic reticulum

High pressure (100-150 MPa) increases the intensity and polarization of fluorescence of FITC-labeled Ca(2+)-ATPase in a medium containing 0.1 mM Ca2+, suggesting a reversible pressure-induced transition from the E1 into an E2-like state with dissociation of ATPase oligomers. Under similar conditions but using unlabeled sarcoplasmic reticulum vesicles, high pressure caused the reversible release of Ca2+ from the high-affinity Ca2+ sites of Ca(2+)-ATPase, as indicated by changes in the fluorescence of the Ca2+ indicator, Fluo-3; this was accompanied by reversible inhibition of the Ca(2+)-stimulated ATPase activity measured in a coupled enzyme system of pyruvate kinase and lactate dehydrogenase, and by redistribution of Prodan in the lipid phase of the membrane, as shown by marked changes in its fluorescence emission characteristics. In a Ca(2+)-free medium where the equilibrium favors the E2 conformation of Ca(2+)-ATPase the fluorescence intensity of FITC-ATPase was not affected or only slightly reduced by high pressure. The enhancement of TNP-AMP fluorescence by 100 mM inorganic phosphate in the presence of EGTA and 20% dimethylsulfoxide was essentially unaffected by 150 MPa pressure at pH 6.0 and was only slightly reduced at pH 8.0. As the enhancement of TNP-AMP fluorescence by Pi is associated with the Mg(2+)-dependent phosphorylation of the enzyme and the formation of Mg.E2-P intermediate, it appears that the reactions of Ca(2+)-ATPase associated with the E2 state are relatively insensitive to high pressure. These observations suggest that high pressure stabilizes the enzyme in an E2-like state characterized by low reactivity with ATP and Ca2+ and high reactivity with Pi. The transition from the E1 to the E2-like state involves a decrease in the effective volume of Ca(2+)-ATPase.

Prolonged exercise induces structural changes in SR Ca(2+)-ATPase of rat muscle

Sarcoplasmic reticulum (SR) isolated from the deep red portion of the gastrocnemius muscle of Sprague-Dawley rats after a single bout of prolonged exercise was shown to have depressed Ca(2+)-stimulated Mg(2+)-dependent ATPase activity over a temperature range of 15 to 42.5 degrees C when compared to SR obtained from control muscle. Inclusion of the calcium ionophore, A23187, failed to restore the depressed ATPase activity from SR of exercised muscle to control values, but it did normalize the stimulatory effect of temperature on ATPase activity. This depression was also manifested as an increased activation energy when the data were converted to an Arrhenius plot. SR vesicles from both groups showed no differences or discontinuities in plots of steady-state fluorescence anisotropy. When the binding characteristics of the fluorescent probe, fluorescein isothiocyanate (FITC), were analyzed, SR vesicles prepared from exercised muscle displayed a 40% reduction in binding capacity with no apparent change in Kd. These findings support the conclusion that a single bout of exercise induces a structural change in the Ca(2+)-ATPase protein of rat red gastrocnemius muscle that is not a direct result of gross lipid alterations or increased muscle temperature.

Purification and photoaffinity labeling of the I-Ak histocompatibility molecule

Photoaffinity labeling was used to evaluate optimal conditions for purification of I-A k histocompatibility molecules in functionally active form. We assessed the biological activity of I-A k primarily by its binding of the hen egg-white lysozyme (HEL) peptide from residues 46-61. [125I]iodo,4-azidosalicyloly(HEL)46-61 (IASA-46-61)-labeled I-A k on B cell hybridoma membranes and their detergent solubilisates, at the alpha chain. Following extensive detergent dialysis, the intensity of this labeling remained unchanged in the case of MEGA 8 and MEGA 9 detergents, but decreased in the case of deoxycholate and n-octylglucoside. Conditions for affinity purifications were assessed on one hand by determining the dissociation conditions of I-A k from various monoclonal antibodies and by determining the denaturation of I-A k under these conditions. Effective dissociation in the absence of detectable denaturation was observed for 10.3.6.2 and 40.LH monoclonal antibody at pH 3.5 and to a lesser extent at low concentrations of ammonium thiocyanate and guanidine thiocyanate at neutral pH. I-A k purified from cell membranes using MEGA 8 and MEGA 9 detergent mixtures and acid elution from 10.3.6.2 Sepharose was efficiently labeled by IASA-46-61. Thus I-A k was active in antigen presentation to a T cell hybridoma when reconstituted in planar membranes. In contrast to I-A k on cell membranes, purified I-A k in detergent showed extensive labeling of the beta chain. The overall labeling intensity and the extent of beta chain labeling substantially changed upon addition of certain lysophosphatides.

Effect of increasing the level of omega-3 fatty acids on rat skeletal muscle sarcoplasmic reticulum

The effect of dietary supplementation with fish oil as compared to corn oil on the lipid dynamics and calcium ATPase activity of rat skeletal sarcoplasmic reticulum was examined. After four-week supplementation with fish oil, the levels of eicosapentaenoic (20:5 omega 3), docosapentaenoic (22:5 omega 3) and docosahexaenoic (22:6 omega 3) acids in the total lipids were 5.3, 5.5 and 28.1% of the total fatty acids, respectively. In contrast, with corn oil only 22:6 was found (8.9%). The level of these fatty acids in phosphatidylethanolamine from the membranes of animals fed fish oil was 4.2 (20:5), 5.4 (22:5) and 49.1% (22:6); and for phosphatidylcholine it was 5.4 (20:5), 4.6 (22:5) and 17.4% (22:6). Again, in corn oil fed animals, only 22:6 was found in appreciable amounts, namely 28.3% in phosphatidylethanolamine and 1.8% in phosphatidylcholine. The steady state fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH) was used to assess lipid order and was found to be only slightly less for membranes from animals supplemented with fish oil (0.120) as compared to those supplemented with corn oil (0.124). The calcium ATPase was found to be unaffected by supplementation consistent with the observed modest changes in lipid order as well as with suggestions that the enzyme is relatively insensitive to the level of unsaturation. It could be argued that if large increases in fatty acyl polyunsaturation in mammalian cell membranes would lead to marked alterations in bulk membrane lipid motional properties, this may not be in the interest of preserving physiological function.(ABSTRACT TRUNCATED AT 250 WORDS)

Saturation transfer electron spin resonance of Ca2(+)-ATPase covalently spin-labeled with beta-substituted vinyl ketone- and maleimide-nitroxide derivatives. Effects of segmental motion and labeling levels

The Ca2(+)-ATPase in native sarcoplasmic reticulum membranes was selectively spin-labeled for saturation transfer electron spin resonance (ESR) studies by prelabeling with N-ethylmaleimide and by using low label/protein ratios. Results with the nitroxide derivative of the standard sulphydryl-modifying reagent, maleimide, were compared with a series of six novel nitroxide beta-substituted vinyl aryl ketone derivatives which differed (with two exceptions) in the substituent at the ketone position. The two exceptions had a different electron withdrawing group at the alpha-carbon, to enhance further the electrophilic character of the beta-carbon. Although differing in their reactivity, all the conjugated unsaturated ketone nitroxide derivatives displayed saturation transfer ESR spectra indicative of much slower motion than did the maleimide derivative. The saturation transfer ESR spectra of maleimide-labeled Ca2(+)-ATPase therefore most likely contain substantial contributions from segmental motion of the labeled group. The effects of the level of spin labeling were also investigated. With increasing degree of spin label incorporation, the linewidths of the conventional ESR spectrum progressively increased and the intensity of the saturation transfer spectrum dropped dramatically, as a result of increasing spin-spin interactions. The hyperfine splittings of the conventional spectrum and the outer lineheight ratios of the saturation transfer spectrum remained relatively unchanged. Extrapolation back to zero labeling level yielded comparable values for the effective rotational correlation times deduced from the saturation transfer spectrum intensities and from the lineheight ratios, for the vinyl ketone label. For the maleimide label the extrapolated values from the integral are significantly lower than those from the lineheight ratios, probably because of the segmental motion. Comparison is made of the effective rotational correlation time for the vinyl ketone label with the predictions of hydrodynamic models for the protein diffusion, in a discussion of the aggregation state of the Ca2(+)-ATPase in the native sarcoplasmic reticulum membrane. The implications for the study of protein rotational diffusion and segmental motion, and of the proximity relationships between labeled groups, using saturation transfer ESR spectroscopy are discussed.

Phospholipids enhance the binding of peptides to class II major histocompatibility molecules

The binding of a lysozyme and ovalbumin peptide to purified class II major histocompatibility molecules in detergents was increased by the addition of certain lipids. Natural lipids from B lymphoma cells enhanced the binding and so did phosphatidylcholine, phosphatidylserine, phosphatidylinositol, and cardiolipin. Phosphatidylethanolamine, sphingomyelin, and cholesterol had no effect. There was no major difference between the effects of a phospholipid and its lyso derivative. As studied with phosphatidylcholine, the increase in peptide binding was also dependent on the fatty acid composition of the lipid. The binding affinity was increased 10- to 50-fold in the presence of lipid as a result of an increase in the association rate while the off-rate remained essentially unchanged. Our results suggest that lipids, directly or indirectly, induce conformational changes in class II molecules that favor their peptide-binding property.

Selective detection of the rotational dynamics of the protein-associated lipid hydrocarbon chains in sarcoplasmic reticulum membranes

We have developed a saturation transfer EPR (ST-EPR) method to measure selectively the rotational dynamics of those lipids that are motionally restricted by integral membrane proteins and have applied this methodology to measure lipid-protein interactions in native sarcoplasmic reticulum (SR) membranes. This analysis involves the measurement of spectral saturation using a series of six stearic acid spin labels that are labeled with a nitroxide at different carbon atom positions. A large amount of spectral saturation is observed for spin labels in native SR membranes, but not for spin labels in dispersions of extracted SR lipids, implying that the motional properties of those lipids interacting with the Ca-ATPase, i.e., the boundary or annular lipid, can be directly measured without the need for spectral subtraction procedures. A comparison of the motional properties of the restricted lipid, measured by ST-EPR, with those measured by digital subtraction of conventional EPR spectra qualitatively agree, for in both cases the Ca-ATPase restricts the rotational mobility of a population of lipids, whose rotational mobility increases as the nitroxide is positioned toward the center of the bilayer. However, the ability of ST-EPR to directly measure the motionally restricted lipid in a model-independent means provides the greater precision necessary to measure small changes in the rotational dynamics of the lipid at the protein-lipid interface, providing a valuable tool in clarifying the relationship between the physical nature of the protein-lipid interface and membrane function.

Biochemical properties of isolated transverse tubular membranes

This review addresses the major biochemical and structural characteristics of isolated transverse tubule (T-tubule) membranes, including methods of isolation and morphology of purified membranes, evaluation of attendant membrane activities, including ion pumps and channels, and structural and compositional analyses of functionally relevant components. Particular emphasis is placed on the Mg2+-ATPase, its localization in the T-system, its unusual kinetic properties, its possible functions, and its potential regulation by diacylglycerol and other biologically-relevant lipids. Conclusions are drawn with respect to the biochemical markers characteristic of T-tubule membranes and the criteria to be applied in the assessment of isolated T-tubule membrane purity.