Adaptive changes in lipid composition of skeletal sarcoplasmic reticulum membranes associated with aging

Spontaneous membrane insertion of a dengue virus NS2A peptide

Non-structural NS2A protein of Dengue virus is essential for viral replication but poorly characterized because of its high hydrophobicity. We have previously shown experimentally that NS2A possess a segment, peptide dens25, known to insert into membranes and interact specifically with negatively-charged phospholipids. To characterize its membrane interaction we have used two types of molecular dynamics membrane model systems, a highly mobile membrane mimetic (HMMM) and an endoplasmic reticulum (ER) membrane-like model. Using the HMMM system, we have been able of demonstrating the spontaneous binding of dens25 to the negatively-charged phospholipid 1,2-divaleryl-sn-glycero-3-phosphate containing membrane whereas no binding was observed for the membrane containing the zwitterionic one 1,2-divaleryl-sn-glycero-3-phosphocholine. Using the ER-like membrane model system, we demonstrate the spontaneous insertion of dens25 into the middle of the membrane, it maintained its three-dimensional structure and presented a nearly parallel orientation with respect to the membrane surface. Both charged and hydrophobic amino acids, presenting an interfacial/hydrophobic pattern characteristic of a membrane-proximal segment, are responsible for membrane binding and insertion. Dens25 might control protein/membrane interaction and be involved in membrane rearrangements critical for the viral cycle. These data should help us in the development of inhibitor molecules that target NS2A segments involved in membrane reorganisation.

Heterogeneous Inhibition in Macroscopic Current Responses of Four Nicotinic Acetylcholine Receptor Subtypes by Cholesterol Enrichment

The nicotinic acetylcholine receptor (nAChR), located in the cell membranes of neurons and muscle cells, mediates the transmission of nerve impulses across cholinergic synapses. In addition, the nAChR is also found in the electric organs of electric rays (e.g., the genus Torpedo). Cholesterol, which is a key lipid for maintaining the correct functionality of membrane proteins, has been found to alter the nAChR function. We were thus interested to probe the changes in the functionality of different nAChRs expressed in a model membrane with modified cholesterol to phospholipid ratios (C/P). In this study, we examined the effect of increasing the C/P ratio in Xenopus laevis oocytes expressing the neuronal α7, α4β2, muscle-type, and Torpedo californica nAChRs in their macroscopic current responses. Using the two-electrode voltage clamp technique, it was found that the neuronal α7 and Torpedo nAChRs are significantly more sensitive to small increases in C/P than the muscle-type nAChR. The peak current versus C/P profiles during enrichment display different behaviors; α7 and Torpedo nAChRs display a hyperbolic decay with two clear components, whereas muscle-type and α4β2 nAChRs display simple monophasic decays with different slopes. This study clearly illustrates that a physiologically relevant increase in membrane cholesterol concentration produces a remarkable reduction in the macroscopic current responses of the neuronal α7 and Torpedo nAChRs functionality, whereas the muscle nAChR appears to be the most resistant to cholesterol inhibition among all four nAChR subtypes. Overall, the present study demonstrates differential profiles for cholesterol inhibition among the different types of nAChR to physiological cholesterol increments in the plasmatic membrane. This is the first study to report a cross-correlation analysis of cholesterol sensitivity among different nAChR subtypes in a model membrane.

Membrane interacting regions of Dengue virus NS2A protein

The Dengue virus (DENV) NS2A protein, essential for viral replication, is a poorly characterized membrane protein. NS2A displays both protein/protein and membrane/protein interactions, yet neither its functions in the viral cycle nor its active regions are known with certainty. To highlight the different membrane-active regions of NS2A, we characterized the effects of peptides derived from a peptide library encompassing this protein’s full length on different membranes by measuring their membrane leakage induction and modulation of lipid phase behavior. Following this initial screening, one region, peptide dens25, had interesting effects on membranes; therefore, we sought to thoroughly characterize this region’s interaction with membranes. This peptide presents an interfacial/hydrophobic pattern characteristic of a membrane-proximal segment. We show that dens25 strongly interacts with membranes that contain a large proportion of lipid molecules with a formal negative charge, and that this effect has a major electrostatic contribution. Considering its membrane modulating capabilities, this region might be involved in membrane rearrangements and thus be important for the viral cycle.

N-terminal AH2 segment of protein NS4B from hepatitis C virus. Binding to and interaction with model biomembranes

HCV NS4B, a highly hydrophobic protein involved in the alteration of the intracellular host membranes forming the replication complex, plays a critical role in the HCV life cycle. NS4B is a multifunctional membrane protein that possesses different regions where diverse and significant functions are located. One of these important regions is the AH2 segment, which besides being highly conserved has been shown to play a significant role in NS4B functioning. We have carried out an in-depth biophysical study aimed at the elucidation of the capacity of this region to interact, modulate and disrupt membranes, as well as to study the structural and dynamic features relevant for that disruption. We show that a peptide derived from this region, NS4BAH2, is capable of specifically binding phosphatidyl inositol phosphates with high affinity, and its interfacial properties suggest that this segment could behave similarly to a pre-transmembrane domain partitioning into and interacting with the membrane depending on the membrane composition and/or other proteins. Moreover, NS4BAH2 is capable of rupturing membranes even at very low peptide-to-lipid ratios and its membrane-activity is modulated by lipid composition. NS4BAH2 is located in a shallow position in the membrane but it is able to affect the lipid environment from the membrane surface down to the hydrophobic core. The NS4B region where peptide NS4BAH2 resides might have an essential role in the membrane replication and/or assembly of the viral particle through the modulation of the membrane structure and hence the replication complex.

Interaction with membranes of the full C-terminal domain of protein NS4B from hepatitis C virus

Hepatitis C virus (HCV) NS4B protein is a transmembrane highly hydrophobic protein responsible for many key aspects of the viral replication process. The C-terminal part of NS4B is essential for replication and is a potential target for HCV replication inhibitors. In this work we have carried out a study of the binding to and interaction with model biomembranes of a peptide corresponding to the C-terminal domain of NS4B, NS4B(Cter). We show that NS4B(Cter) partitions into phospholipid membranes, is capable of rupturing membranes even at very low peptide-to-lipid ratios and its membrane-activity is modulated by lipid composition. NS4B(Cter) is located in a shallow position in the membrane but it is able to affect the lipid environment from the membrane surface down to the hydrophobic core. Our results identify the C-terminal region of the HCV NS4B protein as a membrane interacting domain, and therefore directly implicated in the HCV life cycle and possibly in the formation of the membranous web.

Evolving concepts on the age-related changes in "muscle quality"

The deterioration of skeletal muscle with advancing age has long been anecdotally recognized and has been of scientific interest for more than 150 years. Over the past several decades, the scientific and medical communities have recognized that skeletal muscle dysfunction (e.g., muscle weakness, poor muscle coordination, etc.) is a debilitating and life-threatening condition in the elderly. For example, the age-associated loss of muscle strength is highly associated with both mortality and physical disability. It is well-accepted that voluntary muscle force production is not solely dependent upon muscle size, but rather results from a combination of neurologic and skeletal muscle factors, and that biologic properties of both of these systems are altered with aging. Accordingly, numerous scientists and clinicians have used the term "muscle quality" to describe the relationship between voluntary muscle strength and muscle size. In this review article, we discuss the age-associated changes in the neuromuscular system-starting at the level of the brain and proceeding down to the subcellular level of individual muscle fibers-that are potentially influential in the etiology of dynapenia (age-related loss of muscle strength and power).

Activating and deactivating roles of lipid bilayers on the Ca(2+)-ATPase/phospholamban complex

The physicochemical properties of the lipid bilayer shape the structure and topology of membrane proteins and regulate their biological function. Here, we investigated the functional effects of various lipid bilayer compositions on the sarcoplasmic reticulum (SR) Ca(2+)-ATPase (SERCA) in the presence and absence of its endogenous regulator, phospholamban (PLN). In the cardiac muscle, SERCA hydrolyzes one ATP molecule to translocate two Ca(2+) ions into the SR membrane per enzymatic cycle. Unphosphorylated PLN reduces SERCA's affinity for Ca(2+) and affects the enzymatic turnover. We varied bilayer thickness, headgroup, and fluidity and found that both the maximal velocity (V(max)) of the enzyme and its apparent affinity for Ca(2+) (K(Ca)) are strongly affected. Our results show that (a) SERCA's V(max) has a biphasic dependence on bilayer thickness, reaching maximum activity with 22-carbon lipid chain length, (b) phosphatidylethanolamine (PE) and phosphatidylserine (PS) increase Ca(2+) affinity, and (c) monounsaturated lipids afford higher SERCA V(max) and Ca(2+) affinity than diunsaturated lipids. The presence of PLN removes the activating effect of PE and shifts SERCA's activity profile, with a maximal activity reached in bilayers with 20-carbon lipid chain length. Our results in synthetic lipid systems compare well with those carried out in native SR lipids. Importantly, we found that specific membrane compositions closely reproduce PLN effects (V(max) and K(Ca)) found in living cells, reconciling an ongoing controversy regarding the regulatory role of PLN on SERCA function. Taken with the physiological changes occurring in the SR membrane composition, these studies underscore a possible allosteric role of the lipid bilayers on the SERCA/PLN complex.

Probing ground and excited states of phospholamban in model and native lipid membranes by magic angle spinning NMR spectroscopy

In this paper, we analyzed the ground and excited states of phospholamban (PLN), a membrane protein that regulates sarcoplasmic reticulum calcium ATPase (SERCA), in different membrane mimetic environments. Previously, we proposed that the conformational equilibria of PLN are central to SERCA regulation. Here, we show that these equilibria detected in micelles and bicelles are also present in native sarcoplasmic reticulum lipid membranes as probed by MAS solid-state NMR. Importantly, we found that the kinetics of conformational exchange and the extent of ground and excited states in detergent micelles and lipid bilayers are different, revealing a possible role of the membrane composition on the allosteric regulation of SERCA. Since the extent of excited states is directly correlated to SERCA inhibition, these findings open up the exciting possibility that calcium transport in the heart can be controlled by the lipid bilayer composition. This article is part of a Special Issue entitled: Membrane protein structure and function.

Sarcoplasmic reticulum function in smooth muscle

The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a "one model fits all" approach to this subject, we have tried to synthesize conclusions wherever possible.

A membranotropic region in the C-terminal domain of hepatitis C virus protein NS4B interaction with membranes

We have identified a membrane-active region in the HCV NS4B protein by studying membrane rupture induced by a NS4B-derived peptide library on model membranes. This segment corresponds to one of two previously predicted amphipathic helix and define it as a new membrane association domain. We report the binding and interaction with model membranes of a peptide patterned after this segment, peptide NS4B(H2), and show that NS4B(H2) strongly partitions into phospholipid membranes, interacts with them, and is located in a shallow position in the membrane. Furthermore, changes in the primary sequence cause the disruption of the hydrophobicity along the structure and prevent the resulting peptide from interacting with the membrane. Our results suggest that the region where the NS4B(H2) is located might have an essential role in the membrane replication and/or assembly of the viral particle through the modulation of the replication complex. Our findings therefore identify an important region in the HCV NS4B protein which might be implicated in the HCV life cycle and possibly in the formation of the membranous web.

Properties of the SR Ca-ATPase in an Open Microsomal Membrane Preparation

SR vesicles isolated from rabbit muscle were treated by a SDS incubation and subsequent dialysis to obtain open membrane fragments that allow a direct access to the luminal membrane surface and especially to the ion-binding sites in the P-E(2) conformation of the Ca-ATPase. The open membrane fragments showed about 80% of the enzyme activity in the untreated membranes. Pump function was investigated by using electrochromic styryl dyes. The kinetic properties of cytoplasmic ion binding showed no significant differences between the Ca-ATPases in SR vesicles and in membrane fragments. From pH-dependent Ca(2+) binding it could be deduced that due to the SDS treatment the density of negatively charged lipid was increased by one elementary charge per 12 lipid molecules. Major differences between Ca-ATPase from SR vesicles and membrane fragments were the respective fluorescence amplitudes. This effect is, however, produced by dye-lipid interaction and not by pump function. It was demonstrated that time-resolved kinetics may be study by the use of caged compounds such as caged ATP or caged calcium also in the case of the membrane fragments.

Mass spectrometry in aging research

This review covers the application of mass spectrometric techniques to aging research. Modern proteomic strategies will be discussed as well as the targeted analysis of specific proteins for the correlation of post-translational modifications with protein function. Selected examples will show both the power and also current limitations of the respective techniques. Experimental results and strategies are discussed in view of current theories of the aging process.

Mitochondrial phospholipid bilayer structure is ruined after liver oxidative injury in vivo

The purpose of this study was to investigate whether, after oxidative injury in vivo, liver mitochondrial phospholipids suffered from structural defects similar to those we have previously observed after either chemical oxidation or respiration state IV incubation of isolated mitochondria in vitro. Oxidative injury of the liver was simulated by endogastric administration of CCl4 to rats in variable amounts for different times, under various conditions. Measurements of the phospholipid bilayer packing order were carried out by electron paramagnetic resonance (EPR) spectrometry of oriented planar samples of phospholipids extracted from liver mitochondria, spin labeled with 5-doxylstearoyl-lecithin. Disordering of the bilayer was revealed by the anisotropy loss of EPR spectra and reached a maximum value 4.5 h after CCl4 administration, vanishing thereafter. The observed disorder also increased with the amount of CCl4 administered, showing distinct dose-dependence, while administration of resveratrol soon after carbon tetrachloride decreased bilayer disordering by 50%. On the contrary, the order parameter S of spin labeled lecithin in isolated mitochondrial membranes from intoxicated rats revealed no change in membrane fluidity after oxidative stress. It is concluded that the phospholipid damage leading to disturbed bilayer geometry after oxidative attack already observed in model membranes and in isolated mitochondria in vitro also occurs in a simulated pathological state in vivo, indicating its possible occurrence also in real oxidative stress-linked pathologies as a contribution to the onset/sustaining of related diseases.

Regulation of the calcium release channel from skeletal muscle by suramin and the disulfonated stilbene derivatives DIDS, DBDS, and DNDS

Activation of skeletal muscle ryanodine receptors (RyRs) by suramin and disulfonic stilbene derivatives (Diisothiocyanostilbene-2',2'-disulfonic acid (DIDS), 4,4'-dibenzamidostilbene-2,2'-disulfonic acid (DBDS),and 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS)) was investigated using planar bilayers. One reversible and two nonreversible mechanisms were identified. K(a) for reversible activation (approximately 100 micro M) depended on cytoplasmic [Ca(2+)] and the bilayer composition. Replacement of neutral lipids by negative phosphatidylserine increased K(a) fourfold, suggesting that reversible binding sites are near the bilayer surface. Suramin and the stilbene derivatives adsorbed to neutral bilayers with maximal mole fractions between 1-8% and with affinities approximately 100 micro M but did not adsorb to negative lipids. DIDS activated RyRs by two nonreversible mechanisms, distinguishable by their disparate DIDS binding rates (10(5) and 60 M(-1) s(-1)) and actions. Both mechanisms activated RyRs via several jumps in open probability, indicating several DIDS binding events. The fast and slow mechanisms are independent of each other, the reversible mechanism and ATP binding. The fast mechanism confers DIDS sensitivity approximately 1000-fold greater than previously reported, increases Ca(2+) activation and increases K(i) for Ca(2+)/Mg(2+) inhibition 10-fold. The slow mechanism activates RyRs in the absence of Ca(2+) and ATP, increases ATP activation without altering K(a), and slightly increases activity at pH < 6.5. These findings explain how different types of DIDS activation are observed under different conditions.

In situ localization of cholesterol in skeletal muscle by use of a monoclonal antibody

A common perception is that cholesterol, the major structural lipid found in mammalian membranes, is localized nearly exclusively to the plasma membrane of living cells and that it is found in much smaller quantities in internal membranes. This perception is based almost exclusively on cell fractionation studies, in which density gradient centrifugation is used for purification of discrete subcellular membrane fractions. Here we describe a monoclonal antibody, MAb 2C5-6, previously reported to detect purified cholesterol in synthetic membranes (Swartz GM Jr, Gentry MK, Amende LM, Blanchette-Mackie EJ, and Alving CR. Proc Natl Acad Sci USA 85: 1902-1906, 1988), that is capable of detecting cholesterol in situ in the membranes of skeletal muscle sections. Localization of cholesterol, the dihydropyridine receptor of the T tubule, and the Ca(2+)-ATPase of the sarcoplasmic reticulum (SERCA2) by means of double and triple immunostaining protocols clearly demonstrates that cholesterol is primarily localized to the sarcoplasmic reticulum membranes of skeletal muscle rather than the sarcolemmal or T tubule membranes. The availability of this reagent and its ability to spatially localize cholesterol in situ may provide a greater understanding of the relationship between membrane cholesterol content and transmembrane signaling in skeletal muscle.

Human skeletal sarcoplasmic reticulum Ca2+ uptake and muscle function with aging and strength training

This study investigated the adaptations of skeletal muscle sarcoplasmic reticulum (SR) Ca2+ uptake, relaxation, and fiber types in young (YW) and elderly women (EW) to high-resistance training. Seventeen YW (18-32 yr) and 11 EW (64-79 yr) were assessed for 1) electrically evoked relaxation time and rate of the quadriceps femoris; and 2) maximal rates of SR Ca2+ uptake and Ca2+-ATPase activity and relative fiber-type areas, analyzed from muscle biopsies of the vastus lateralis. EW had significantly slower relaxation rates and times, decreased SR Ca2+ uptake and Ca2+-ATPase activity, and a larger relative type I fiber area than did YW. A subgroup of 9 young (YWT) and 10 elderly women (EWT) performed 12 wk of high-resistance training (8 repetition maximum) of the quadriceps and underwent identical testing procedures pre- and posttraining. EWT significantly increased their SR Ca2+ uptake and Ca2+-ATPase activity in response to training but showed no alterations in speed of relaxation or relative fiber-type areas. In YWT none of the variables was altered after resistance training. These findings suggest that 1) a reduced SR Ca2+ uptake in skeletal muscle of elderly women was partially reversed with resistance training and 2) SR Ca2+ uptake in the vastus lateralis was not the rate-limiting mechanism for the slowing of relaxation measured from electrically evoked quadriceps muscle of elderly women.

Age-related chemical modification of the skeletal muscle sarcoplasmic reticulum Ca-ATPase of the rat

Much emphasis has been placed on the description of age-related changes in skeletal muscle physiology. The present paper summarizes the chemical characterization of age-related post-translational modifications of the rat skeletal muscle sarcoplasmic reticulum (SR) Ca-ATPase isoforms SERCA1 and SERCA2a obtained from 5- and 28-month-old male Fischer 344 rats. Whereas the SERCA1 isoform shows an age-dependent loss of Cys and Arg, the SERCA2a isoform displays a loss of Cys but also a significant accumulation of 3-nitrotyrosine. The in vitro exposure of SR vesicles particularly rich in SERCA1 (>90%) from 5-month-old rats to low levels of peroxyl radicals yielded SR vesicles with physical properties of the SR Ca-ATPase identical to those observed for the SR Ca-ATPase obtained from 28-month-old rats. The peroxyl radical-modified SR Ca-ATPase showed a loss of Cys and Arg but also of Ser and Met, indicating that peroxyl radicals, though a good model oxidant to generate 'aged' SR vesicles, may not be the only oxidant responsible for the chemical modification of the SR Ca-ATPase in vivo. In fact, efficient thiol modification of the SERCA1 was also observed after the exposure to peroxynitrite. Peroxynitrite selectively nitrated the tyrosine residues of the SERCA2a isoform even in the presence of an excess of SERCA1. Thus, peroxynitrite may be responsible for the age-dependent modification of the SR Ca-ATPase in vivo.

Decrease in Ca-ATPase activity in aged synaptosomal membranes is not associated with changes in fatty acyl chain dynamics

We have examined lipid peroxidation (LPO) and fatty acid acyl chain dynamics in synaptosomal membranes isolated from aged rat (Fischer 344 x Brown Norway F1 hybrids) brains, correlating these results with measurements of enzymatic activity of the synaptic plasma membrane Ca2(+)-ATPase (PMCA). Calcium-dependent ATPase activity in these membranes exhibits progressive decreases with a maximal loss of activity with age of approximately 35%. The sensitivity of this membrane-bound ion transporter to the lipid composition of the surrounding membrane, as well as the high abundance of oxidatively sensitive polyunsaturated fatty acyl chains in synaptosomal membranes, suggests that this age-related loss in catalytic turnover may result from LPO-mediated protein modification and/or changes in the physical structure of the bilayer. However, high-performance liquid chromatography analysis of 2,4-dinitrophenylhydrazone derivatives reveals no significant age-related increases in the content of reactive aldehydes (malondialdehyde, formaldehyde, acetaldehyde or acetone) which comprise breakdown products of lipid peroxidation. Electron paramagnetic resonance measurements employing 5- and 12-stearic acid spin labels with the nitroxide reporter groups at two depths in the bilayer were used to assess the fatty acyl chain dynamics (fluidity) of synaptosomal membranes. The resulting spectra demonstrate anisotropic lipid dynamics of two populations of lipids, i.e. lipids in direct association with membrane proteins (boundary lipids) and bulk lipids that do not directly associate with proteins. The nanosecond dynamics of both lipid populations is unaltered with age indicating that any compositional changes occurring with age are insufficient to result in alterations in bilayer fluidity relevant to PMCA activity. Thus, the observed age-related decline in PMCA activity may be explained by direct modification of membrane protein.

Decreased conformational stability of the sarcoplasmic reticulum Ca-ATPase in aged skeletal muscle

Sarcoplasmic reticulum (SR) membranes purified from young adult (4-6 months) and aged (26-28 months) Fischer 344 male rat skeletal muscle were compared with respect to the functional and structural properties of the Ca-ATPase and its associated lipids. While we find no age-related alterations in (1) expression levels of Ca-ATPase protein, and (2) calcium transport and ATPase activities, the Ca-ATPase isolated from aged muscle exhibits more rapid inactivation during mild (37 degrees C) heat treatment relative to that from young muscle. Saturation-transfer EPR measurements of maleimide spin-labeled Ca-ATPase and parallel measurements of fatty acyl chain dynamics demonstrate that, accompanying heat inactivation, the Ca-ATPase from aged skeletal muscle more readily undergoes self-association to form inactive oligomeric species without initial age-related differences in association state of the protein. Neither age nor heat inactivation results in differences in acyl chain dynamics of the bilayer including those lipids at the lipid-protein interface. Initial rates of tryptic digestion associated with the Ca-ATPase in SR isolated from aged muscle are 16(+/- 2)% higher relative to that from young muscle. indicating more solvent exposure of a portion of the cytoplasmic domain. During heat inactivation these structural differences are amplified as a result of immediate and rapid further unfolding of the Ca-ATPase isolated from aged muscle relative to the delayed unfolding of the Ca-ATPase isolated from young muscle. Thus age-related alterations in the solvent exposure of cytoplasmic peptides of the Ca-ATPase are likely to be critical to the loss of conformational and functional stability.

In vivo aging of rat skeletal muscle sarcoplasmic reticulum Ca-ATPase. Chemical analysis and quantitative simulation by exposure to low levels of peroxyl radicals

Sarcoplasmic reticulum (SR) Ca-ATPase of young adult (5 months) and aged (28 months) Fischer 344 male rat skeletal muscle was analyzed for posttranslational modifications as a result of biological aging and their potential functional consequences. The significant differences in the amino acid composition were a 6.8% lower content of sulfhydryl groups and a ca. 4% lower content of Arg residues of the Ca-ATPase from old as compared to young rats. Based on a total of 24 Cys residues the difference in protein thiols corresponds to a loss of 1.5 mol Cys/mol Ca-ATPase as a result of in vivo aging. The loss of Cys residues was not accompanied by a loss of enzyme activity though the 'aged' Ca-ATPase was more sensitive to heat inactivation, aggregation, and tryptic digestion. A comparison of the total sulfhydryl content of all SR proteins present revealed a 13% lower amount for SR vesicles isolated from aged rats. Compared to the alterations of Cys and Arg, there was only a slight and probably physiologically insignificant increase of protein carbonyls with aging, i.e. from 0.32 to 0.46 mol carbonyl groups per mol of Ca-ATPase. When SR vesicles from young rats were exposed to AAPH-derived peroxyl radicals, there was a loss of ca. 1.38 x 10(-4) M total SR sulfhydryl groups per 4 mg SR protein/ml (corresponding to ca. 25%) and a loss of 9.6 x 10(-5) M Ca-ATPase sulfhydryl groups (corresponding to ca. 31%) per 1.6 x 10(-5) M initiating peroxyl radicals, indicating that the stoichiometry of sulfhydryl oxidation was > or = 6 oxidized thiols per initiating AAPH-derived peroxyl radical. Besides Cys, the exposure to AAPH-derived radicals caused a slight loss of Ca-ATPase Arg, Met, and Ser residues. Most importantly, the SR Ca-ATPase exposed to this low concentration of peroxyl radicals displayed physical and functional properties quantitatively comparable to those of SR Ca-ATPase isolated from aged rats, i.e. no immediate loss of activity, increased susceptibility to heat inactivation, aggregation, and tryptic digestion. Moreover, a comparison of kinetically early tryptic fragments by HPLC-electrospray MS and N-terminal sequencing revealed that similar peptide fragments were produced from 'aged' and AAPH-oxidized Ca-ATPase which were not (or kinetically significantly later) generated from the 'young' Ca-ATPase, suggesting some conformational changes of the Ca-ATPase as a result of aging and AAPH-exposure. All except one of these peptides originated from locations remote from the nucleotide-binding and calcium-binding sites. The latter results suggest that aging and AAPH-exposure may target similar Cys residues, mainly at locations remote from the nucleotide-binding and calcium-binding sites, rationalizing the fact that Cys oxidation did not immediately cause inactivation of the Ca-ATPase. Our results provide a quantitative estimate of a net concentration of reactive oxygen species, here peroxyl radicals, which induces physical and chemical alterations of the SR Ca-ATPase quantitatively comparable to those induced by in vivo aging.

In K562 and HL60 cells membrane ageing during cell growth is associated with changes in cholesterol concentration

In a cell culture model of aging we have previously shown that there is an age-related decrease in the lipid dynamics of the proerythropoetic K562 cell membranes, as determined by the generalized polarization (GP) of the phase-sensitive lipid probe 2-dimethylamino-6-lauroylnaphthalene (Laurdan) (T. Parasassi, M. Di Stefano, G. Ravagnan, O. Sapora and E. Gratton. Exp. Cell Res., 202 (1992) 432-439). In the present study we also extended our observations to the lymphoblastoid HL60 cell line. In both K562 and HL60 cells during the four days after the last cell culture medium renewal the GP Laurdan value increased in a linear fashion indicating a time-dependent decrease in lipid dynamics. The initial membrane physical properties were almost completely restored upon renewal of the cell culture medium. We measured lipid composition, including individual and total phospholipids, free and esterified cholesterol at the first ('young') and at the fourth ('aged') day after culture medium renewal. We found that the decreased membrane lipid 'fluidity' at the fourth day of cell growth was associated with a 40% increase in cholesterol concentration in both cell lines. This increase in cholesterol concentration was reversible 24 h following the culture medium change. We conclude that in K562 and HL60 cells the 'age-related' decrease in membrane lipid dynamics is mediated by an 'age-related' increase in cell cholesterol content.

Benzophenone-sensitized photooxidation of sarcoplasmic reticulum membranes: site-specific modification of the Ca(2+)-ATPase

Benzophenone (BP) was used as a photosensitizer to initiate lipid peroxidation in model and native biological membranes at concentrations of BP that do not perturb bilayer structure, as assessed by stearic acid spin label dynamics. Illumination of BP partitioned into sarcoplasmic reticulum membranes (SR) results in an exponential decay of BP and a linear accumulation of conjugated dienes and other products of lipid peroxidation as observed previously for micelles of linoleic acid [Marcovic and Patterson. Photochem. Photobiol. 58:329-334, 1993]. Lipid peroxidation was substantially inhibited in the presence of membrane-spanning proteins in SR compared to protein-free lipid vesicles, suggesting the competitive reaction of the initiator (triplet BP) and BP-derived radical species with protein groups. Modification of the predominant integral membrane protein, the Ca(2+)-ATPase, was demonstrated by changes in Ca(2+)-ATPase amino acid composition as well as by its functional inhibition. The rate of calcium transport showed an immediate exponential decay to completion, while calcium-dependent ATPase activity exhibited an initial lag before modest inactivation. These results are consistent with the respective localization of calcium transport sites within membrane-spanning peptides and the ATP-binding site within the cytosolic domain of the Ca(2+)-ATPase, further suggesting that photosensitization of BP models oxidative stress inside the hydrophobic interior of the SR membrane.