The bound nucleotide of the isolated myofibril

Proteomics of the heart

Mass spectrometry-based proteomics is a sophisticated identification tool specializing in portraying protein dynamics at a molecular level. Proteomics provides biologists with a snapshot of context-dependent protein and proteoform expression, structural conformations, dynamic turnover, and protein-protein interactions. Cardiac proteomics can offer a broader and deeper understanding of the molecular mechanisms that underscore cardiovascular disease, and it is foundational to the development of future therapeutic interventions. This review encapsulates the evolution, current technologies, and future perspectives of proteomic-based mass spectrometry as it applies to the study of the heart. Key technological advancements have allowed researchers to study proteomes at a single-cell level and employ robot-assisted automation systems for enhanced sample preparation techniques, and the increase in fidelity of the mass spectrometers has allowed for the unambiguous identification of numerous dynamic posttranslational modifications. Animal models of cardiovascular disease, ranging from early animal experiments to current sophisticated models of heart failure with preserved ejection fraction, have provided the tools to study a challenging organ in the laboratory. Further technological development will pave the way for the implementation of proteomics even closer within the clinical setting, allowing not only scientists but also patients to benefit from an understanding of protein interplay as it relates to cardiac disease physiology.

The nature of the muscle-relaxing factor. I. An improved assay system

Analysis of the effects of deoxycholate and benzalkonium chloride on granule relaxing activity, myofibril and glycerinated fiber responses to ATP, and the sensitivity of these actomyosin-containing systems to relaxing factors have established that it is possible to differentially inactivate intrinsic granule relaxing factor. It is therefore possible to use these agents to treat myofibrils and glycerinated muscle fibers so that they may be used for the assay of muscle-relaxing factor preparations. The destruction of intrinsic relaxing factor can be produced by treating actomyosin with 1 mM deoxycholate or 0.005 per cent benzalkonium chloride for 5 minutes at room temperature.

Tropomodulin caps the pointed ends of actin filaments

Many proteins have been shown to cap the fast growing (barbed) ends of actin filaments, but none have been shown to block elongation and depolymerization at the slow growing (pointed) filament ends. Tropomodulin is a tropomyosin-binding protein originally isolated from red blood cells that has been localized by immunofluorescence staining to a site at or near the pointed ends of skeletal muscle thin filaments (Fowler, V. M., M. A., Sussman, P. G. Miller, B. E. Flucher, and M. P. Daniels. 1993. J. Cell Biol. 120: 411-420). Our experiments demonstrate that tropomodulin in conjunction with tropomyosin is a pointed end capping protein: it completely blocks both elongation and depolymerization at the pointed ends of tropomyosin-containing actin filaments in concentrations stoichiometric to the concentration of filament ends (Kd < or = 1 nM). In the absence of tropomyosin, tropomodulin acts as a "leaky" cap, partially inhibiting elongation and depolymerization at the pointed filament ends (Kd for inhibition of elongation = 0.1-0.4 microM). Thus, tropomodulin can bind directly to actin at the pointed filament end. Tropomodulin also doubles the critical concentration at the pointed ends of pure actin filaments without affecting either the rate of extent of polymerization at the barbed filament ends, indicating that tropomodulin does not sequester actin monomers. Our experiments provide direct biochemical evidence that tropomodulin binds to both the terminal tropomyosin and actin molecules at the pointed filament end, and is the long sought-after pointed end capping protein. We propose that tropomodulin plays a role in maintaining the narrow length distributions of the stable, tropomyosin-containing actin filaments in striated muscle and in red blood cells.

The effect of troponin C substitution on the Ca(2+)-sensitive ATPase activity of vertebrate and invertebrate myofibrils by troponin Cs with various numbers of Ca(2+)-binding sites

The effect of four different classes of troponin C with different numbers of Ca(2+)-binding sites was investigated on the Ca(2+)-activation profiles of the ATPase of troponin C-depleted myofibrils prepared from vertebrate fast skeletal (rabbit), vertebrate cardiac (bovine) and invertebrate crustacean tail striated (crayfish, lobster) muscles. Troponin C from vertebrate sources [fast skeletal (rabbit, chicken) with four Ca(2+)-binding sites, and cardiac (bovine, chicken) with three Ca(2+)-binding sites] activated the Ca(2+)-ATPase of troponin C-depleted myofibrils from the vertebrate skeletal or cardiac muscles, but did not activate the invertebrate troponin C-depleted crustacean myofibrils. On the other hand, two kinds of crustacean (crayfish, lobster) troponin C with two Ca(2+)-binding sites activated only crustacean troponin C-depleted myofibrils. One invertebrate molluscan (Akazara scallop) troponin C with one Ca(2+)-binding site did not activate the Ca(2+)-ATPase of the troponin C-depleted myofibrils from either vertebrate or crustacean striated muscles. The results obtained from the four kinds of combinations of troponin C and troponin C-depleted myofibrils from vertebrate skeletal and cardiac muscles demonstrated that the characteristic cooperativity of the Ca(2+)-activation profiles of both vertebrate skeletal and cardiac myofibrils was determined by the skeletal or cardiac origin of troponin C molecules, irrespective of the animal species, and the Ca(2+)-affinity of the myofibrillar ATPase was related to the skeletal or cardiac origin of both the troponin C and myofibrils. These findings indicated that each of the four classes of troponin C has its own characteristic Ca(2+)-activation profile for each kind of myofibril examined in the present study.

Use of cytosolic metabolite patterns to estimate free magnesium in normoxic myocardium

Cytosolic free magnesium (Mgf) is considered relatively constant. To test this concept, Mgf was estimated during hyperkalemic ventricular akinesis, normal and maximum adrenergic stimulation, and sulfate loading of the normoxic perfused guinea-pig heart. The Mgf estimates utilized a new sliding scale derived from the Mg(2+)-dependence of glyceraldehyde-3-phosphate dehydrogenase/phosphoglycerate kinase (GAPDH/PGK). The pseudo constant K'GAPDH.K'PGK was measured as ([creatine phosphate][3-phosphoglycerate][lactate]KLDH)/([creatine][Pi] [glyceraldehyde 3-phosphate][pyruvate]KCK), which varied with magnesium due to KCK (CK, LDH = creatine kinase, lactate dehydrogenase). However, the correct magnesium dependencies of the true constants KGAPDH.KPGK and KCK were taken from the literature. The [Mg2+] at which pseudo K'GAPDH.K'PGK equalled true KGAPDH.KPGK was the best estimate of Mgf.Mgf fell to approximately 0.13 mM in hyperkalemic arrest from a control of approximately 0.6 mM, rising to approximately 0.85 mM only during maximum adrenergic stress. Mgf increased further to approximately 1.3 mM during sulfate loading which induced ATP catabolism. Mgf and ATP were reciprocally related. Thus; (1) myocardial free [Mg2+] judged from GADPH/PGK mass-action relations changed appreciably only under extreme physiological states; (2) ATP was a major chelator of Mg2+ in perfused myocardium, i.e., acute ATP pool size reduction may be associated with increments in Mgf.

Determination of free creatine and phosphocreatine concentrations in the isolated perfused rat heart by 1H- and 31P-NMR

To measure free creatine in the isolated perfused rat heart, the concentration of phosphocreatine, and phosphocreatine plus creatine (sigma Cr) were measured by 31P- and 1H-NMR, respectively. Quantification was performed in the presence and absence of an intraventricular balloon filled with a known amount of PCr, which acted as an external standard. Total (free plus bound) phosphocreatine and creatine were measured by HPLC analysis of extracts from the same hearts, freeze-clamped at the end of the perfusions. A greater concentration of creatine (mumol/g dry wt.) in the perfused rat heart was measured by HPLC analysis (40.3 +/- 2.38 (11)) as compared to NMR (34.6 +/- 1.95 (11)), whilst no significant difference was observed in the measurement of phosphocreatine between the two assay methods. Consequently, a greater sigma Cr was measured by HPLC. This work suggests that the majority of Cr in the heart is NMR visible and unbound, so available to interact with creatine kinase. The lower free ADP concentration calculated from NMR measurements (53.3 +/- 3.80 microM (9)) was not significantly different from that determined by HPLC analysis (56.9 +/- 5.90 microM (9)). This suggests that the concentration of free ADP in the heart is higher than values where it can regulate oxidative phosphorylation most effectively.

The effect of Mg2+ on the Ca2+ binding to troponin C in rabbit fast skeletal myofibrils

The effect of Mg2+ on the Ca2+ binding to rabbit fast skeletal troponin C and the CA2+ dependence of myofibrillar ATPase activity was studied in the physiological state where troponin C was incorporated into myofibrils. The Ca2+ binding to troponin C in myofibrils was measured directly by 45Ca using the CDTA-treated myofibrils as previously reported (Morimoto, S. and Ohtsuki, I. (1989) J. Biochem. 105, 435-439). It was found that the Ca2+ binding to the low and high affinity sites of troponin C in myofibrils was affected by Mg2+ competitively and the Ca2(+)- and Mg2(+)-binding constants were 6.20 x 10(6) and 1.94 x 10(2) M-1, respectively, for the low affinity sites, and 1.58 x 10(8) and 1.33 x 10(3) M-1, respectively, for the high affinity sites. The Ca2+ dependence of myofibrillar ATPase was also affected by Mg2+, with the apparent Ca2(+)- and Mg2(+)-binding constants of 1.46 x 10(6) and 276 x 10(2) M-1, respectively, suggesting that the myofibrillar ATPase was modulated through a competitive action of Mg2+ on Ca2+ binding to the low affinity sites, though the Ca2+ binding to the low affinity sites was not simply related to the myofibrillar ATPase.

Subcellular distribution of malate-aspartate cycle intermediates during normoxia and anoxia in the heart

The subcellular distribution of adenine nucleotides, phosphocreatine and intermediates of the malate-aspartate cycle was investigated in adult rat heart myocytes under normoxia and anoxia. Cytosolic and mitochondrial concentrations of metabolites were determined by a fractionation method using digitonin. Under normoxia, cytosolic/mitochondrial gradients were found for ATP (c/m = 4), AMP (c/m less than 0.01), citrate (c/m = 0.5), aspartate (c/m = 3), glutamate (c/m = 2), while phosphocreatine and glutamine were confined to the cytosolic space. No gradients were found for malate and 2-oxoglutarate. The results show that the transport of electrons from the cytosol into the mitochondria is supported by the glutamate gradient and by a high glutamate/aspartate ratio inside the mitochondria (Glu/Asp = 15) which is maintained by the energy-dependent Glu-Asp exchange across the mitochondrial membrane. Under anoxia, cytosolic glutamate is transaminated with pyruvate, yielding alanine and 2-oxoglutarate, which is oxidized to succinate inside the mitochondria and leaves the cell. The data indicate that stimulation of transamination is caused by a mass action effect following a decrease in cytosolic 2-oxoglutarate which may be due to succinate-2-oxoglutarate exchange across the mitochondrial membrane. Inhibition of the energy-dependent inward transport of glutamate may support this process.

Characteristic structures of actin gels induced with hepatic actinogelin or with chicken gizzard alpha-actinin: implication for their function

We studied the properties of actinogelin, a Ca2+-regulated actin cross-linking protein isolated from Ehrlich tumor cells or rat liver. Chicken gizzard alpha-actinin was used as a Ca2+-insensitive control. Actinogelin, which has very high gelation activity under low Ca2+ conditions, was found using electron microscopic or fluorescence studies to induce formation of a characteristic structure in which actin filaments and bundles radiate to (or converge from) all directions from spot-like core structures. A similar structure was induced with actinogelin, even in the presence of 0.7 saturation of tropomyosin. No such structure was detected with actinogelin under high Ca2+ conditions, and only a few were found with gizzard alpha-actinin. Because reconstituted structures are similar to those observed intracellularly, actinogelin may be important in the formation of similar microfilament organization in the cells. It seems also important that these structures are reconstituted with only two purified protein components, i.e., actinogelin and actin. Immunocompetition studies showed that actinogelin and gizzard alpha-actinin partially shared antigenicity, and their molecular shape and peptide maps were similar. Their amino acid compositions [Kuo et al., 1982], subunit and domain structures, and binding sites on actin [Mimura and Asano, 1987] are also very similar. Therefore, it is concluded that actinogelin belongs to alpha-actinin superfamily proteins. Furthermore, the presence of functionally different subfamilies concerned with Ca2+ sensitivity, gelation-efficiency, and others is discussed. Actinogelin, which induces networks of actin filaments, may be classified as high gelation type.

Cytosolic adenylates and adenosine release in perfused working heart. Comparison of whole tissue with cytosolic non-aqueous fractionation analyses

Free cytosolic adenylates were examined in relation to adenosine plus inosine released from perfused working guinea-pig hearts. Whole-tissue adenylate data from freeze-clamped hearts were quantitatively compared with corresponding values obtained by subcellular fractionation of homogenized myocardium in non-aqueous media. Adenosine and inosine in venous cardiac effluents were measured by high-performance liquid chromatography. Hearts, perfused at their natural flows, were subjected to various workloads, substrates and catecholamines to alter myocardial energy metabolism and respiration over a wide physiological range. Non-aqueous cytosolic ATP and creatine phosphate (CrP) accounted for more than 80% of the respective total myocardium content. The cytosolic CrP/Pi ratio was in near-quantitative agreement with the overall tissue CrP/Pi ratio when the latter parameter was corrected for extracellular Pi. This was conclusive evidence that ATP, CrP and Pi were predominantly located in the cytosol of the well-oxygenated cardiomyocyte. Measured myocardial oxygen uptake (MVO2) was reciprocally related to the phosphorylation state of CrP [( CrP]/[Cr] X [Pi]) and hence that of ATP [( ATP]/[ADP] X [Pi]) assuming the creatine kinase at near-equilibrium at a near-constant pH of 7.2. On the other hand, calculated mean free cytosolic ADP concentrations increased essentially linearly up to threefold with increasing MVO2 in the presence of virtually unchanged or only slightly decreased ATP levels; this was found both according to the whole tissue and the special subcellular fractionation data. Employing the myokinase mass-action ratio and substituting total cardiac ADP by the mean free cytosolic ADP concentrations, the mean free cytosolic AMP concentrations proved to be in the nanomolar range, i.e. up to three orders of magnitude lower than the overall tissue AMP content. We propose, therefore, that in the normoxic heart, AMP is located predominantly in the mitochondrial compartment. Nevertheless, both free cytosolic AMP concentration and release of adenosine plus inosine were apparently square or even higher-power functions of the rate of cardiac respiration. On the other hand, the mean purine nucleoside release seemed linearly correlated (r = 0.920) with the calculated free cytosolic AMP concentration. Our observations seem to suggest that the concentrations of free ADP and AMP in the cytosol are major determinants of the production of inosine and coronary vasodilator adenosine.(ABSTRACT TRUNCATED AT 400 WORDS)

Myosin at the apical pole of ciliated epithelial cells as revealed by a monoclonal antibody

A monoclonal antibody (CC-212), obtained in a fusion experiment in which basal bodies from quail oviduct were used as immunogen, has been shown to label the apical pole of ciliated cells and to react with a 200-kD protein. This monoclonal antibody was demonstrated to be an anti-myosin from smooth muscle or from nonmuscular cells using the following criteria: On Western blots it reacted with the myosin heavy chains from gizzard and platelet extracts and from cultured cell line extracts, but did not react with striated muscle myosin heavy chains. By immunofluorescence it decorated the stress fibers of well-spread cells with a characteristic striated pattern, while it did not react with myotubes containing organized myofibrils. On native ciliated cells as well as on Triton-extracted ciliated cortices from quail oviduct, this monoclonal antibody decorated the apical pole with a stronger labeling of the periphery of the apical area. Ultrastructural localization was attempted using the immunogold technique on the same preparation. Myosin was associated with a filamentous material present between striated rootlets and the proximal extremities of the basal bodies. No labeling of the basal body itself or of axoneme was observed.

Metabolic recovery after exercise and the assessment of mitochondrial function in vivo in human skeletal muscle by means of 31P NMR

It has been suggested that the rate of phosphocreatine resynthesis after exercise is an index of mitochondrial oxidative metabolism in intact muscle. To investigate this hypothesis, the time courses of metabolite recovery following mild and more severe dynamic exercise of human forearm muscle were compared by means of 31P NMR. Severe exercise resulted in greater net hydrolysis of phosphocreatine and greater intracellular acidosis than light exercise. The rate of phosphocreatine resynthesis was significantly slower during recovery from the more severe exercise. To explain this it was noted that, as a consequence of the high activity of creatine kinase in the sarcoplasm, the [phosphocreatine] at any time is a function of the intracellular pH. Calculations demonstrate that the difference between rates of phosphocreatine recovery after the two exercise protocols was primarily determined by the rates of recovery of the intracellular pH to normal rest values. It is concluded that the calculated rate of recovery of the cytosolic free [ADP] to its pre-exercise concentration may provide a more specific measure of mitochondrial oxidative activity.

Separation of muscle proteins

This review covers various methods used in the separation and isolation of individual muscle contractile proteins. It is shown which methods have been most useful for the separation of contractile proteins and their fragments and in extending our knowledge of muscle biochemistry and physiology.

Muscle gelsolin: isolation from heart tissue and characterization as an integral myofibrillar protein

A 92-kDa polypeptide present in rabbit and dog cardiac muscle was purified to homogeneity and some of its properties were investigated using biochemical and cytochemical approaches. The protein was found to be similar, if not identical to macrophage gelsolin; it cross-reacts immunologically with anti-rabbit macrophage gelsolin antibody, has a Ca2+-sensitive shortening effect on the actin filaments as judged by the high shear viscometry and sedimentation experiments, and has a similar amino acid composition. In addition, immunoblot and SDS polyacrylamide gel analysis of cardiac muscle extracts obtained at high and low ionic strength showed that this protein is tightly bound to myofibrils, both in the absence and presence of Ca2+, in ventricular as well as in atrial muscle cells. Indirect immunofluorescence microscopy revealed a striated gelsolin staining pattern analogous to that previously observed for the skeletal muscle gelsolin, suggesting that in the muscle cell this protein is sharing the same localisation as actin. Because of its severing and nucleating properties the gelsolin may play a major role in the organization, assembly and turnover of the thin filaments within the muscle cells.

Structural differences between atrial and ventricular myosins from normal human hearts

Comparisons were made between myosins isolated from the right and left ventricles and the atria of normal human hearts. Parameters examined included electrophoretic mobilities of native molecules, K+ and Ca2+ dependent enzymatic activities, light chain composition, peptide patterns from partial proteolytic digests of entire heavy chains or rods, and maps of complete digests of specific 21 and 25 kilodalton heavy chain fragments. Human ventricular and atrial myosins differ in all parameters except in the charge of molecules. Structural differences between cardiac myosins derived from the two sources were apparent in both the head and tail portions of the heavy chains. With respect to the above parameters no differences were found between myosins from left and right human ventricles.

Quantitative determination of myosin and actin in rabbit skeletal muscle

The myosin and actin content of muscle tissue and purified myofibrils from rabbit psoas muscle has been determined. Myofibrils were purified using Percoll gradients, which allowed rapid separation from nuclei and connective tissue proteins. Myosin and actin were quantitated by amino acid analysis of the appropriate bands from sodium dodecyl sulfate/polyacrylamide gels. Muscle tissue contained 94 and 619 nmol/g wet weight of myosin and actin, respectively, while myofibrils had 0.82 and 5.37 mumol/g protein. Thus myosin contributed 43% and actin 22% of the myofibril protein mass. The value of 2.5 myosins per 14.3 nm repeat as calculated from these results suggests that thick filament models with mixtures of two and three crossbridges per repeat should be considered.

Evidence for new forms of cardiac myosin heavy chains in mechanical heart overloading and in ageing

Heavy chains of myosin rods and subfragment 1 were isolated from normal hearts and from mechanically overloaded hearts of young and older rats. These myosin heavy-chain fragments were cleaved by cyanogen bromide or partially proteolysed by pronase and by chymotrypsin after denaturation with sodium dodecyl sulfate. The peptides, analyzed by electrophoresis on a one-dimensional polyacrylamide slab gel, varied depending on the origin of the cardiac myosin heavy chains. Some bands present in the peptide patterns of the normal heart of young rats were missing from the pattern of greatly hypertrophied hearts and vice versa. We conclude that mechanical overloading of the heart stimulates the synthesis of cardiac myosin 'isozyme' with a heavy-chain primary structure which is different from that observed in the normal heart of young rat. The patterns from myosin heavy-chain peptides from the hearts of older rats were different from those for peptides from young rat hearts; these results also indicate the presence of a new myosin heavy chain specific to ageing. No difference was detected between the peptide patterns of heavy chains isolated from hypertrophied hearts of young and older rats, and those isolated from normal hearts of older rats.

Investigation of the short-time autolysis of rat hearts by means of SDS. Polyacrylamide gel electrophoresis and electron microscopy

The short-time autolysis of hearts was regarded as a model of ischaemic heart failure. Therefore, isolated rat hearts were subjected to 30--120 min autolysis in a Locke solution at 37 degrees C. Electron microscopic examinations and myofibrillar preparations were made from the autolysed heart ventricles. The myofibrillar proteins were resolved by SDS-polyacrylamide gel electrophoresis. After 30 min autolysis the amount of a protein of 192,000 daltons greatly increased. At the same time on the electron micrographs the focal destruction of filament destruction on the A filament area and the mitochondrial structure altered too. After 60 min autolysis another protein of 36,400 daltons appeared. On the electron micrographs the focal desintegration of Z membranes and the focal destruction of I filaments can be observed. After 120 min autolysis further proteolytic products could not be detected by gel electrophoresis but on the electron micrographs the destruction of Z membranes and I filaments became more pronounced.

Energy relationships between cytosolic metabolism and mitochondrial respiration in rat heart

Isolated rat heart was perfused with Langendorff's retrograde perfusion method, while the oxygen consumption and the left ventricular pressure were monitored continually. The steady-state contents of metabolites in the cardiac tissue, freeze clamped under various work-load conditions, were determined and the concentrations of free cytosolic ADP and AMP were calculated from the near equilibrium in creatine phosphokinase and adenylate kinase reactions. Increasing respiratory rate with increasing load was accompanied by a fall in the cytosolic free [ATP]/[ADP][Pi] but little change in the mitochondrial free [NAD+]/[NADH]. The free energy of ATP hydrolysis was calculated from the concentrations of the adenine nucleotides and compared with the values computed from the measured turnover number for cytochrome c and redox state of the mitochondrial NAD couple according to a mathematical model. The agreement between the two values was good over a wide range of metabolic conditions, which provides further support for the proposed near-equilibrium model of mitochondrial respiration with control exerted at the cytochrome oxidase-oxygen reaction.

[The post mortem alterations of myofibrilar proteins and adenosintriphosphatase activities of the skeletal muscles (author's transl)]

Immediately after the death of rabbits and at different times within 48 hours, we took out a part of the psoas muscle, from which we made myofibrilar preparations. The carcasses providing the muscle samples were held at two different temperatures. One group was held for 48 hours at 25 degrees C, imitating room temperature. The other group was held for 12 hours at 25 C degrees and at 25 C degrees and 12 hours at 10 C degrees, imitating daily temperature changes. Each myofibrilar sample was subjected to SDS-polyacrylamide gel electrophoresis. In addition we determined the Ca++ activated and the EGTA inhibited ATPase specific activity of the myofibrils. We found that within 48 hours the myofibrilar proteins were subjected to some characteristic proteolytic changes, which were dependant on the environmental temperature. The most interesting change was found in carcasses held constantly at 25 C degrees for 48 hours, where the EGTA inhibited ATPase activity was increased to about seven times its initial value, reflecting impairment of the troponin complex.

Damage of skeletal muscle in rats by immunoglobulins. I. Pathophysiological data and lightmicroscopic observations

An experimental model of the autoimmune process was developed to study the pathophysiological and structural changes in immunomyopathies. Immune-sera derived from three muscle proteins--myofibril, myosin and actin--produced immunomyopathies for study. The pathophysiological data show that the general muscle efficiency began to decrease with the early stages of treatment and reflected a severe disorder to muscle function with the progression of time. Parallel changes were also observed by light microscopy.