Studies on Myosin from Red and White Skeletal Muscles of the Rabbit

Fiber-type traps: revisiting common misconceptions about skeletal muscle fiber types with application to motor control, biomechanics, physiology, and biology

Skeletal muscle is a highly complex tissue that is studied by scientists from a wide spectrum of disciplines, including motor control, biomechanics, exercise science, physiology, cell biology, genetics, regenerative medicine, orthopedics, and engineering. Although this diversity in perspectives has led to many important discoveries, historically, there has been limited overlap in discussions across fields. This has led to misconceptions and oversimplifications about muscle biology that can create confusion and potentially slow scientific progress across fields. The purpose of this synthesis paper is to bring together research perspectives across multiple muscle fields to identify common assumptions related to muscle fiber type that are points of concern to clarify. These assumptions include 1) classification by myosin isoform and fiber oxidative capacity is equivalent, 2) fiber cross-sectional area (CSA) is a surrogate marker for myosin isoform or oxidative capacity, and 3) muscle force-generating capacity can be inferred from myosin isoform. We address these three fiber-type traps and provide some context for how these misunderstandings can and do impact experimental design, computational modeling, and interpretations of findings, from the perspective of a range of fields. We stress the dangers of generalizing findings about "muscle fiber types" among muscles or across species or sex, and we note the importance for precise use of common terminology across the muscle fields.

Tripolyphosphate hydrolysis by bovine fast and slow myosin subfragment 1 isoforms

Polyphosphates are used in the meat industry to increase the water holding capacity of meat products. Tripolyphosphate (TPP) is a commonly used polyphosphate and it is metabolized into pyrophosphate and monophosphate in meat. The enzymes responsible for its metabolism have not been fully characterized. The motor domain of myosin (subfragment 1 or S1) is a likely candidate. The objectives of this study were to determine if bovine S1 hydrolyzes TPP, to characterize the TPPase activity of the fast (cutaneous trunci) and slow (masseter) isoforms, and to determine the influence of pH on S1 TPPase activity. S1 hydrolyzed TPP and in comparison with ATP as substrate, it hydrolyzed TPP 16-32% more slowly. Fast S1 hydrolyzed both substrates faster compared to slow S1 and the difference between the isoforms was greater with TPP as the substrate. The V(max) was 0.94 and 5.0 nmol Pi/mg S1 protein/min while the K(m) was 0.38 and 0.90 mM TPP for slow and fast S1, respectively. Pyrophosphate was a strong inhibitor of TPPase activity with a K(i) of 88 and 8.3 microM PPi for fast and slow S1 isoforms, respectively. Both ATPase and TPPase activities were influenced by pH with the activity being higher at low pH for both fast and slow S1 isoforms. The activity at pH 5.4 was 1.5 to 4-fold higher than that at pH 7.6 for the different isoforms and substrates. These data show that myosin S1 readily hydrolyzes TPP and suggest that it is a major TPPase in meat.

Mammalian skeletal muscle fiber type transitions

Mammalian skeletal muscle is an extremely heterogeneous tissue, composed of a large variety of fiber types. These fibers, however, are not fixed units but represent highly versatile entities capable of responding to altered functional demands and a variety of signals by changing their phenotypic profiles. This adaptive responsiveness is the basis of fiber type transitions. The fiber population of a given muscle is in a dynamic state, constantly adjusting to the current conditions. The full range of adaptive ability spans fast to slow characteristics. However, it is now clear that fiber type transitions do not proceed in immediate jumps from one extreme to the other, but occur in a graded and orderly sequential manner. At the molecular level, the best examples of these stepwise transitions are myofibrillar protein isoform exchanges. For the myosin heavy chain, this entails a sequence going from the fastest (MHCIIb) to the slowest (MHCI) isoform, and vice-versa. Depending on the basal protein isoform profile and hence the position within the fast-slow spectrum, the adaptive ranges of different fibers vary. A simple transition scheme has emerged from the multitude of data collected on fiber type conversions under a variety of conditions.

Myosin polymorphism and differential expression in adult human skeletal muscle

1. Myosin heavy chain (HC) and light chain (LC) isoforms are expressed in a tissue-specific and developmentally-regulated manner in human skeletal muscle. 2. At least seven myosin HC isoforms are expressed in skeletal muscle of the adult. 3. Histochemically-delineated fibre types (based on the stability of myofibrillar actomyosin adenosine triphosphatase activity) in limb muscles correlate with the myosin HC content. 4. Alterations in the phenotypic expression of myosin provides a mechanism of adaptation to stresses placed upon the muscle (e.g. increased and decreased usage).

In vitro non-enzymatic glycosylation of myofibrillar proteins

1. Glycation is non-enzymatic modification of proteins by sugars in which not only structural but also biological properties of proteins are altered. 2. Our in vitro experiments show that incubation of myofibrillar proteins with ribose results in sugar attachment to proteins and at the same time myofibrillar ATPase activity is lowered. 3. DETAPAC, aminoguanidine and 2-mercaptoethanol all partially block myofibrillar protein glycation and ATPase activity is less inactivated. 4. The dependence of ATPase activity of myofibrils incubated with ribose on the amount of 2-mercaptoethanol present suggests that also modification of SH groups is involved in enzyme inactivation. 5. Electrophoretic studies revealed that heavy chains of myosin, actin, and tropomyosins are proteins which are mainly glycated in vitro.

Comparison of calcium release from sarcoplasmic reticulum of slow and fast twitch muscles

The mechanism of Ca2+ release from the sarcoplasmic reticulum (SR) of slow and fast twitch muscle was compared by examining biochemical characteristics, ryanodine binding, Ca2+ efflux, and single Ca2+ channel properties of SR vesicles. Although many features of the Ca2+ release channel were comparable, two functional assays revealed remarkable differences. The comparable properties include: a high molecular weight protein from both types of muscle was immunologically equivalent, and Scatchard analysis of [3H]ryanodine binding to SR showed that the Kd was similar for slow and fast SR. In the flux assay the sensitivity to the agonists caffeine, doxorubicin, and Ca2+ and the antagonists Mg2+, ruthenium red, and tetracaine differed only slightly. When SR vesicles were incorporated into lipid bilayers, the single-channel conductances of the Ca2+ release channels were indistinguishable. The distinguishing properties are: When Ca2+ release from passively 45Ca(2+)-loaded SR were monitored by rapid filtration, the initial rates of Ca2+ release induced by Ca2+ and caffeine were three times lower in slow SR than in fast SR. Similarly, when Ca2+ release channels were incorporated into lipid bilayers, the open probability of the slow SR channel was markedly less, mainly due to a longer mean closed time. Our results indicate that slow and fast muscle have ryanodine receptors that are biochemically analogous, yet functional differences in the Ca2+ release channel may contribute to the different time to peak contraction observed in intact slow and fast muscles.

Relationship of primary and secondary myogenesis to fiber type development in embryonic chick muscle

The formation of fast and slow myotubes was investigated in embryonic chick muscle during primary and secondary myogenesis by immunocytochemistry for myosin heavy chain and Ca2(+)-ATPase. When antibodies to fast or slow isoforms of these two molecules were used to visualize myotubes in the posterior iliotibialis and iliofibularis muscles, one of the isoforms was observed in all primary and secondary myotubes until very late in development. In the case of myosin, the fast antibody stained virtually all myotubes until after stage 40, when fast myosin expression was lost in the slow myotubes of the iliofibularis. In the case of Ca2(+)-ATPase, the slow antibody also stained all myotubes until after stage 40, when staining was lost in secondary myotubes and in the fast primary myotubes of the posterior iliotibialis and the fast region of the iliofibularis. In contrast, the antibodies against slow muscle myosin heavy chain and fast muscle Ca2(+)-ATPase stained mutually exclusive populations of myotubes at all developmental stages investigated. During primary myogenesis, fast Ca2(+)-ATPase staining was restricted to the primary myotubes of the posterior iliotibialis and the fast region of the iliofibularis, whereas slow myosin heavy chain staining was confined to all of the primary myotubes of the slow region of the iliofibularis. During secondary myogenesis, the fast Ca2(+)-ATPase antibody stained nearly all secondary myotubes, while primaries in the slow region of the iliofibularis remained negative. Thus, in the slow region of the iliofibularis muscle, these two antibodies could be used in combination to distinguish primary and secondary myotubes. EM analysis of staining with the fast Ca2(+)-ATPase antibody confirmed that it recognizes only secondary myotubes in this region. This study establishes that antibodies to slow myosin heavy chain and fast Ca2(+)-ATPase are suitable markers for selective labeling of primary and secondary myotubes in the iliofibularis; these markers are used in the following article to describe and quantify the effects that chronic blockade of neuromuscular activity or denervation has on these populations of myotubes.

Lysosomal proteinase-sensitive regions in fast and slow skeletal muscle myosins

We investigated the limited proteolysis of fast and slow myosins purified from rabbit psoas major and semimembranosus proprius muscles, respectively, by the main lysosomal proteinases: cathepsins B, H, L, and D. In EDTA containing buffer, cathepsin D cleaved both myosins only at the rod-S1 junction leading to the formation of two S1 fragments of slightly higher Mr than the three forms obtained with chymotrypsin. On addition of MgCl2 instead of EDTA, myosin hydrolysis was markedly reduced. In contrast, irrespective of the presence of either MgCl2 or EDTA, cathepsin B hydrolysed both myosins into HMM and LMM. Cathepsin L digested myosins more extensively than cathepsins B and D and the main fragments generated were, in decreasing order of importance, rod, S1, S2, HMM, and LMM. In the incubation conditions tested, cathepsin H displayed nondetectable action on myosins. As fast and slow myosin digest patterns were compared, the main differences observed concerned the size of the proteolytic products and the rate of hydrolysis, which was about 4-fold higher for the fast than for the slow isoform. This appeared consistent whatever enzyme was considered.

Adult human masseter muscle fibers express myosin isozymes characteristic of development

Masseter muscle biopsies were obtained from nine patients undergoing orthognatic surgery or surgery for parotid tumors. A detailed enzyme-histochemical and immunocytochemical study of these muscles was performed using antibodies specific to the various isozymes of the myosin heavy chain (MHC) in order to identify the MHC isozymes that were present in the different fiber types. The contractile proteins in these same biopsies were analyzed by two-dimensional gel electrophoresis, native pyrophosphate gel electrophoresis, and by an immunopolypeptide mapping approach. These studies have shown that there is a very heterogeneous distribution of the myosin isozymes, with many fibers containing more than one myosin type. We also present evidence that in addition to adult fast and slow myosin, the human masseter muscle contains two proteins, neonatal MHC and embryonic myosin light chain, that are characteristic of developing muscle.

Heterogeneity of myofibrillar proteins in lobster fast and slow muscles: variants of troponin, paramyosin, and myosin light chains comprise four distinct protein assemblages

Fast and slow muscles from the claws and abdomen of the American lobster Homarus americanus were examined for adenosine triphosphatase (ATPase) activity and for differences in myofibrillar proteins. Both myosin and actomyosin ATPase were correlated with fiber composition and contractile speed. Four distinct patterns of myofibrillar proteins observed in sodium dodecyl sulfate-polyacrylamide gels were distinguished by different assemblages of regulatory and contractile protein variants. A total of three species of troponin-T, five species of troponin-I, and three species of troponin-C were observed. Lobster myosins contained two groups of light chains (LC), termed "alpha" and "beta." There were three alpha-LC variants and two beta-LC variants. There were no apparent differences in myosin heavy chain, actin, and tropomyosin. Only paramyosin showed a pattern completely consistent with muscle fiber type: slow fibers contained a species (105 kD) slightly smaller than the principle variant (110 kD) in fast fibers. It is proposed that the type of paramyosin present could provide a biochemical marker to identify the fiber composition of muscles that have not been fully characterized. The diversity of troponin and myosin LC variants suggests that subtle differences in physiological performance exist within the broader categories of fast- and slow-twitch muscles.

Functionality of muscle proteins in gelation mechanisms of structured meat products

Recent advances in muscle biology concerning the discoveries of a large variety of proteins have been described in this review. The existence of polymorphism in several muscle proteins is now well established. Various isoforms of myosin not only account for the difference in physiological functions and biochemical activity of different fiber types or muscles, but also seem to differ in functional properties in food systems. The functionality of various muscle proteins, especially myosin and actin in the gelation process in modal systems which simulate structured meat products, is discussed at length. Besides, the role of different subunits and subfragments of myosin molecule in the gelation mechanism, and the various factors affecting heat-induced gelation of actomyosin in modal systems are also highlighted. Finally, the areas which need further investigation in this discipline have been suggested.

Ca2+-activated force-generating properties of mammalian skeletal muscle fibres: histochemically identified single peeled rabbit fibres

Single peeled (sarcolemma removed) rabbit skeletal muscle fibres, identified histochemically from their myofibrillar ATPase and oxidative staining patterns, were characterized according to their Ca2+-activated steady-state force-generating properties at normal intracellular pH (7.0) and under acidotic (pH 6.5) conditions. Maximum force-generating capacity of each fibre was assessed by measuring steady-state isometric force generation at saturating Ca2+ concentration at both pH values. The Ca2+ sensitivity of each fibre was ascertained by determining the percentage of maximum force generated at each of several subsaturating Ca2+ concentrations at both pH values. Fibres were selected from soleus, tibialis anterior and adductor magnus muscles. At subsaturating Ca2+ concentrations only two functional groups of fibres were distinguishable, corresponding to the histochemical classifications type I and type II. Type I fibres were more sensitive to Ca2+ and less depressed by acidosis than type II fibres in the subsaturating range of Ca2+ concentrations. At saturating Ca2+ concentrations, the acidotic depression of maximum force was significantly less for type I fibres than type II nonoxidative fibres regardless of their muscle of origin. Type II oxidative fibre maximum force properties depended upon the muscle of origin and demonstrated subgroups of these fibres that were different from type II nonoxidative fibres and similar to type I fibres.

Differentiation of fiber types in wing muscles during embryonic development: effect of neural tube removal

The embryonic precursors of the avian slow (type I and III) and fast (type II) fibers can be distinguished from each other early in muscle formation (stage 28, V. Hamburger and H. L. Hamilton, J. Morphol, 88, 49-92, 1951) on the basis of the differential sensitivity of their myosin ATPases. To test the neural dependence of fiber type differentiation, the source of motor innervation was eliminated by excision of the brachial neural tube at stages 16-18 before muscles are innervated. Removal of the brachial neural tube did not affect the number of primary myotubes in a sample muscle of the forelimb (ulnimetacarpalis dorsalis, UMD) up until stage 36. Myosin ATPase staining at a variety of pHs revealed the typical patterns of fiber types in muscles of neural-tube free embryos in stages 35-37. These muscles included the anterior latissimus dorsi, brachialis, and UMD which showed presumptive type III staining (type IIIEMB), the pronator superficialis and flexor carpi ulnaris which showed embryonic type II staining (type IIEMB), and the triceps brachii muscles which showed characteristic arrangements of both type IEMB and type IIEMB fibers. The normal patterns of type IEMB and type IIEMB myotubes were also seen in muscles containing a heterogeneous mixture of fiber types such as the biceps brachii, extensor metacarpi radialis, and adductor indicis muscles, although the intensity of acid-stable ATPase staining of the type IEMB myotubes in these muscles was lower than in innervated muscles. It is concluded that the earliest differentiation of muscle fiber types is independent of the nervous system.

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.

Unusual features of the Ca2+-ATPase activity of myosin from fast skeletal muscle of the frog: effect of actin and SH1 thiol group modification

The K+-ATPase and actin-activated Mg2+-ATPase activity of myosin from fast skeletal muscle of the frog, Rana esculenta or Rana temporaria, are comparable to the respective activities of rabbit fast skeletal muscle. On the other hand, the Ca2+-ATPase activity of the same preparations of frog myosin is 6-7-fold lower than that of myosin from rabbit muscle. Various control experiments indicate that the small extent of Ca2+ stimulation is an intrinsic property of frog muscle myosin. Unlike myosin from rabbit muscle, the Ca2+-ATPase activity of frog myosin is strongly activated by actin; at high actin concentrations it approaches the level of the Ca2+-ATPase activity of rabbit myosin. The levels of Ca2+-ATPase activity of frog and rabbit myosins also become comparable upon modification of myosin SH1 thiol groups; this means that the modification of the SH1 groups results in a much higher activation of the Ca2+-ATPase of frog myosin than that of rabbit myosin. The results suggest a difference in the active site conformation in frog and rabbit muscle myosins. The effects of actin and SH1 group modification are discussed in terms of allosteric changes which diminish the difference in the active site conformation of the two myosins. We have also observed a difference in the reactivity of thiol groups which are not essential for the enzymatic activity in frog and rabbit myosin, indicating structural differences in regions other than the active site.

Myosin transformation in hypertrophied rat muscle

Chronic hypertrophy (X = 88%) of the fast-twitch rat plantaris muscle was induced by surgical ablation of the synergistic gastrocnemius. Alterations in the catalytic activity and the light chain composition of myosin extracted from the hypertrophied plantaris muscle were examined. The hypertrophied muscle exhibited a decreased Ca2+-activated myosin ATPase activity and an increase in the content of LCS1 and LCS2 light chains when compared to its contralateral control. These changes, which occur in the absence of a direct manipulation of the muscle's innervation, suggest that the histochemically observed increase in the percent of alkaline labile fibres found in hypertrophied muscle are the result of the expression of a different myosin phenotype.

Characterization of myosin heavy chain by cyanogen bromide peptide maps

Procedures have been developed for the preparation of pure myosin heavy chain (h-myosin) by preparative gel electrophoresis, and for the characterization of h-myosin by cyanogen bromide peptide mapping. Major sources of error are the oxidation of methionine and the proteolytic splitting of the chain during purification. These errors have been eliminated. A peculiar feature is the doubling or quadrupling of a peptide of molecular weight 17 000. The results show structural differences between isomyosins derived from myonal types within the same animal, as well as interspecies differences.

Myosin isoenzymes in adult rat skeletal muscles

The myosin isoenzymic content of several adult rat muscles has been analyzed by electrophoresis under non-dissociating conditions. Fast-twitch fibres, whether of the oxidative type, such as the red masseter, or of the glycolytic type, such as the white tensor fasciae latae, are all shown to contain three isomyosins with respective mobilities identical in both types of muscles. These three isoenzymes, which, as in the case of myosins from avian fast muscles, represent alkali light chain hetero-and homodimers with similar large subunits, occur in somewhat variable relative proportions depending on the muscle. No obvious correlation was established between the type of the fast fibres--either oxidative or glycolytic--and the type of the myosins. Besides the three fast isoenzymes, other muscles, such as the predominantly fast latissimus dorsi and the mixed diaphragm, are shown to contain one myosin species of lower electrophoretic mobility; this supplementary isoenzyme comigrates with the major component of the predominantly slow-twich soleus muscle, but differs from the avian slow-tonic isoform. Ca2+ ATPase determinations on gel indicate that the fast isomyosins all display similar activity, which is five to ten times higher, depending on the experimental conditions of the assay, than the activity shown by the slow isoenzymes. Altogether, at least five isoenzymes, corresponding to one "slow-twich", one "slow-tonic", and three "fast-twitch" myosin species, were detected in rat skeletal muscles.U

The relation between myosin Ca2+ - and K+ -adenosine triphosphatase activity of heart muscles in mammals of different size

1. The influence of KCl and CaCl2 on ATPase activity of ventricular myosin of the mouse, rat, rabbit and cow, the temperature dependence of ATPase and the effect of pCMB treatment and tryptic digestion on ATPase activity of these myosins were studied. 2. Ca2+ - and K+ -ATPase activities of myosins were inversely related to body size of the animal species; when K+ -ATPase activities were measured in the absence of EDTA, the body size/ATPase dependence was only slightly apparent. 3. The influence of temperature, the effect of pCMB and the influence of tryptic digestion on Ca2+ - ATPase activity distinguished the compared myosins. 4. There was a marked alteration of the effect of myosin treatment with pCMB or trypsin on K+ -ATPase activity of these myosins and in this case differences in K+ -ATPase activities were less pronounced.

Dynamic aspects of structural proteins in vertebrate skeletal muscle

In this review, our current knowledge on the structural proteins of vertebrate skeletal muscle is briefly outlined. Structural proteins include the contractile proteins (actin and myosin), the major regulatory proteins (troponin and tropomyosin), the minor regulatory proteins (M-protein, C-protein, F-protein, I-protein, and actinins), and the scaffold proteins (connectin, desmin, and Z-protein). In addition, the relative turnover rates of the muscle proteins (M-protein greater than or equal to troponin greater than soluble protein as a whole greater than tropomyosin not equal to alpha-actinin greater than myosin greater than 10S-actinin greater than actin) are discussed. The changes in the turnover of muscle proteins are compared in denervated and dystrophic muscles. The properties of the various proteases in muscle, including alkaline protease, calcium-activated neutral protease (CANP), and acidic protease (cathepsins), and the structural alterations of myofibrils by these proteases are also described. Finally, the role of proteases and their inhibitors in diseased muscle is summarized, with focus on CANP and its inhibitors, leupeptin and E-64.

Immunological and biochemical evidence for atrial-like isomyosin in thyrotoxic rabbit ventricle

Immunological, structural and enzymatic characteristics of atrial and ventricular myosin from euthyroid rabbits were analyzed and compared with ventricular myosin from hyperthyroid animals. (1) Specific antibodies against bovine atrial myosin were found to react selectively in double-immunodiffusion and enzyme immunoassay with both rabbit atrial myosin and ventricular myosin from thyroxine-treated animals. These specific anti-bovine atrial myosin antibodies reacted with the heavy chains of thyrotoxic ventricular myosin when examined by enzyme immunoassay combined with SDS-polyacrylamide gel electrophoresis. (2) In one-dimensional SDS-polyacrylamide gel electrophoresis, no difference could be demonstrated in the light chain pattern of ventricular myosin from euthyroid and hyperthyroid rabbit hearts. One-dimensional analysis of myosin heavy chains after chymotryptic digestion in the presence of SDS showed significant differences between the two ventricular myosins. Also, the peptide maps from atrial myosin resembled the pattern of peptides found with ventricular heavy chains from hyperthyroid rabbits. The steady state rate, the alkali stability and the pH sensitivity of Ca2+-ATPase activity of thyrotoxic ventricular myosin were very similar to those of atrial myosin. (3) These results provide direct immunochemical and biochemical evidence for the existence of an atrial-like isomyosin in thyrotoxic rabbit ventricles.

Actomyosin ATPase. II. Fiber typing by histochemical ATPase reaction

A new method for fiber typing based on staining for actomyosin Ca,Mg-ATPase is presented. Inclusion of ethanol in the medium enhanced the differentiation of type 2A and 2B fibers. With this technique, type 1, 2A, and 2B fibers can be distinguished in a single-step procedure for human biopsy samples.

Actomyosin ATPase. I. Quantitative measurement of activity in cryostat sections

A method for the quantitative study of the actomyosin ATPase activity (Ca,MG-ATPase) in thin sections cut in a cryostat is presented. This method is based on the liberation of 32P from [gamma 32P]ATP or 45Ca phosphate precipitation. The advantage of this method lies in the requirement for only a small muscle sample and the availability of serially cut sections for other assays including Ca uptake by sarcoplasmic retciulum and histochemical tests for oxidative and glycolytic enzymes. The actomyosin ATPase activity for various types of muscles determined by this method showed the same sequence found in isolated protein, that is, fast-twitch skeletal greater than slow-twitch skeletal greater than cardiac. The Ca,Mg-ATPase of cryostat sections showed Ca sensitivity. The fact that the sections retained Ca sensitivity at 37 degrees C, in contrast to myofibrils, which have been reported to lose Ca sensitivity at this temperature, indicates that the structural integrity of the contractile and regulatory apparatus is preserved to a higher degree in sections than in isolated myofibrils.

An electrophoretic study of native myosin isozymes and of their subunit content

Myosin polymorphism in muscles has been studied by a variety of electrophoretic techniques, in non-dissociating and in dissociating conditions. The analysis of myosin isozymes in the native state was achieved in pyrophosphate buffer and required only minute amounts of protein; identical results were obtained with purified or crudely extracted myosin. The determination of the subunit content of each isozyme was done in the presence of sodium dodecyl sulphate or urea for light chain, and in a phenol, acetic acid and urea system for heavy chain screening. Electrophoresis in non-dissociating conditions has led to the separation of up to a dozen of myosin isozymes, differing in mobilities by as much as 30%. Muscle specificity of myosin was clearly established. Apart from a few exceptions, all the muscles tested were shown to contain more than one myosin species; fast-twitch muscles for instance all contained the same three isozymes, but in variable ratios. Class specificity of myosin appeared related to the relative proportions of isozymes in a given muscle. A second electrophoresis in dissociating solvents of the myosin bands first resolved in pyrophosphate buffer has then allowed a further characterization of the various isozymes. The differences in mobilities observed in the native state were shown to come either from the light chains, or from the heavy chains, or from both. The first case was illustrated by the three species present in fast muscles, which were shown to correspond to three alkali light-chain isozymes, the heterodimer representing in some instances up to 40% of the total. Next to light-chain muscle type specificity, electrophoresis in the phenol, acetic acid, urea system has led to the detection of differences in the heavy chains of fast, slow and cardiac myosins. The application of these various electrophoretic techniques to the analysis of the modification of myosin isozymes during development or in pathology studies can be considered.

Enzymatic activities and ATP-induced fluorescence enhancement of myosin from fast and slow skeletal and cardiac muscles

The maximal ATP-induced enhancement of fluorescence and the dependence of this enhancement on ATP concentration were determined for myosins from fast and slow skeletal and cardiac muscle of the rabbit. With myosins from slow and cardiac muscle modifications in the preparative procedure and chromatography on DEAE-Sephadex were required to obtain preprations which were free of actin, which exhibited the maximal fluorescence enhancement and which bound two moles of ATP per mole of myosin. Since the fluorescence enhancement of cardiac and slow muscle myosins is labile at slightly alkaline pH, it was also necessary to minimize incubation at pH greater than 7 in order to attain the maximal enhancement. With fast muscle myosin the changes in preparative procedure, together with chromatography, led to a 50 to 100% increase in the steady-state rate of ATP hydrolysis and fluorescence enhancement, without changing the maximal binding of ATP. From a comparison of the rate of steady-state hydrolysis of ATP with the rate of decay of the enhanced fluorescence, it appears that for all three myosins, both ATP binding sites have the same enzymatic activity, the steady-state rate per site being slower for cardiac and slow muscle myosins than for fast muscle myosin.

The relationship of mechanical Vmax to myosin ATPase activity in rabbit and marmot ventricular muscle

Papillary muscle mechanics and ventricular myosin calcium-activated ATPase activity were measured in the same heart as a function of temperature (8--28 degrees) in rabbits and marmots, in order to examine further the hypothesis that the velocity of cardiac muscle shortening at zero load (Vmax) is correlated with myosin ATPase activity. There was a similar Q10 for Vmax in each muscle type, as measured with isotonic afterloaded quick-releases at 30--33% time-to-peak tension; the calcium activated ATPase of myosin in the two muscle types also was similar. The least squares linear regression of rabbit Vmax on calcium-activated myosin ATPase activity was the same as in the marmot, so all the data were pooled to yield a linear regression (Y = 0.47 +/- 3.82X) with a high correlation between the two variables [r = 0.95, P less than 0.01 (ANOV)]. Furthermore, the correlation proved to be predictive of cardiac Vmax and myosin ATPase activity levels in other experiments where these two measurements decreased below normal as a result of hypertrophic growth. Consequently, the quantitative relationship between Vmax and myosin ATPase defined here may prove to be predictive of the ability of cardiac muscle to release bond energy.

Myosin changes in experimental 2,4-dichlorophenoxyacetate myopathy

Young rats treated with 10 to 14 daily injections of 2,4-dichlorophenoxyacetate (2,4-D) developed a myopathy mainly involving fast muscles. Myosin isolated from the gastrocnemius muscles of treated and normal control animals differed in several respects. The Ca2+- and Mg2+-mediated ATPases were higher in myopathic muscle myosin than in normals. Alkylation of thiols by N-ethylmaleimide (NEM) induced an increase of Ca2+-activated ATPase that was higher in normal than in myopathic myosin. Trinitrophenylation of reactive amino groups by 2,4,6-trinitrobenzene sulfonate (TBS) induced on increase in Mg2+-mediated ATPase in both preparations, but the increase was higher in normals. Although the heavy- and light-chain pattern was identical in normal and myopathic myosin, during storage at 0 degrees C the relative amount of myopathic L2 light chain decreased. Myosins fragmented either by limited proteolysis with trypsin and chymotrypsin or by specific cleavage at tryptophanyl and cysteinyl peptide bonds showed differences on sodium dodecylsulfate (SDS)-polyacrylamide-gel electrophoresis. The results indicate that there is a change in the heavy chains of myosin isolated from the gastrocnemius muscle in 2,4-D-induced rat myopathy.

Morphological and biochemical correlates of skeletal muscle contractility in the cat. II. Physiological and biochemical studies

Isometric twitch characteristics and biochemical parameters of isolated myosin and sarcoplasmic reticulum have been compared in three cat hind limb muscles. The fast twitch caudofemoralis and the slow twitch soleus are almost pure muscles as judged from histochemical studies. Isolated myosin from the caudofemoralis is not only 2- to 3-fold higher in its ATPase activities than that of the soleus, but also in non-dissociated forms has greater electrophoretic mobility than the soleus myosin. Purified myosins from fast muscles as well as soleus exhibited three light chains upon electrophoresis. However, the intact non-solubilized myosins differed in electrophoretic mobilities. The sarcoplasmic reticulum fraction isolated from caudfemoralis exhibits faster rates of Ca++ binding and uptake than soleus, and when fit to a two component model, the caudofemoralis SR exhibits a higher amount of a fast binding site than does soleus SR, features reflected in differences in the relaxation time of the two muscles. In contrast, the fast twitch tibialis anterior has been shown to be a gradient of fiber types and its isometric twitch may be separated by selective nerve stimulation, into a fast and a slow twitch component. Our findings that myosin fractions, as well as sarcoplasmic reticulum fractions isolated from these two components differ with respect to their biochemical characteristics add support to the possibility of a dual function in this muscle.

Comparative studies on myosin ATPase of a flying and nonflying bird

Myosin was purified from the flight muscles of a flying (pigeon) and a nonflying (fowl) bird. Ki (ADP) of myosin ATPase of pigeon is higher, but the Km (ATP) is lower than that of fowl. The specific activity (mumole of Pi liberated/min/mg protein) is higher for the fowl. A0.5 (CaCl2) of myosin of both pigeon and fowl is similar. However, the two proteins differ in their interactions with ADP, ATP and p-chloromercuribenzoate. The two proteins have the same tyrosine, tryptophan and sulfhydryl contents. The electrophoretic patterns of the two myosins on SDS-polyacrylamide gels are different. These studies show significant molecular differences in the myosin derived from the flight muscles of a flying (pigeon) and a nonflying (fowl) bird.

Myosin polymorphism in human skeletal muscles

Myosins isolated from individual human muscles (primarily normal muscles) were investigated with respect to their structural and catalytic properties. The results indicate unexpected elements of uniformity shared by the several myosins, such as a three-banded, electrophoretic pattern of light chains in sodium dodecylsulfate (SDS) gels and a low degree of alkaline lability. The pH activity profile and the effect of KCl on myosin ATPase activities were also found to be the same for the myosins from predominantly fast (e.g., vastus lateralis and rectus abdominis) and slow (e.g,, soleus and pectoralis minor) muscles. Coelectrophoretic experiments lend further credence to the interrelationship between human myosin light chains and the light chains of rabbit fast-muscle myosin. However, several kinds of circumstantial evidence, such as that derived from the study of myosin in nemaline myopathy, suggest that one shoould exercise caution in interpreting these results. On the other hand, human muscle myosins, like those of other mammalian species, can be divided into two main categories according to the peptide composition of tryptic heavy meromyosin (HMM) and the banding pattern of light meromyosin (LMM) paracrystals. These results, which are indicative of differences in the primary structure of the heavy chains, allow us to identify these heavy chains as the main site of heterogeneity among myosins in human mucles.

Histochemical characterization of the red fibres in pigeon pectoralis muscle

Red fibres of the pigeon pectoralis muscle showed high ATPase reaction at pH 9.4. Veronal-acetate pretreatment completely inhibited the ATPase reaction in these red fibres but not in type I fibres of the gastrocnemius. The former are type II red muscle fibres and hence are unlike type I red, the so-called slow-twitch muscle fibres.