Cathepsin activity in normal and dystrophic human muscle

Metabolic status of patients with muscular dystrophy in early phase of the disease: In vitro, high resolution NMR spectroscopy based metabolomics analysis of serum

AIMS

Proton Nuclear Magnetic Resonance (NMR) based metabolomics analysis is extensively used to explore the metabolic profiling of biofluids. This approach was used for the analysis of metabolites in serum of patients with major types of muscular dystrophy in early phase of the disease.

MATERIAL AND METHODS

Proton NMR spectroscopy based qualitative (assignment of metabolites) and quantitative (quantification of metabolites) analysis of metabolites in native serum of patients with Duchenne muscular dystrophy (DMD) [n=88; n represent the number], Becker muscular dystrophy (BMD) [n=40], facioscapulohumeral dystrophy (FSHD) [n=22], limb girdle muscular dystrophy (LGMD)-2B [n=35] and myotonic dystrophy (DM) [n=21] as compared to normal subjects [n=50] were performed.

KEY FINDINGS

Quantity of branched chain amino acids was elevated in serum of patients with DMD, BMD, FSHD and DM-1 as compared to normal subjects. Acetate level was elevated in serum of patients with DMD, BMD, FSHD, LGMD-2B and DM-1 as compared to normal subjects. Level of glutamine was reduced in serum of patients with DMD, BMD, LGMD-2B, FSHD and elevated in DM-1 patients as compared to normal subjects. Quantity of tyrosine was increased in serum of BMD patients as compared to normal subjects. There was a reduction in the level of lysine in serum of FSHD, LGMD-2B and DM-1 patients as compared to normal subjects. Citrate level was reduced in serum of FSHD patients, but elevated in LGMD-2B patients. Lactate level was reduced in serum of LGMD-2B patients and histidine was reduced in serum of patients with FSHD as compared to normal subjects.

SIGNIFICANCE

Outcome of this study may be useful as supportive information for the existing diagnostic methods of the muscular dystrophy.

Quantitative determination and localization of cathepsin D and its inhibitors

A literature survey was performed of the methods of quantitative assessment of the activity and concentration of cathepsin D and its inhibitors. Usefulness of non-modified and modified proteins and synthetic peptides as measurement substrates was evaluated. The survey includes also chemical and immunochemical methods used to determine the distribution of cathepsin D and its inhibitors in cells and tissues.

Demonstration of cathepsins B, H and L in xenografts of normal and Duchenne-muscular-dystrophy muscles transplanted into nude mice

The activities and contents of the lysosomal cysteine proteinases cathepsins B, H and L were examined in xenografts of biopsied muscles transplanted from age-matched normal subjects and Duchenne-muscular-dystrophy (DMD) patients into nude mice. The activity of cathepsin B increased 9-fold and that of B-plus-L increased 24-fold in the first week after transplantation in normal muscle xenografts. By the third week, the activity of cathepsin B increased a total of 20-fold and B-plus-L increased to 36-fold the original level. The activity levels of cathepsin B, B-plus-L, H and D, and acid phosphatase in normal and DMD xenografts were not significantly different when compared 2 weeks after transplantation. However, the protein content of cathepsin B in DMD muscle xenografts was more than 3-fold that of normal xenografts at 2 weeks. The profile of cathepsin H activity in normal muscle xenografts was different than those of cathepsins B and B-plus-L. In the first week, the cathepsin H diminished sharply to about one-third of the biopsied muscle level and then, by 3 weeks after transplantation, it had increased slightly to about half the original level. The amount of endogenous cysteine-proteinase inhibitor changed in parallel with the activity of cathepsins B and B-plus-L. Cathepsins B and H, but not cathepsin L, were found immunohistochemically in regenerating muscle fibres of normal and DMD xenografts 2 weeks after transplantation. Staining of cathepsin B in DMD xenografts was slightly stronger than that in normal subjects. There was no immunostaining in degenerating or necrotic muscle fibres 2 weeks after transplantation. Western-blot analysis revealed that the cathepsin B band at 29 kDa was increased in normal xenografts 2 and 3 weeks after transplantation. Also, 2 weeks after transplantation the staining intensity of this band was slightly stronger in DMD xenografts than in normal xenografts. These results suggest that cathepsin B participates in the regeneration of transplanted muscle, both normal and DMD, and in the DMD muscle fibre-wasting processes, during regeneration.

An epoxysuccinic acid derivative(loxistatin)-induced hepatic injury in rats and hamsters

Loxistatin is a possible therapeutic agent of muscular dystrophy. A single oral administration of loxistatin to male rats caused focal necrosis of the liver with inflammatory cell infiltration. The severity of the lesions was dose-dependent up to 200 mg/kg and also manifest by an increase in serum alanine aminotransferase and aspartate aminotransferase activities. Hepatic glutathione (GSH) levels decreased with a maximum 20% depletion within 5 hr after the oral administration of loxistatin. Pretreatment with diethyl maleate did not potentiate the loxistatin-induced hepatic injury. On the other hand, the hepatoprotective effect of cysteamine was observed when cysteamine was administered 24 hr before loxistatin dosing, but the effect was not observed when the antidote was administered concomitantly with loxistatin. Pretreatment of rats with phenobarbital or trans-stilbene oxide provided partial protection against the hepatotoxic effect of loxistatin. Pretreatment with SKF-525A resulted in increased hepatic injury, while pretreatment with piperonyl butoxide, cimetidine, or 3-methylcholanthrene had no effect on hepatic damage by loxistatin. Five hours after [14C]loxistatin administration to rats, the covalent binding of the radioactivity to proteins was greatest in the liver, followed by the kidney, then muscle and blood to a lesser extent. [14C]Loxistatin acid, the pharmacologically active form of loxistatin, irreversibly bound to rat liver microsomal proteins; more binding occurred when the NADPH-generating system was omitted and when the microsomes were boiled first. GSH did not alter the extent of irreversible binding, whereas N-ethylmaleimide decreased the binding of [14C]loxistatin acid to rat liver microsomal proteins by 75%. Unlike the rat, administration of loxistatin to hamsters caused neither hepatic injury nor hepatic GSH depletion even at a high dose (500 mg/kg). Both the distribution and covalent binding of radioactivity in the hamster liver were one-third of those in rats following [14C]loxistatin dosing. These results suggest that loxistatin causes species-specific hepatotoxicity and that, at least in part, some of the toxic effects of loxistatin are mediated by the nonenzymatic covalent binding of loxistatin acid to thiol residues on cellular macromolecules.

High molecular weight kininogen, the extracellular inhibitor of thiol proteases, is deficient in hamsters with muscular dystrophy

High molecular weight kininogen has been shown to be the principal plasma inhibitor of cellular thiol proteases including cathepsins B, H and L and calpains 1 and 2. Since these same enzymes have been reported to be elevated in animals with muscular dystrophy, we studied plasmas from hamsters with muscular dystrophy and compared these to normal hamster plasma. The ability of plasma to inhibit purified platelet calpain was assayed and found to be 62% of normal. Since low molecular weight kininogen can also inhibit calpain, the coagulant activity of kininogen, an activity unique for high molecular weight kininogen, was determined in dystrophic hamster plasma and found to be 69% of normal in close agreement with the calpain inhibitory activity. The contribution of the other plasma calpain inhibitor alpha 2-macroglobulin appeared small since inactivation with methylamine did not alter the ability to inhibit calpain in either normal or dystrophic plasma. We conclude that there is a selective deficiency of plasma high molecular weight kininogen in dystrophic hamsters, an abnormality which could play a role in the pathogenesis of this disorder.

Elevations of cathepsin B and cathepsin L activities in forelimb and hind limb muscles of genetically dystrophic mice

The combined activities of cathepsin B and cathepsin L were studied in the forelimb and hind limb muscles of dystrophic mice. The activities of these proteases in the forelimb and hind limb muscles of young and adult dystrophic mice were significantly higher than those in normal mice. However, clinical involvement of dystrophy appeared in the hind limbs but not in the forelimbs. We therefore suggest that the increase in protease activity begins at a very early age and that the clinical involvement is not linked with the increase in cathepsin B and L.

Beneficial effect of chymostatin on dystrophic mice

We studied the effect of chymostatin on dystrophic mice (C57BL/6J-dy). The locomotor activity of normal mice increased markedly, attaining a plateau at 8 weeks of age, whereas in dystrophic mice, it increased until the age of 7 weeks, and thereafter decreased gradually. Serum levels of creatine phosphokinase were significantly higher in dystrophic mice compared with normal mice, and dystrophic mice had a reduced muscle protein content. When 3-week-old dystrophic mice received chymostatin (1 mg/kg, i.p.), the decrease in locomotor activity was retarded, serum enzyme levels decreased significantly, and muscle protein content increased significantly. In addition, the survival time of treated dystrophic mice was prolonged. The locomotor activity, serum enzyme levels, and muscle protein content of normal mice were not affected by chymostatin. Therefore, we posit that chymostatin retarded the progression of dystrophy in mice.

Modification of glucocorticoid-induced changes in myofibrillar protein turnover in rats by protein and energy deficiency as assessed by urinary excretion of Ntau-methylhistidine

The effects of differing degrees of experimental protein-energy malnutrition on the response of myofibrillar protein turnover rates to administration of corticosteroid has been studied in two experiments on rats. The basal control diet, offered ad lib. in each case, contained 40 g protein/kg, and other groups received diets containing 62 X 5, 95 or 220 g protein/kg at 0 X 67, 1 or 1 X 5 times the level of the control energy intake. Daily administration of 25 or 30 mg corticosterone/kg body-weight after 18 d pre-feeding caused an increase in plasma protein, glucose and insulin concentrations, but a decrease in the corticosterone: insulin values. Liver size and protein content increased, as did the fractional excretion of dietary nitrogen as urea-N in all treated groups. However, whereas a fall in food intake and body-weight occurred in one experiment the reverse occurred in the other. Ntau -Methylhistidine excretion was 12% lower for rats receiving 40 v. 220 g protein/kg diet and excretion was increased by only 57 v. 90% respectively, when the two groups of rats were given 30 mg corticosterone/kg per d. Rats which received 25 mg corticosterone/kg per d and up to 95 g protein/kg diet increased excretion of Ntau -methylhistidine by an average 35%. The fractional degradation rate of myofibrillar protein (kd) was reduced by about 10% by the low-protein diet from 3 X 1 to 2 X 8%/d. During corticosterone treatment the increment in kd for rats on this diet was only 60% of that for rats receiving the 220 g protein/kg diet, i.e. an increase of 1 X 8 v. 3 X 0%/d. Energy restriction further reduced kd during low-protein intake but did not affect the response to the corticosterone. Variations in dietary protein from 40 to 95 g/kg had little effect on the increase in kd during steroid treatment. The effect of corticosterone on calculated synthesis rates (ks) differed markedly between experiments. While ks fell by 50-65% in rats which lost weight on treatment, it rose by up to 60% in rats where carcass non-collagen-protein accretion remained unchanged or increased, despite an increase in kd. Protein deficiency decreases the catabolic response to glucocorticoid, but the net metabolic response appears crucially dependent on changes in food intake or the stage of growth of the rat or both. A net anabolic response with increased fractional rates of myofibrillar protein breakdown, synthesis and accretion was observed in growing rats fed on relatively-low-protein diets and given 25 mg corticosterone/kg per d.(ABSTRACT TRUNCATED AT 400 WORDS)

Enhanced muscle protein degradation and amino acid release from the hemicorpus of acutely uremic rats

Protein synthesis and degradation and net uptake and release of amino acids and minerals were investigated in the perfused hemicorpus of acutely uremic and control Sprague-Dawley rats. Rats underwent bilateral nephrectomy or sham surgery and were studied 30 hr after surgery. The uremic rats displayed greater urea N appearance (net urea generation), lower plasma and muscle concentrations of most amino acids, and increased muscle protein degradation as compared to control rats. Muscle protein synthesis was slightly but not significantly decreased in the uremic animals. There was greater net release of phenylalanine, tyrosine, alanine, total nonessential amino acids, total amino acids, potassium and phosphorus from the perfused hemicorpus of uremic rats and greater release of citrulline from sham rats. Muscle ATP, creatine phosphate, cyclic-AMP, and activities of cathepsin B1, cathepsin D, and alkaline protease were not different in the uremic and sham rats. These data provide evidence that acutely uremic rats sustain increased muscle protein wasting which is due to enhanced protein degradation. The increased protein degradation does not appear to be due to enhanced activities of muscle cathepsin B1, cathepsin D or alkaline protease.

Morphometric and biochemical studies of peripheral nerves in amyotrophic lateral sclerosis

Phrenic nerves of 11 patients with amyotrophic lateral sclerosis studied postmortem contained only 33% of the normal number of large myelinated fibers (9 controls; p less than 0.001). In the phrenic nerves of these patients, there were 18% fewer large myelinated fibers in the distal segment than in the proximal segment (p less than 0.025). The ratio of axonal circumference to myelin lamellae in large myelinated fibers in the distal segment was 34% greater than that in control fibers (p less than 0.002). The proportion of acute axonal degeneration was the same at all levels (48.0 +/- 13.7%). Sural nerves of 21 patients with amyotrophic lateral sclerosis had more acute axonal degeneration and 30% fewer myelinated fibers (p less than 0.05) than controls; evidence of degeneration also extended to unmyelinated fibers. The amount of axonal transport of acetylcholinesterase in 9 sural nerves determined in vitro was reduced by 24% (p less than 0.05) and the apparent transport rate was reduced by 44% (p less than 0.01) compared with 4 controls. These findings show that in amyotrophic lateral sclerosis a small degree of dying-back change and of distal axonal atrophy is superimposed on the degeneration of motor neuron cell bodies, and that the disease effects spread beyond the motor neurons.

Calmodulin content and Ca-activated protease activity in dystrophic hamster muscles

As part of a study on the implication of elevated Ca2+ levels in the myofibrillar degeneration seen in dystrophic muscle, the content of calmodulin and the activity of Ca2+-activated neutral protease (CANP) have been measured in normal and dystrophic (UM-X7.1) hamsters. Calmodulin levels, expressed as micrograms +/- SEM per gram wet weight were highest in brain (385 +/- 24.7), followed by tongue (93.88 +/- 3.93), heart (42.13 +/- 2.93), and skeletal muscle (31.69 +/- 1.42). No significant increases in calmodulin were observed in the dystrophic tissues thus suggesting that the Ca2+ accumulations observed in dystrophic muscles are unrelated to changes in a calmodulin levels. Because of the complexity of regulation of CANP, a time-dependent study was done using extracts of skeletal, heart, and tongue muscles. Marginal increases in dystrophic CANP were seen in skeletal muscle at all times studied and in the heart and tongue at initial time points only. The data are discussed in terms of rising levels of Ca2+ in muscles of the UM-X7.1 hamster being sufficient to increase CANP activity (without increasing content) to where it causes Z-line dissolution.

Content of methylhistidines in normal and pathological human skeletal muscles

The content of 3-methylhistidine (3-MH) and 1-methylhistidine (1-MH) was measured in muscle biopsy specimens from 13 normal controls, 19 patients with Duchenne muscular dystrophy, 8 limb-girdle disease patients, and 23 disease controls with different forms of muscular pathology. 3-MH and 1-MH concentrations in normal human muscle did not appear to be influenced by sex, body weight, and age, at least for subjects in the 10--60 year age group examined. Skeletal muscle 1-MH levels did not significantly differ from mean control values in any of the pathologies investigated. In the patient population examined, the mean 3-MH level per unit of noncollagen protein (NCP) was significantly lower than normal in Duchenne dystrophy only, the reduction being related to disease severity. The significantly lower concentrations of 3-MH in muscle of Duchenne patients indicate the importance of measuring 3-MH in diseased muscle to obtain reliable estimates of the myofibrillar protein catabolic rate.

Limited benefit to genetically dystrophic chickens from a synthetic proteinase inhibitor: Ep475

Chickens with inherited muscular dystrophy (Line 413) were treated in two separate trials with daily intraperitoneal injections of 10% DMSO-water solutions containing the proteinase inhibitors, Ep475 and E64. Drug therapy in each case significantly prolonged the functional ability of the treated chickens. Diluent control chickens around day 35 ex ovo characteristically reached a maximum ability to right from the supine position in a standardized functional test for muscle weakness. Subsequently, the control chickens were found to decline progressively in their ability to right. Treatment with the proteinase inhibitors had no effect on the typically elevated levels of plasma creatine kinase activity. In a histological analysis of the affected pectoralis major muscle, drug treatment had no effect on the relative distribution of degenerating, and vacuolated fibers, inflammatory cells, and abnormal fiber diameters. An exception was seen in decreased necrotic fibers of chickens treated with high doses of Ep475. Moreover, both inhibitors had positive effects on two biochemical abnormalities common to the dystrophic pectoralis muscle: increase in noncollagen protein, and reduction in total calcium.

Immunocytochemical studies of cathepsin D in human skeletal muscle

The distribution of cathepsin D, an acidic endopeptidase, was localized by immunocytochemistry in human skeletal muscle obtained from 34 persons with a variety of neuromuscular disorders. Normal human skeletal muscle contained small amounts of cathepsin D, all of which was found close to the sarcolemmal membrane. Immunoreactive cathepsin D was present in the cytoplasm of many infiltrating phagocytic cells and was increased in skeletal muscle fibers from patients with muscular dystrophies, inflammatory myopathies, rhabdomyolysis, acid maltase deficiency, and neurogenic atrophy. In cases of Duchenne type muscular dystrophy, the increase in cathepsin D was especially prominent in small regenerating fibers, in which it was visualized at the ultrastructural level in lysosome-like organelles and extralysosomal locations. The function of cathepsin D in skeletal muscle is unclear, but the present findings suggest a possible role in muscle regeneration and repair. Such a role would necessitate careful selection of drugs which interfere with proteolytic activity if they are to be used as therapeutic agents in treating neuromuscular diseases.

Effect of corticosterone on myofibrillar protein turnover in diabetic rats as assessed by Ntau-methylhistidine excretion

The effect of corticosterone on myofibrillar protein breakdown in diabetic rats was investigated in order to assess the possible counteracting effects of the secondary rise in plasma insulin concentrations which normally accompanies such treatment. N(tau)-Methylhistidine excretion, an index of myofibrillar protein breakdown, was compared before and after corticosterone treatment (4.0 mg/100 g body wt. per day) of normal control, adrenalectomized, 10-day-streptozotocin-diabetic and adrenalectomized diabetic rats. Diabetic rats received 1.5 units of insulin/100 g body wt. per day throughout the experiment and showed marked hyperglycaemia and glucosuria during corticosterone treatment, whereas non-diabetic rats had only mild hyperglycaemia but elevated insulin concentrations. Corticosterone treatment increased the average rate of myofibrillar protein breakdown by 68% and 95% respectively in non-diabetic and diabetic rats. Net loss of muscle non-collagen protein for the same 7-day period was greater in diabetic than in non-diabetic animals (4.15 versus 2.84% per day), and the calculated average synthesis rates were lowest in diabetic rats. Adrenalectomy had little effect except to decrease slightly the rate of muscle protein breakdown. These results show that the rise in plasma insulin concentrations that accompanies exogenous corticosterone administration to non-diabetic rats diminishes the catabolic effect of this glucocorticoid on muscle. Insulin appears to antagonize the effects of the glucocorticoid by attenuating the increased rates of myofibrillar protein breakdown and, to a lesser extent, by limiting the decrease in synthesis rates.

In vivo effects of protease inhibitors on chickens with hereditary muscular dystrophy

Beginning on day 4 ex ovo, and every 3 d thereafter, genetically dystrophic Line 413 chickens were given intraperitoneal injections (4 mg/kg body wt) of a protease inhibitor, leupeptin, pepstatin, or antipain. Experimental chickens received protease inhibitors dissolved in a water:ethanol:dimethyl sulfoxide solution (50:40:10, vol:vol:vol). Control untreated animals received diluent injections. Untreated dystrophic chickens typically reach around day 30 ex ovo a maximum ability to right from the supine position in a standardized functional test for muscle weakness. After day 30 ex ovo, the dystrophic chickens are found to decline progressively in their ability to right, compared with normal, nondystrophic controls, which have an unimpaired ability to right. Concomitantly, dystrophic chickens exhibit characteristically high levels of plasma creatine phosphokinase enzyme activity. In addition, an increased frequency of degenerating, regenerating, and vacuolated myofibers, and inflammatory cells appear in the affected pectoralis major muscles from the dystrophic chicken. Throughout the duration of the trial, there was no major enhancement in the functional righting ability of dystrophic chickens receiving any one of the protease inhibitors tested. However, there was a significant reduction in the abnormally high levels of plasma creatine phosphokinase in the treated chickens. Also, there was an apparent reduction in the mean number of vacuolated fibers in the pectoralis muscle from the protease inhibitor-treated birds. No significant reductions were observed in the relative frequency of degenerating and regenerating myofibers or inflammatory cells. In addition to the plasma creatine phosphokinase decrease, however, therapeutic benefit was seen in 31.0, 30.5, and 14.8% increases in the wet weight (and total noncollagen protein) of pectoralis muscle from dystrophic chickens receiving leupeptin, pepstatin or antipain, respectively.

Increased rates of myofibrillar protein breakdown in muscle-wasting diseases

The excretion of endogenous creatinine and 3-methylhistidine by subjects with muscle diseases has been measured in order to assess muscle mass and fractional rates of myofibrillar protein degradation. Increases in the rates of myofibrillar protein breakdown were observed in all subjects with Duchenne, Becker, autosomal recessive Duchenne-like, and limb-girdle muscular dystrophy; dystrophia myotonica; myotonia congenita; peroneal muscular atrophy; myasthenia gravis; and central core disease; in some cases of spinal muscular atrophy; but in no cases of facioscapulohumeral muscular dystrophy of dystonia musculorum deformans. All increases in myofibrillar protein breakdown were associated with reductions in muscle proportion below the normal. Muscle-wasting diseases may respond to therapy directed towards an inhibition of muscle protease activity; the efficacy of such therapy can be monitored by the 3-methylhistidine-to-creatinine excretion ratio.

Esterase activity of rat muscle

The esterasic capacity of a series of skeletal muscles in response to three hemisuccinate ester drugs was investigated in rats and compared to that on alpha-naphthylacetate as a reference esterase substrate. Marked variations between different muscles and between given muscles of animals of different sex were observed, indicative of a complex heterogeneity in muscular expression of esterase activity.

Activity of some proteolytic enzymes in normal and dystrophic human muscle

1. The following proteolytic enzymes were measured in muscles of control subjects and patients with muscular dystrophies and related neuromuscular diseases: an elastase-like enzyme, carboxypeptidase A, carboxypeptidase B and pyroglutamyl peptidase. 2. Elastase-like enzyme and carboxypeptidase B did not show significant alterations in various disease conditions that were examined. 3. Carboxypeptidase A was moderately elevated in dystrophic as well as other diseased muscles. 4. Pyroglutamyl peptidase was not markedly altered in any disease condition except that is was slightly lower in dystrophic muscles.

Increased calcium-activated neutral protease activity in muscles of dystrophic hamsters and mice

A Ca2+-activated neutral protease activity was examined in muscles of normal and dystrophic hamsters and mice. Light grey and golden brown strains of normal and B10 14.6 strain of dystrophic hamsters were used. Normal and dystrophic mice were of the Bar Harbor 129 ReJ strain. Enzyme activity was measured in the post myofibrillar fraction (homogenate) and in the 75,000 x g pellet (particulate fraction) and supernatant using purified myofibrils. In normal and dystrophic hamsters or mice, the Ca2+-activated neutral protease was most active in the supernatant followed by the homogenate and particulate fractions. As compared to fractions from normal muscle, enzyme activity was significantly elevated in all 3 fractions from dystrophic muscles of hamsters and mice. Both homogenate and supernatant fractions from muscles of normal hamsters had significantly higher enzyme activity than those of normal mice. Enzyme activity was similar in the particulate fraction. Similarly enzyme activity in the 3 fractions from dystrophic hamster and mouse muscles showed no significant difference. It is suggested that the Ca2+-activated neutral protease may be involved in muscle fibre necrosis in muscular dystrophy.

Muscle breakdown and lysosomal activation (biochemistry)

Muscle tissue levels of lysosomal catheptic enzymes, such as cathepsins D, A, B1, C, and dipeptidyl peptidase II, were measured in control subjects and patients with muscular dystrophies, polymyositis, and certain denervating diseases. The results show that, in general, the activities of these enzymes are increased in muscles of patients with muscular dystrophies and other diseases. The increases in cathepsin D and autolytic activities are not significant until the late stage of the disease process. Cathepsins A, B1, and C are, however, significantly elevated in mildly affected dystrophic and other diseased muscles. Of these catheptic enzymes, cathepsin B1 displays the highest rise at an early stage, suggesting that it may be one of the rate-controlling enzymes of proteolysis. Dipeptidyl peptidase II is increased slightly in dystrophic and other myopathic muscles but is unchanged in denervated muscle. These data clearly implicate the lysosomal group of proteinases as largely responsible for mediating muscle breakdown in the muscular dystrophies and certain other muscle and neuromuscular diseases in man.

Muscular dystrophy and activation of proteinases

Evidence is presented for the existence of many different systems of proteolytic enzymes in human skeletal muscle. These include the lysosomal system of cathepsins as well as proteinases and peptide hydrolases that are optimally active at neutral and alkaline pH ranges. The majority of proteolytic enzymes examined are found to show increased activity in dystrophic human muscle. Moreover, a high initial rise is observed in cathepsin B1, a thiol-dependent endopeptidase of lysosomes, and in dipeptidyl peptidase IV, a membrane-associated peptidase. In addition, a calcium-activated neutral proteinase is found to be significantly elevated in muscle from patients with Duchenne dystrophy. The possible roles of these proteinases in intracellular protein catabolism and muscle wasting are discussed.

Muscle transketolase in normal foetuses and Duchenne dystrophy

The activity of the enzyme transketolase (EC 2.2.1.1) or the pentose phosphate metabolizing enzyme (PPME) was investigated in the quadriceps muscles of patients with Duchenne muscular dystrophy (DMD), polymyositis and spinal muscular atrophy (SMA) and in normal foetuses. The enzyme activity was significantly elevated in these muscle disorders with highest and lowest levels seen in DMD and SMA groups, respectively. The enzyme activity was increased early in DMD muscle and appeared to increase with progression of the disease. Normal foetal muscle contained high transketolase activity which is comparable to that seen in DMD patients.

Muscular dystrophy: inhibition of degeneration in vivo with protease inhibitors

The protease inhibitors leupeptin and pepstatin were used in vivo in genetically dystrophic chickens to determine their effects on the histological and biochemical changes observed in this disease. These compounds appear to delay the degeneration of muscle tissue which is characteristic of this disorder and thus may have potential therapeutic value in the treatment of muscular dystrophy.

Age-related changes in protein turnover and ribonucleic acid of the diaphragm muscle of normal and dystrophic hamsters

Diaphragm muscles of dystrophic hamsters were found to be larger than those of control animals at two of three ages studied. The additional growth of these afflicted muscles correlated with large increases in protein synthesis and concentrations of RNA. Protein breakdown was also increased in the dystrophic muscles, but to a smaller extent than synthesis.

Arylamidases in normal and diseased human muscle

Human skeletal muscle homogenates were found to contain enzymes that catalyze the hydrolysis of beta-naphthylamides of leucine, arginine and lysine, known substrates for neutral and basic arylamidases. They also contained a trace of activity towards alpha-aspartyl-beta-naphthylamide. The muscle arylamidases were found to be inhibited by p-chloromercuribenzoate, Hg2+ and puromycin. Leucyl, arginyl and lysysl arylamidases were slightly activated by cobalt ions. When compared to controls, no significant differences in muscle arylamidase activities were observed in patients with muscular dystrophies and certain denervating diseases.

Determination in vitro of the rate of protein synthesis and degradation in human-skeletal-muscle tissue

The present studies were aimed to evaluate the possibility to use a system for estimation in vitro of the biosynthesis and degradation rates of human skeletal muscle protein. A previously characterized human skeletal muscle preparation was used. Amino acids and insulin stimulated significantly the incorporation rate of leucine into proteins. The effect of amino acids was more pronounced than that of insulin. The stimulatory effect of insulin could be decreased by amino acids. Insulin did not influence the tissue uptake or the oxidation rate of leucine. The release of [14C]leucine deriving from degradation of prelabelled skeletal muscle fibre proteins was linear for at least 2.5 h of incubation and optimal with leucine at concentrations beyond 12.5 mmol/1 or in the presence of puromycin in the incubation medium. The rate of the release of radioactivity was significantly inhibited by amino acids and at borderline significance by insulin but not by puromycin. The specific radioactivity in prelabelled proteins decreased significantly in the presence of puromycin suggesting that leucine derived from protein degradation was reutilized in vitro. This reutilization was found to be 9 +/- 1% of leucine released from degradation of proteins in 30 subjects. A statistically significant positive correlation between the cathepsin D activity in human skeletal muscle tissue and the degradative rate of prelabelled muscle proteins in vitro was observed. The results indicate that biosynthesis and degradation of skeletal muscle proteins in this system in vitro were subjected to control mechanisms. It is suggested that the release of radioactivity from prelabelled muscle fibre proteins during incubation probably only reflects the degradation of some rapidly-turning-over proteins.