Intracellular localization of two distinct Ca2+-proteases (calpain I and calpain II) as demonstrated by using discriminative antibodies

Intracellular localization of two molecular species of calpain (Ca2+-dependent cysteine proteinase) was studied by immunocyto- and histochemical methods employing antibodies strictly monospecific for the respective antigens. Apparent immunological cross-reactivity between the larger subunits of calpain I (low Ca2+-requiring form) and calpain II (high Ca2+-requiring form) was calculated to be 15-17%, and two steps of affinity chromatography were needed to obtain antibodies which can discriminate between the two proteases. Indirect immunofluorescent staining of cultured PK 15 cells revealed diffuse staining of the cytoplasm with both antibodies against calpain I and calpain II. Preincubation with Ca2+-ionophore had no effect on the staining patterns. Sections of porcine kidney were stained by the avidin-biotinylated peroxidase complex method. The proximal and distal tubules and collecting duct were stained, but the glomerulus, macula densa, and vascular vessels were not stained by either anti-calpain I or anti-calpain II antibodies.

Purification and characterization of a protein inhibitor of calcium-dependent proteases from rat liver

Soluble extracts of rat liver contain a protein inhibitor of calcium-dependent proteases. The inhibitor has an apparent Mr = 250,000 and is separated from the calcium-dependent proteases by gel-filtration chromatography in the presence of EGTA. The inhibitor has been purified by affinity chromatography using a calcium-dependent protease covalently linked to Affi-Gel 15. The inhibitor specifically binds to this affinity resin in a calcium-dependent manner and elutes in the presence of EDTA or EGTA. The purified inhibitor appears as a single protein with Mr = 125,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Presumably it is a dimer under nondenaturing conditions. The inhibitor inhibits each of two calcium-dependent proteases from rat liver and from other tissues and species. However, it has no effect on any other protease tested.

Multiple forms of phosphoinositide-specific phospholipase C of different relative molecular masses in animal tissues. Evidence for modification of the platelet enzyme by Ca2+-dependent proteinase

The Mr distribution of phosphoinositide-specific phospholipase C in the supernatants isolated from a variety of animal tissues was analysed by high-performance gel-filtration chromatography. In most tissues, at least four peaks of activity were resolved. However, different tissues showed quite marked differences in the distribution of activity between these peaks. In rat heart, lung and kidney, the predominant form had Mr approx. 90000, whereas the predominant form in brain had Mr approx. 290000. In liver, the Mr-90000 form predominated, but this tissue also contained relatively large amounts of a form of Mr approx. 150000. Phospholipase C in these tissues from other animal species gave similar distributions of activity between the peaks. In supernatants prepared from platelets sonicated in the presence of leupeptin (0.5 mM) or EGTA (20 mM), the Mr-290000 form predominated. However, when leupeptin or EGTA (inhibitors of Ca2+-dependent proteinase) was omitted from the sonication buffer, the Mr-290000 form appeared to be replaced by a form of Mr 100000. Similar changes in Mr were not demonstrated with the other tissues. These results may be relevant to the intracellular regulation of phospholipase C, since Ca2+-dependent proteolysis has been reported to occur during platelet activation.

Two cytosolic, Ca2+-dependent, neutral proteinases from rabbit liver: purification and properties of the proenzymes

Two Ca2+-requiring proteinases have been purified from rabbit liver cytosol and shown to be present in isolated hepatocytes. They differ in relative molecular mass, with the major and minor forms, Mr = 150,000 and Mr = 200,000, accounting for 75 and 18% of the total cytosolic neutral proteinase activity, respectively. Both are recovered as inactive proenzymes that can be converted to the active, low-Ca2+-requiring proteinases by incubation with Ca2+ and substrate [S. Pontremoli, E. Melloni, F. Salamino, B. Sparatore, M. Michetti, and B. L. Horecker (1984) Proc. Natl. Acad. Sci. USA 81, 53-56. Each proenzyme is composed of two subunits, with molecular masses of 80 and 100 kDa, respectively. Activation of the proenzymes was found to correlate with their dissociation into subunits. The optimum pH for conversion of the proenzymes to the active proteinases in the presence of 5 mM Ca2+ and 2 mg/ml of denatured globin was approximately 7.5, and the same pH optimum was observed for the digestion of denatured globin by the activated proteinases. Following activation, each proteinase was observed to undergo autolytic inactivation at rates that were dependent on the concentration of both Ca2+ and the digestible substrate. A model is proposed for the activation of the proenzymes and the subsequent inactivation of the active proteinases.

Regulation of the Ca2+-dependent neutral proteinases from rabbit liver by an endogenous inhibitor

An endogenous inhibitor of neutral Ca2+-dependent proteinases has been isolated from rabbit liver cytosol. The inhibitor is a heat-stable, 240-kDa, tetrameric protein. It is dissociated into its 60-kDa subunits by high concentrations of Ca2+ (0.1-1 mM), but not by lower concentrations in the physiological range. Inhibition of the 150-kDa proteinase of rabbit liver [Melloni, E., Pontremoli, S., Salamino, F., Sparatore, B., Michetti, M. and Horecker, B.L. (1984) Arch. Biochem. Biophys. 232, 505-512] requires the monomeric form of the inhibitor, and occurs only at the high concentrations of Ca2+ which also cause dissociation of the dimeric 150-kDa proteinase into its 80-kDa subunits. The molecular weight of the inactive proteinase-inhibitor complex was estimated by the equilibrium gel penetration method to be 140 kDa, suggesting that it contains one subunit of proteinase and one of inhibitor. The mechanism of interaction of the inhibitor with the 200-kDa proteinase at high concentrations of Ca2+ is identical to that observed for the 150-kDa proteinase, namely dissociation of both proteinase and inhibitor into subunits and formation of an inactive 160-kDa proteinase-inhibitor complex. However, unlike the 150-kDa proteinase, which does not interact with the inhibitor at low Ca2+ concentrations, the 200-kDa proteinase is also inhibited at low concentrations of Ca2+. Under these conditions, the high-molecular-weight complex (greater than 400 kDa) formed between the tetrameric inhibitor and the dimeric proteinase prevents conversion of the 200-kDa proenzyme to the active, low-Ca2+-requiring form.

Characterization of the single peptide generated from the amino-terminus end of alpha- and beta-hemoglobin chains by the Ca2+-dependent neutral proteinase

Human erythrocyte Ca2+-dependent neutral proteinase catalyzes a limited proteolysis of isolated globin chains. The rate of hydrolysis is very rapid using heme-deprived alpha- or beta-globin chains and is reduced to one-fifth with their corresponding native forms. In both cases, the proteinase specifically cleaves a single peptide bond, this resulting in the removal from the amino-terminus end of an octapeptide in beta-globin and of an undecapeptide in alpha-globin. Both peptides have been isolated, their amino acid composition has been characterized and the susceptible site of cleavage has been identified. Hemoglobin variants show a different rate of digestion as compared to that of normal chains. The alpha-Hasharon [alpha 47(CE5) Asp----His] undergoes rapid digestion, while the beta-G San Josè chain [beta 7(A4) Glu----Gly], which carries the mutation near the site of cleavage, reveals a high degree of resistance to proteolytic degradation by the neutral proteinase.

Cleavage of fibrinogen by human platelet calcium-activated protease

In lysates of washed human platelets produced by sonication or by addition of nonionic detergent, fibrinogen (Mr 340,000) was rapidly degraded, under conditions favorable to activation of the endogenous calcium-activated protease (CAP), to a core derivative (Mr 280-290,000) composed of partially degraded A alpha chains (Mr 47,000, 46,000, and 34,000) and B beta chains (Mr 56,000), and apparently intact gamma chains (Mr 53-54,000). Extensive degradation occurred within one minute at 4 degrees C, ambient temperature or at 37 degrees C, and was inhibited by leupeptin, EDTA, EGTA, or N-Ethylmaleimide, but not by soybean trypsin inhibitor, hirudin, aprotonin, benzamidine, phenylmethylsulfonyl fluoride or epsilon-aminocaproic acid. Purified plasma fibrinogen exposed to lysates containing active protease was cleaved in an identical fashion. The cleavage pattern of A alpha chains produced by this platelet protease activity is different from that produced by plasmin in vitro or that found in fibrinogen catabolites in vivo, and is unlike that produced by any cellular fibrinolytic enzyme yet described. In view of this finding, as well as the striking differential inhibitory effect of the agents cited above, we conclude that the degradation of platelet fibrinogen observed in these studies is due to direct proteolysis by platelet CAP.

High molecular weight proteins in cardiac and skeletal muscle junctional sarcoplasmic reticulum vesicles bind calmodulin, are phosphorylated, and are degraded by Ca2+-activated protease

A unique set of high molecular weight proteins was identified in junctional sarcoplasmic reticulum (SR) vesicles isolated from both cardiac muscle and skeletal muscle. These high Mr proteins were not present in free SR vesicles isolated from either tissue, nor were they observed in purified sarcolemmal fractions. The junctional SR high Mr proteins migrated as doublets in sodium dodecyl sulfate-polyacrylamide gels and exhibited apparent Mr values between 290,000 and 350,000. The high Mr proteins bound calmodulin; they were the principal proteins labeled in the cardiac and skeletal muscle SR subfractions by azido-125I-calmodulin. The high Mr proteins were also substrates for an endogenous Ca2+-calmodulin-dependent protein kinase activity, as well as exogenously added catalytic subunit of cAMP-dependent protein kinase. In addition, the junctional SR high Mr proteins were the major SR proteins degraded by a Ca2+-activated protease purified from smooth muscle. Control experiments verified the separation of junctional SR vesicles and free SR vesicles from both muscle types. Junctional SR vesicles were enriched in calsequestrin, and they exhibited Ca2+ uptake which was stimulated up to 10-fold by either ryanodine or ruthenium red. Free SR vesicles were deficient in calsequestrin and were insensitive to these two agents. Localization of the cardiac and skeletal muscle high Mr proteins to the junctional SR, coupled with demonstration of their nearly identical biochemical properties, suggests that the proteins are homologous and are likely to have similar functions in both types of striated muscle.

Evidence for membrane-associated calpain I in human erythrocytes. Detection by an immunoelectrophoretic blotting method using monospecific antibody

Low and high Ca2+-requiring forms of Ca2+-dependent cysteine proteinase are known as calpain I and calpain II, respectively. We have obtained, for the first time, monospecific antibodies for calpain I and for calpain II. Using these antibodies and an electrophoretic blotting method, we have found that a small, but reproducible, amount of calpain I was associated with human erythrocyte membranes while the bulk of the protease was contained in the cytosol. Most of membrane-associated calpain I was extractable with 1% Triton X-100, but not with 0.1% detergent. In the presence of 0.1 mM Ca2+ and 5 mM cysteine, membrane-associated calpain I degraded the membrane protein band 4.1 preferentially and band 3 protein only slowly. The Ca2+-induced autodigestion of the membrane preparation was inhibited by leupeptin but not by a cytosolic calpain inhibitor, calpastatin, added to the incubation medium. No calpain II was detected in either erythrocyte cytosol or membranes when anti-calpain II antibody was used under the same conditions as those for the detection of calpain I.

The biochemistry of memory: a new and specific hypothesis

Recent studies have uncovered a synaptic process with properties required for an intermediate step in memory storage. Calcium rapidly and irreversibly increases the number of receptors for glutamate (a probable neurotransmitter) in forebrain synaptic membranes by activating a proteinase (calpain) that degrades fodrin, a spectrin-like protein. This process provides a means through which physiological activity could produce long-lasting changes in synaptic chemistry and ultrastructure. Since the process is only poorly represented in the brain stem, it is hypothesized to be responsible for those forms of memory localized in the telencephalon.

Enzyme immunoassay of calpain I and calpastatin and its application to the analysis of human erythrocyte hemolysate

A highly sensitive sandwich enzyme immunoassay for a Ca2+-dependent cysteine proteinase (calpain I) and its specific endogenous inhibitor protein (calpastatin) was developed. The calpain I and calpastatin used as immunogens were purified from human erythrocytes. Anti-calpastatin antisera having sufficiently high titer were obtained only when the immunogen was purified by preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The assay method was principally based on the report by M. Imagawa et al. (1982, J. Appl. Biochem. 4, 41-57), using a specific antibody-coated polystyrene ball and horseradish peroxidase-conjugated Fab' fragment of the antibody. The sensitivity was 0.1 ng of calpain I or calpastatin per assay tube. Starting with 50 microliter of the hemolysate from human erythrocytes, the method permitted direct and simultaneous determination of calpain I and calpastatin, without prior separation of these two enzymatically counteracting components by chromatography. The present method as applied to the erythrocytes from 14 healthy adults gave 120-170 micrograms for calpain I and 164-211 micrograms for calpastatin per gram of hemoglobin, respectively.

Brain fodrin: substrate for calpain I, an endogenous calcium-activated protease

The calcium-activated thiol-protease calpain I, which is present in cytosolic and membrane preparations from rat brain, was tested for its capacity to degrade the neuronal spectrin-like protein fodrin. In the presence of micromolar calcium concentrations purified calpain I degraded both purified fodrin and the fodrin present in hippocampal and cerebellar membranes. Fodrin was identified as a high molecular weight protein present in brain membranes by the following criteria: (i) comigration on NaDodSO4/polyacrylamide gels with purified fodrin, (ii) reactivity with antibodies to purified fodrin, and (iii) a proteolytic map following calpain activation comparable to that found after calpain-mediated degradation of purified fodrin. The fodrin breakdown was selective in that calpain I did not affect at least 15 other membrane-associated polypeptides. Fodrin degradation by the protease was rapid and was accompanied by the appearance of a lower molecular weight breakdown product. Calpain I had a high affinity for fodrin, with a Km for degradation of about 50 nM. Purified calpain I also degraded purified spectrin and the spectrin present in erythrocyte membranes. Calpain I-mediated degradation of spectrin-like proteins could provide a mechanism by which brief increases in intracellular free calcium levels modify the structure of the submembraneous cytoskeleton and the distribution of cell surface receptors and alter cell shape.

Lysosomal and nonlysosomal proteolytic activities in experimental diabetic cardiomyopathy

The role of cardiac lysosomal and nonlysosomal protease alterations in the development of the cardiomyopathy that occurs in genetically diabetic C57BL/KsJ db/db mice has been examined. The db/db mice and age-matched controls were sacrificed between 7 and 24 weeks of age. Cathepsin D activity, myofibrillar alkaline protease (MAP) activity (including serine protease activity), and Ca2+-activated protease activity were determined by using [3H]acetyl-casein as substrate. There is a significant decrease in cathepsin D, MAP, and serine protease activities in the myocardium of 7- to 20-week old diabetic mice with a rebound of these activities toward normal levels by 24 weeks of age. Cathepsin D and MAP activities are inversely related to heart weight in diabetic mice with the higher levels being recorded in association with the most pronounced decrease in heart weight. In contrast, Ca2+-activated protease activity in the hearts of diabetic mice does not differ significantly from controls throughout the period of observation. The results suggest that both lysosomal cathepsin D and nonlysosomal MAP may mediate the accelerated cardiac muscle degradation that occurs in the late stage of diabetic cardiomyopathy in the db/db mice.

Action of calpain on the basic estrogen receptor molecule of porcine uterus

Basic estrogen receptor (ER) molecule (vero-ER) of the cytosol of porcine uterus was purified 1,200-fold after successive chromatographies on phenyl-Sepharose, hydroxylapatite, and DEAE-cellulose, followed by Sephadex G-150 gel filtration. The purified vero-ER was completely free from endogenous protease and ER-binding factor. The action of Ca2+-dependent cysteine proteinase (calpain) on vero-ER was studied by utilizing the purified receptor and calpains from porcine uterus (endogenous calpain), porcine kidney, and human erythrocytes. Proteolysis of vero-ER was followed by monitoring the disappearance of the binding capability of vero-ER with "8S" ER-forming factor. Vero-ER was proteolyzed by both the endogenous and the exogenous calpains in the presence of Ca2+. The calpains did not attack vero-ER in the absence of Ca2+. The results indicated the absolute requirement by calpain for Ca2+ for the limited hydrolysis of vero-ER. Uterine cytosol was shown to contain, in parallel with calpain, a protease which does not require Ca2+ for the limited proteolysis of vero-ER. The strongly hydrophobic domain of vero-ER, recently shown to be indispensable for the nuclear translocation of vero-ER (Murayama, A. & Fukai, F. (1983) FEBS Lett. 158, 255), was preferentially destroyed by both the Ca2+-requiring and -nonrequiring enzymes. It was assumed that calpain might intervene in the estrogen action by diminishing irreversibly the amount of the cytoplasmic ER capable of translocating into the nucleus.

Comparison of low and high calcium requiring forms of the calcium-activated neutral protease (CANP) from rabbit skeletal muscle

Two distinct calcium-dependent neutral proteases (CANPs) with different sensitivities to calcium ions were purified concurrently by almost the same procedures from rabbit skeletal muscle and their enzymatic properties were compared (sensitivity to various divalent metal ions, the pH dependency and heat-stability of the activity, and the hydrolytic activity towards various substrates). They were further compared chemically in terms of the state of thiol groups, the amino acid compositions of subunits and the peptide fragments by digestion with S. aureus V8 protease. The low calcium requiring form of CANP (microCANP) was more sensitive to other divalent metal ions such as Sr2+ and Ba2+ than the high calcium requiring form of CANP (mCANP). The comparison of the pH dependency of these CANP activities showed that microCANP was active in a broader pH range than mCANP and the former was more heat-stable than the latter. Both CANPs had similar affinity to various substrates, but the hydrolytic velocity was several times higher with microCANP than with mCANP. Although they were inhibited by thiol protease inhibitors to the same extent, the states of thiol groups in them were quite different. The thiol group involved in the catalytic activity of the enzyme was exposed without adding Ca2+ in microCANP, whereas the group in mCANP became exposed only when sufficient Ca2+ was added. The large subunits of these two CANPs were different in their amino acid compositions and in the peptide fragment patterns produced by S. aureus V8 protease but the small subunits were indistinguishable from each other. These results led us to conclude that these two CANPs are quite different in nature and are not in a simple relationship, i.e., one of them is not derived from the other by autolysis or modification.

Ca2+-activated proteinase in the rat. Quantification by immunoassay in the uterus during pregnancy and involution, and in other tissues

Rat uteri were taken at various stages of pregnancy and involution post partum, and several other tissues were taken from pregnant and non-pregnant animals. Portions of each tissue were homogenized in the presence of proteinase inhibitors, and the amounts of the high-Ca2+-requiring Ca2+-activated proteinase in the supernatants were measured by a two-site immunoradiometric assay using 125I-immunoglobulin G. The proteinase was shown, by protein blotting, to be immunologically identical in all tissues. The amounts in the various tissues, expressed in units of proteinase activity/g wet wt., were: lung, 95; kidney and small intestine, 42; liver, 20; brain, heart and skeletal muscle, 13. Uterine wet weight increased at the end of pregnancy by about 8-fold, but the amounts of proteinase per uterus increased by about 22-fold; alternatively, expressed in units of proteinase activity/g wet wt., the mean uterine values were: non-pregnant, 28.6; term-pregnant, 77.0. As the wet weight of the uterus fell rapidly during involution, the amounts of proteinase activity remained relatively high. The data suggest that the Ca2+-activated proteinase may have some role in tissue resorption during uterine involution, but the high proteinase activity present before parturition must be regulated in ways which are not yet clear.

Large-scale purification of porcine calpain I and calpain II and comparison of proteolytic fragments of their subunits

Large-scale purification of calpain [Ca2+-dependent cysteine proteinase; EC 3.4.22.17] from porcine tissues is described. The methods used included chromatographies on DEAE-cellulose, Ultrogel AcA 34, Blue Sepharose CL-6B, and DEAE Bio-Gel A which yielded homogeneous enzyme proteins: 27.0 mg of calpain I (low Ca2+-requiring form) from 5 liters of blood with 17,900-fold purification and 57.6 mg of calpain II (high Ca2+-requiring form) from 1.5 kg of kidneys with 5,800-fold purification. Porcine calpains I and II are half-maximally activated at 2.8 microM and 150 microM Ca2+, respectively. They are composed of large and small subunits: Mr 83,000 and 29,000 for calpain I and Mr 80,000 and 29,000 for calpain II. Gel-electrophoretic analysis of the digest with a-chymotrypsin or Staphylococcus aureus V8 protease revealed that the large subunits of calpains I and II are markedly different in structure whereas the small subunits are most likely identical. Mono-specific antibodies directed toward the respective large and small subunits were used for immunoblotting experiments which established not only the identity among several porcine tissues of calpain I but also that of calpain II. several porcine tissues of calpain I but also that of calpain II.

Degradation of actin and vimentin by calpain II, a Ca2+-dependent cysteine proteinase, in bovine lens

Calpain II, a high Ca2+-requiring form of Ca2+-dependent cysteine proteinase (EC 3.4.22.17), isolated from bovine lens was found to cleave actin and vimentin, two major cytoskeletal elements of the lens. Polyacrylamide gel electrophoresis revealed that actin (Mr 43 000) was broken down through intermediary products of approximate Mr 42 000 and 40 000, while vimentin (Mr 57 000) was rapidly cleaved into several fragments ranging from Mr 44 000 to 20 000. The cleavage was dependent on Ca2+ and could be blocked by calpastatin , a calpain-specific inhibitor. These findings suggest that calpain might play a role in age-related degradation of the lens cytoskeleton.

Kinase activities associated with calcium-activated neutral proteases

Studies on the phosphorylation of calcium-activated neutral protease (CANP) revealed the presence of kinase activities closely associated with purified CANP preparations. The kinase activity in uCANP (CANP with high affinity for calcium) was cAMP-independent whereas the kinase activity in mCANP (CANP with low affinity for calcium) was cAMP-dependent, inhibited by kinase specific inhibitor and abolished when the mCANP was preincubated in calcium. The CANP-associated kinase(s) phosphorylate uCANP and mCANP , causing modulation of their proteolytic activities.

Conversion of myelin-associated glycoprotein (MAG) to a smaller derivative by calcium activated neutral protease (CANP)-like enzyme in myelin and inhibition by E-64 analogue

Using the immunoblot technique, we found that an incubation of purified human myelin in 10 mM Tris-HCl buffer at pH 7.5 resulted in the conversion of the myelin-associated glycoprotein (MAG) to a smaller derivative (dMAG). Exogenously added 5 mM CaCl2 accelerated the conversion of MAG. In buffer containing more than 100 microM of EGTA, the conversion was inhibited. In addition, the existence of endogenous calcium in purified myelin was confirmed using atomic absorption spectroscopy. The conversion was also inhibited partially by one of the thiol protease inhibitors, E-64 analogue (E-64-a). These observations suggest that the conversion of MAG is mediated by calcium-activated neutral protease (CANP)-like enzyme.

Limited proteolysis of bovine lens alpha-crystallin by calpain, a Ca2+-dependent cysteine proteinase, isolated from the same tissue

A Ca2+-dependent cysteine proteinase (calpain, EC 3.4.22.17) was found in the cystosolic fraction of bovine lens and purified to apparent homogeneity. The purified enzyme required 1 mM Ca2+ for its full activation and was composed of two subunits of Mr 80 000 and 29 000 as demonstrated by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate (SDS). This enzyme, when activated by Ca2+, degraded both A- and B-chains of alpha-crystallin, which were isolated also from bovine lens. SDS-gel electrophoresis of the digest revealed that the A-chain (Mr 19 500) was broken down to produce an 18-kDa polypeptide fragment and the B-chain (Mr 22 500) to produce a 19.5-kDa polypeptide fragment. No further cleavage occurred even upon prolonged incubation or after the second addition of the enzyme, indicating the uniquely limited proteolysis of each chain protein. The existence of calpastatin, an endogenous inhibitor protein specific for calpain, was also demonstrated in bovine lens cytosol.

Lack of tissue-specificity of calcium-activated neutral proteases from skeletal muscle and lung of rabbit

Calcium-activated neutral proteases (CANPs) with high sensitivity (microCANP) and low sensitivity (mCANP) to calcium ions were purified individually from rabbit skeletal muscle and rabbit lung and compared as to their electrophoretic properties, calcium requirements and peptide mapping of fragments produced by S. aureus V8 protease digestion of separated subunits. All of the results suggested that there is no difference between the microCANPs as well as between the mCANPs obtained from the two tissues, with respect to the chemical and enzymatic properties. However, the contents of CANPs in these tissues were different.

Control of protein degradation in muscle by prostaglandins, Ca2+, and leukocytic pyrogen (interleukin 1)

Protein degradation in skeletal muscle increases with fever and sepsis. Our studies indicate that prostaglandin E2 (PGE2) is an important regulator of muscle proteolysis that seems to signal this increase in fever. When rat skeletal or cardiac muscles were incubated with arachidonate, rates of protein breakdown rose and protein balance became more negative. Aspirin or indomethacin, which prevented synthesis of PGE2, markedly reduced this effect. By itself PGE2 stimulated proteolysis without altering protein synthesis. PGE2 seems to increase proteolysis in the lysosomes, inasmuch as leupeptin and Ep-475 inhibit this response. These inhibitors inactivate lysosomal thiol proteases in the muscles without affecting the Ca2+-activated protease. (In fact, complete inactivation of the latter enzyme with mersalyl did not reduce overall proteolysis in the muscles). When muscles from feverish rats were incubated in vitro, they showed greater protein breakdown and PGE2 synthesis than muscles from normal animals. Addition of indomethacin eliminated this difference. Leukocytic pyrogen (interleukin 1), a protein released by monocytes that signals the onset of fever, also seems to signal increased muscle PGE2 synthesis and muscle proteolysis. This protein enhanced both processes dramatically in the isolated muscles. These findings suggest that cyclooxygenase inhibitors may be useful in the treatment of patients showing excessive protein breakdown.

Ca2+-dependent neutral proteinase from human erythrocytes: activation by Ca2+ ions and substrate and regulation by the endogenous inhibitor

Ca2+-dependent neutral proteinase purifies from human erythrocytes as an inactive proenzyme, that can be converted in an active low Ca2+ requiring form either by high concentrations of Ca2+ (0.1-1 mM) in the absence of the substrate, or by low concentrations of Ca2+ (1-5 microM) in the presence of digestible substrates. Activation requires dissociation to constituent inactive proenzyme subunits which are then converted to the active proteinase species still retaining their monomeric structure. The activation process produced by high Ca2+ concentrations is controlled by the endogenous inhibitor which also dissociates into constituent subunits in order to exert its inhibitory effect. An additional regulation of the activated proteinase involves an autoproteolytic process, Ca2+ and substrate dependent, producing enzyme inactivation.

Properties of smooth muscle vinculin

Vinculin, isolated from turkey gizzard smooth muscle, was purified by chromatography on CM-cellulose after isolation from a DEAE-cellulose column. Two-dimensional gel electrophoretic analysis of crude muscle fractions demonstrated that: 1) much of the approximately 130,000-dalton protein present in smooth muscle did not co-isoelectrically focus with the purified 130,000-dalton vinculin and 2) the purified vinculin consisted of three major, closely spaced isoelectric variants that were present only in small amounts in the original smooth muscle sample. Purified vinculin sedimented as a single peak with a sedimentation coefficient S0 20,w of 5.9. Circular dichroism spectra of purified vinculin indicated a considerable degree of secondary structure, with an alpha-helical content of approximately 50% as measured at 208 nm. The ultraviolet absorption spectrum of vinculin gave a measured E1%(278) of 4.64. Digestion of vinculin, much of which is located at the cytoplasmic surface of the cell membrane, with Ca2+-activated neutral protease purified from skeletal muscle yielded major fragments with molecular weights determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of 98,000, 85,000, and 26,000. The factor(s) in DEAE-cellulose-purified vinculin responsible for decreasing the low shear viscosity of actin was removed and found in a crude fraction isolated by CM-cellulose chromatography. The purified vinculin had a small, but positive effect on the MgCl2-induced polymerization of actin as measured by low shear viscometry.