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

Comparing the impact of an acute exercise bout on plasma amino acid composition, intraerythrocytic Ca(2+) handling, and red cell function in athletes and untrained subjects

The N-methyl d-aspartate receptors (NMDARs) mediating Ca(2+) uptake upon stimulation with glutamate and glycine were recently discovered in red blood cells (RBC) of healthy humans. Activation of these receptors with agonists triggered transient Ca(2+)-dependent decrease in hemoglobin oxygen affinity in RBC suspension. The aim of this study was to assess the potential physiological relevance of this phenomenon. Two groups formed by either healthy untrained volunteers or endurance athletes were subjected to a stepwise incremental cycling test to exhaustion. Plasma glutamate levels, activity of the NMDARs, and hemoglobin O2 affinity were measured in blood samples obtained before and after the exercise in both groups. Increase in plasma glutamate levels following exercise was observed in both groups. Transient Ca(2+) accumulation in response to the NMDAR stimulation with NMDA and glycine was followed by facilitated Ca(2+) extrusion from the RBC and compensatory decrease in cytosolic Ca(2+) levels. Short-term activation of the receptors triggered a transient decrease in O2 affinity of hemoglobin in both groups. These exercise-induced responses were more pronounced in athletes compared to the untrained subjects. Athletes were initially presented with lower basal intracellular Ca(2+) levels and hemoglobin oxygen affinity compared to non-trained controls. High basal plasma glutamate levels were associated with induction of hemolysis and formation of echinocytes upon stimulation with the receptor agonists. These findings suggest that glutamate release occurring during exhaustive exercise bouts may acutely facilitate O2 liberation from hemoglobin and improve oxygen delivery to the exercising muscle.

Clinical severity of β-thalassaemia/Hb E disease is associated with differential activities of the calpain-calpastatin proteolytic system

Earlier observations in the literature suggest that proteolytic degradation of excess unmatched α-globin chains reduces their accumulation and precipitation in β-thalassaemia erythroid precursor cells and have linked this proteolytic degradation to the activity of calpain protease. The aim of this study was to correlate the activity of calpain and its inhibitor, calpastatin, with different degrees of disease severity in β-thalassaemia. CD34(+) cells were enriched from peripheral blood of healthy individuals (control group) and patients with mild and severe clinical presentations of β(0)-thalassaemia/Hb E disease. By ex vivo cultivation promoting erythroid cell differentiation for 7 days, proerythroblasts, were employed for the functional characterization of the calpain-calpastatin proteolytic system. In comparison to the control group, enzymatic activity and protein amounts of μ-calpain were found to be more than 3-fold increased in proerythroblasts from patients with mild clinical symptoms, whereas no significant difference was observed in patients with severe clinical symptoms. Furthermore, a 1.6-fold decrease of calpastatin activity and 3.2-fold accumulation of a 34 kDa calpain-mediated degradation product of calpastatin were observed in patients with mild clinical symptoms. The increased activity of calpain may be involved in the removal of excess α-globin chains contributing to a lower degree of disease severity in patients with mild clinical symptoms.

Generation of peptides by human erythrocytes: Facts and artifacts

Previously reported data on peptide composition of human erythrocyte lysate were obtained under conditions that did not exclude proteolytic degradation of hemoglobin in the process of peptide isolation. Comparative chromatographic analysis of the diluted erythrocyte lysate incubated in acidic conditions with or without proteolytic enzyme inhibitors showed that several peptides earlier identified as intraerythrocyte ones in fact result from hemoglobin degradation by erythrocyte acidic protease(s) during incubation of the lysate. A rational scheme excluding postlysis proteolysis was developed for isolation of peptide fraction. Further analysis resulted in determination of structure and content of about 50 endogenous intraerythrocyte hemoglobin fragments. A primary endopeptidase splitting of alpha- and beta-globin chains followed by consecutive exopeptidase trimming of primary fragments is suggested as a degradation mechanism. The intraerythrocyte peptides were shown to differ from peptides excreted by the erythrocytes to the extracellular medium in the primary culture. It was also found that intraerythrocyte peptides cannot play the role of precursors of hemoglobin fragments present in tissue extracts.

On the sequential determinants of calpain cleavage

The structural clues of substrate recognition by calpain are incompletely understood. In this study, 106 cleavage sites in substrate proteins compiled from the literature have been analyzed to dissect the signal for calpain cleavage and also to enable the design of an ideal calpain substrate and interfere with calpain action via site-directed mutagenesis. In general, our data underline the importance of the primary structure of the substrate around the scissile bond in the recognition process. Significant amino acid preferences were found to extend over 11 residues around the scissile bond, from P(4) to P(7)'. In compliance with earlier data, preferred residues in the P(2) position are Leu, Thr, and Val, and in P(1) Lys, Tyr, and Arg. In position P(1) ', small hydrophilic residues, Ser and to a lesser extent Thr and Ala, occur most often. Pro dominates the region flanking the P(2)-P(1)' segment, i.e. positions P(3) and P(2)'-P(4)'; most notable is its occurrence 5.59 times above chance in P(3)'. Intriguingly, the segment C-terminal to the cleavage site resembles the consensus inhibitory region of calpastatin, the specific inhibitor of the enzyme. Further, the position of the scissile bond correlates with certain sequential attributes, such as secondary structure and PEST score, which, along with the amino acid preferences, suggests that calpain cleaves within rather disordered segments of proteins. The amino acid preferences were confirmed by site-directed mutagenesis of the autolysis sites of Drosophila calpain B; when amino acids at key positions were changed to less preferred ones, autolytic cleavage shifted to other, adjacent sites. Based on these preferences, a new fluorogenic calpain substrate, DABCYLTPLKSPPPSPR-EDANS, was designed and synthesized. In the case of micro- and m-calpain, this substrate is kinetically superior to commercially available ones, and it can be used for the in vivo assessment of the activity of these ubiquitous mammalian calpains.

Opioid peptides derived from hemoglobin: hemorphins

Investigation of hemoglobin peptic hydrolysate has revealed the presence of biologically active peptides with affinity for opioid receptors. Two peptides, VV-hemorphin-7 and LVV-hemorphin-7, were resolved by a combination of size exclusion and reversed phase HPLC. A new spectroscopic method based on the second order derivative spectra analysis of aromatic amino acids has been developed. This method allows qualitative and quantitative evaluation of hemorphins generated by peptic hemoglobin hydrolysis. Using this method, a kinetic study of hemorphins appearance has been undertaken. In this paper, we also evidenced the generation of VV-hemorphin-7 from globin by peritoneal macrophages. In regard to this result, the putative physiological role of hemorphins is discussed.

Identification and characterization of membrane-bound calpains in clathrin-coated vesicles from bovine brain

Calpains are intracellular cysteine proteases activated in a Ca(2+)-dependent manner. Previously, we found that the differentiation of K562 cells induced by 4 beta-phorbol 12-myristate 13-acetate treatment is accompanied by an increase in m-calpain levels and, at the same time, m-calpain becomes localized on the inside of plasma membranes, coated pits, and coated vesicles [Nakamura, M., Mori, M., Morishita, Y., Mori, S. & Kawashima, S. (1992) Exp. Cell Res. 200, 513-522]. We also reported that mu-calpain plays an essential role in morphological changes and membrane fusion of erythrocytes through the degradation of spectrin, a lining protein [Hayashi, M., Saito, Y. & Kawashima, S. (1992) Biochim. Biophys. Res. Commun. 182, 939-946]. Thus, calpains are implicated in endocytosis and/or exocytosis, processes stimulated by Ca2+ and involving intracellular membrane fusion. In this study, we report the biochemical characterization of calpains as components of purified coated vesicles from bovine brain. It was found by Western-blot analysis and chemical cross-linking of proteins that calpains are bound to the membranes of coated vesicles, and not to the coats. The binding of m-calpain to vesicles is Ca(2+)-dependent, while that of mu-calpain is less dependent on the presence of Ca2+. We also identified substrate proteins for calpains in coated vesicles. Upon activation of endogenous calpains, component proteins of coated vesicles such as the clathrin light chain, tubulins, and adaptins, but not the clathrin heavy chain, are highly sensitive to calpain digestion. In the case of exogenously added calpains, low concentrations degraded the same protein components. The degradation pattern differs slightly between added mu-calpain and m-calpain. These results strongly suggest that calpains are involved in the formation of coated vesicles and/or vesicle fusion to endosomes.

Differential distribution of calpain in human lymphoid cells

Calpain, a calcium-activated neutral proteinase, is ubiquitously present in human tissues. To determine if lymphoid cells implicated in pathogenesis of demyelination may harbor calpain in a functionally active form, we determined both muCalpain and mCalpain activities in human lymphoid cell lines. DEAE-cellulose and phenylsepharose column chromatography were used to isolate the enzyme from the natural inhibitor, calpastatin. Lymphocytic lines (CCRF-CEM, MOLT-3, MOLT-4, M.R.) showed predominance of muCalpain (55-80%) whereas the monocytic line (U-937) showed predominance of mCalpain (77%). Proportion and subcellular distribution of both isoforms varied among cell lines. Calpains isolated from U-937 cells degraded myelin basic protein. These results indicate that human lymphoid cells harbor functionally active calpain that can degrade myelin components in vitro. The study suggests a degradative role for calpain in demyelinating diseases.

Dynamic changes in the distribution of the calcium-activated neutral protease in human red blood cells following cellular insult and altered Ca2+ homeostasis

Mechanistic studies were conducted to examine the relationship between oxidative membrane protein damage, altered Ca2+ homeostasis, and changes in the levels of plasma membrane-bound Ca(2+)-activated neutral protease, microCANP. Alterations in the levels of plasma membrane-bound microCANP in erythrocytes and hemolysate following cumene hydroperoxide (CHP) insult were monitored using SDS-PAGE and immunoblot analyses. Free radical scavengers, antioxidant and EGTA effects on membrane-bound microCANP levels in CHP-treated cells and hemolysate were also examined. CHP (2 mM) addition to red cells caused a significant decrease/loss in intensity of numerous protein bands in the SDS-PAGE pattern, to include bands 1, 2, 2.1, 4.1, 4.2, and an approximately 60-kDa protein. N-acetylcysteine (20 mM), dithiothreitol (50 mM), and dimethylthiourea (50 mM) diminished CHP-mediated membrane protein damage; in contrast, dimethylfuran (50 mM) exacerbated CHP-mediated membrane protein damage. Dimethylsulfoxide (50 mM) was without significant effect. The free radical scavengers and antioxidants differentially affected membrane-bound microCANP levels largely in parallel with their ability to modulate membrane protein damage. Immunoblot analysis of 1 mM CHP-treated red cells revealed a time-dependent loss of membrane-bound microCANP, with a complete loss of microCANP monitored at 8 hr. Treatment of erythrocytes with CHP also resulted in concentration-dependent alterations in the level of membrane-bound microCANP: at 0.5 or 1.0 mM CHP a decreased level of membrane-bound microCANP was detected relative to control, whereas an increase in the level of bound enzyme was monitored from 2 to 4 mM CHP. CHP addition to hemolysate produced a decrease in membrane-bound microCANP levels comparable to that observed with erythrocytes; addition of the Ca2+ chelator EGTA or Calpain Inhibitor I (N-acetyl-leucyl-leucyl-leucyl-nor-leucinal) to hemolysate effectively inhibited this decrease. In contrast, treatment of erythrocytes with Ca2+ in the presence of the Ca2+ ionophore A23187 resulted in change in the SDS-PAGE protein bands and membrane-bound microCANP levels that were comparable to those produced by CHP. Inclusion of EGTA in this system prevented microCANP binding. These data provide evidence for membrane damage and concomitant dynamic alterations in membrane-bound microCANP levels in the red cell or hemolysate following oxidative insult, and show that this process can be modulated by free radical scavengers and antioxidant, simulated by treating cells with Ca2+ in the presence of ionophore, and inhibited by EGTA or Calpain Inhibitor I.

Enhanced proteolysis and changes in membrane-associated calpain following phenylhydrazine insult to human red cells

Phenylhydrazine-mediated protein damage in human red cells has been assessed using HPLC, one- and two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and immunoblot analysis of major membrane proteins. The association of the Ca(2+)-activated neutral protease, calpain, with membrane proteins following hydrazine insult was also examined using immunoblot analysis. HPLC amino acid analysis of red cell suspensions was employed to quantify proteolysis. Phenylhydrazine (4 mM) increased the rate of leucine, lysine, and histidine release by approximately 12-, 7-, and 5-fold, respectively. N-acetylcysteine (20 mM), dithiothreitol (50 mM), and dimethylthiourea (50 mM) decreased the rate of phenylhydrazine-stimulated amino acid release by approximately 30-50%; in contrast, the free radical scavengers and antioxidants dimethylfuran (50 mM) and dimethyl sulfoxide (50 mM) were without significant effect. The calcium chelator, EGTA (10 mM) inhibited phenylhydrazine-stimulated proteolysis by approximately 30%. Phenylhydrazine (4 mM) caused attenuation of the major membrane protein bands present in the SDS-PAGE pattern and extensive smearing of a band in the region of approximately 28 kDa. Free radical scavengers and antioxidants failed to ameliorate significantly membrane protein damage in phenylhydrazine-treated cells as judged by SDS-PAGE. Immunoblot analysis of spectrin confirmed these results. Two-dimensional SDS-PAGE of membrane proteins following phenylhydrazine treatment, however, revealed the appearance of new protein spots and a loss of existing protein spots as compared to control. Western blot analysis of membrane-associated calpain (79 kDa (proenzyme), 77- and 75-kDa forms) was also performed. Phenylhydrazine-treated red blood cells exhibited concentration- and time-dependent changes in the level of membrane-associated procalpain relative to control. The inhibitors N-acetylcysteine, dithiothreitol, dimethylthiourea, and dimethyl sulfoxide in the presence of phenylhydrazine appeared to preserve the level of procalpain in association with the membrane proteins, but only N-acetylcysteine and dithiothreitol protected the 77- and 75-kDa forms. In contrast, dimethylfuran in the presence of phenylhydrazine caused a substantial decrease in all three forms of membrane-associated calpain. In phenylhydrazine-treated hemolysate, the level of the 77- and 75-kDa forms of membrane-associated calpain was decreased relative to control. These forms were absent when EGTA (10 mM) was included in the incubation and the level of proenzyme was decreased. These data suggest that calpain is recruited to the membrane following hydrazine insult, undergoes a Ca(2+)-dependent conversion to the active forms, and may be involved in the degradation of damaged cytosolic and membrane protein(s).

Band 3 protein degradation by calpain is enhanced in erythrocytes of old people

Band 3 protein is a major erythrocyte transmembrane glycoprotein. We compared the degradation of band 3 protein by calpain I (a cytoplasmic, micromolar-Ca2(+)-requiring thiol proteinase) in the cells from old individuals (greater than 70 years old) to that in the cells from young ones (20-30 years old). In the young, little degradation of band 3 protein occurred in calpain-treated erythrocyte ghosts. In the old, significant band 3 protein degradation was found in erythrocyte ghosts treated similarly. The difference between young and old in the susceptibility of band 3 protein to calpain was retained in membrane vesicles (membranes stripped of peripheral proteins by NaOH) and in chymotrypsin-generated 60 kDa fragment (CH-60). The isolated N-terminal cytoplasmic 43 kDa fragment was degraded by calpain to a similar extent in old and in young. The separated 17 kDa membrane domain of the CH-60 and the trypsin-generated C-terminal 55 kDa membrane-spanning fragment were not degraded by calpain I in the young, nor in the old. Thus the N-terminal cytoplasmic domain is the domain degraded by calpain I. Enhanced sensitivity in the old is observed in intact band 3 protein and in CH-60, the isolated cytoplasmic domain being equally susceptible in young and old. The observed age-related enhanced sensitivity to calpain is consistent with the presence of modifications in band 3 protein and alterations in the association with the calpain-calpastatin system. Band 3 protein has several important functions, with modifications in the protein having implications for altered cell behaviour in the old individual.

Calmodulin-binding proteins as calpain substrates

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Pre-Golgi degradation of newly synthesized T-cell antigen receptor chains: intrinsic sensitivity and the role of subunit assembly

The T cell antigen receptor (TCR) is a multisubunit complex composed of at least seven transmembrane chains. The predominant species in most T cells has the composition alpha beta gamma delta epsilon zeta 2. The roles of subunit assembly in transport out of the ER and in the recently described process of pre-Golgi degradation of newly synthesized TCR chains were analyzed in a T-cell line deficient in the synthesis of delta chains (delta 2) and in COS-1 fibroblasts transfected with genes encoding individual TCR chains. Studies with the delta-deficient T-cell line showed that, in the absence of delta, the other TCR chains were synthesized at normal rates, but, instead of being transported to the cell surface, they were retained in the ER. Analysis of the fate of TCR chains retained in the ER showed that they were degraded at vastly different rates by a nonlysosomal pathway. Whereas the alpha chains were degraded rapidly, gamma, zeta, and epsilon were relatively long-lived. To analyze whether this selective degradation was because of different intrinsic susceptibility of the individual chains to degradation or to the formation of resistant oligomers, TCR chains were expressed alone or in combinations in COS-1 fibroblasts. These studies showed that (a) individual TCR chains were degraded at different rates when expressed alone in COS-1 cells, and (b) sensitive chains could be stabilized by coexpression with a resistant chain. Taken together, these observations indicate that both intrinsic sensitivity and subunit assembly play a role in determining the rates at which newly synthesized TCR chains are degraded in the ER.

Degradation of proteins with blocked amino groups by cytoplasmic proteases

The effect of blocking amino groups on the susceptibility of BSA and calmodulin to high molecular weight protease (HMP) and calpain, the two major cytosolic proteases, was studied. Both proteases hydrolyzed methylated vs. unmodified BSA more slowly. Methylation of BSA resulted in the accumulation of proteolytic intermediates, especially of larger sizes. However, similar fragments were generated from unmodified BSA indicating that rates of hydrolysis rather that sites of proteolytic cleavage were altered. Calmodulin from Dictyostelium discoideum was hydrolyzed rapidly by HMP whereas brain and muscle calmodulins which have a epsilon-N-trimethyl residue on the single surface lysine were relatively stable.

Proteolysis of the calcium-dependent protease inhibitor by myocardial calcium-dependent protease

Bovine heart peak II calcium-dependent protease was capable of hydrolyzing its specific inhibitor protein at high molar ratios of protease to inhibitor. The proteolysis was inhibited by leupeptin and required millimolar calcium. Thus, it appeared to be attributable to the calcium-dependent protease and not to possible contaminating proteases in the purified preparations of inhibitor or calcium-dependent protease. Incubation of the purified inhibitor with the calcium-dependent protease produced a discrete pattern of inhibitor fragments on Western blots developed with an inhibitor-specific monoclonal antibody. Traces of similar or identical lower molecular weight immunoreactive material could be observed in Western blots of bovine heart extracts, and the immunoreactivity present as these lower molecular weight forms could be increased by incubation of the extracts with calcium ion. These results suggest that the inhibitor can be proteolyzed to low molecular weight forms which can be detected in cardiac tissue extracts, and that calcium-dependent protease(s) may be responsible for this phenomenon.

Binding to erythrocyte membrane is the physiological mechanism for activation of Ca2+-dependent neutral proteinase

In the presence of micromolar concentrations of Ca2+ the catalytic 80 kDa subunit of human erythrocyte procalpain binds to the cytosolic surface of the erythrocyte membrane. Binding is rapid, highly specific and is reversed by the removal of Ca2+. In the bound form the 80 kDa catalytic subunit undergoes a rapid conversion to calpain, the active 75 kDa Ca2+-requiring proteinase. The activated proteinase produces extensive degradation of membrane components, particularly of band 4.1 and 2.1 proteins. Binding to membranes may represent an obligatory physiological mechanism for the conversion of procalpain to calpain.

A dual role for the Ca2+-requiring proteinase in the degradation of hemoglobin by erythrocyte membrane proteinases

Binding of hemoglobin chains to erythrocyte membranes is an obligatory step in the conversion of hemoglobin to acid-soluble products by erythrocyte proteinases. This binding requires limited proteolysis of the hemoglobin chains and also modification of the inner surface of the erythrocyte membrane, both of which result from the action of a soluble Ca2+-requiring neutral proteinase. Final digestion of the bound hemoglobin chains in the membrane complex results from the action of intrinsic membrane endopeptidases. Regulation of the activity of the Ca2+-requiring proteinase by the substrate provides a mechanism for the initiation of selective protein turnover.