Insights into the degradation of human elastin by matrilysin-1

Human matrilysin-1 (MMP-7) is one of the most potent elastases besides macrophage elastase in the family of matrix metalloproteinases (MMPs). It has been reported to provide macrophages with the highest elastinolytic capacity and plays key roles in diseases such as emphysema and cancer. Describing the enzymatic turnover of matrix components helps to understand the molecular basis of disease processes. Hence, in this work, the cleavage behavior of MMP-7 with respect to its natural substrate human elastin was investigated using mass spectrometric (MS) techniques and molecular modeling. Elastin peptides in the range of 500-8000 Da released through the action of MMP-7 were analyzed and domains susceptible to proteolytic attack by MMP-7 were identified. MMP-7 was found to mainly cleave in N- and C-terminal regions of elastin’s precursor, which involves linkages in domains encoded by exons 2, 3, 5-7, 26, and 30-33. In contrast, only few cleavages were found in the central part of the precursor and no cleavages in regions in elastin that are involved in cross-linking. MMP-7 shows a strong preference for Leu in P₁′ and also accepts Val, Gly, and Pro at this position, whereas Ala is not preferred at P₁′. Analysis by molecular modeling revealed that not only the size of the amino acid residue in P₁′ but also the orientation of the neighboring P₁ residue and, thus, the orientation of the peptide bond that is cleaved influences the cleavage preference of MMP-7. Overall, this study provides an important insight into the degradation of human elastin by MMP-7 and may aid in the development of approaches to treat elastin-degrading diseases.     

In vitro degradation of human tropoelastin by MMP-12 and the generation of matrikines from domain 24

Degradation of elastic fibers in tissues can result in the development of disorders that include aneurysms, atherosclerosis, and loss of skin elasticity. Tropoelastin is the precursor of the cross-linked elastin and its expression is triggered by elastin-degrading factors as a response to damage. Factors like UV radiation not only increase the expression of tropoelastin but also potent metalloelastases such as macrophage elastase (MMP-12). The development of elastin-degrading diseases, moreover, is a chronic process during which elastin and tropoelastin are repeatedly exposed to attacks by MMP-12. Hence, in this work we report the in vitro susceptibility of tropoelastin and the potential of MMP-12 to generate matrikines. This work provides evidence that tropoelastin is substantially and rapidly degraded by MMP-12 even at very dilute enzyme concentrations. MMP-12 cleaves at least 86 sites in tropoelastin. Analysis of the generated peptides revealed that some small peptides contained the motif GXXPG that may enable them to bind with the elastin binding protein (EBP). Furthermore, using synthesized peptides it was confirmed that several sites in the sequence encoded by exon 24 which contains repetitive units of biologically active VGVAPG domains are susceptible to attack by MMP-12, provided that the active subsites in MMP-12 (S₄ to S₄′) are occupied. Such cleavage events have lead to the generation of ligands that may bind to EBP.     

Characterization of peptides resulting from digestion of human skin elastin with elastase

Several pathological disorders are associated with abnormalities in elastic fibers, which are mainly composed of elastin. Understanding the biochemical basis of such disorders requires information about the primary structure of elastin. Since the acquisition of structural information for elastin is hampered by its extreme insolubility in water or any organic solvent, in this study, human skin elastin was digested with elastase to produce water-soluble peptides. Tandem mass spectrometry (MS/MS) experiments were performed using conventional electrospray ionization (ESI) and nano-ESI techniques coupled with ion trap and quadrupole time-of-flight (qTOF) mass analyzers, respectively. The peptides were identified from the fragment spectra using database searching and/or de novo sequencing. The cleavage sites of the enzyme and, for the first time, the extent and location of proline hydroxylation in human skin elastin were determined. A total of 117 peptides were identified with sequence coverage of 58.8%. It has been observed that 25% of proline residues in the sequenced region are hydroxylated. Elastase cleaves predominantly at the C-terminals of the amino acids Gly, Val, Leu, Ala, and Ile, and to a lesser extent at Phe, Pro, Glu, and Arg. Our results confirm a previous report that human skin elastin lacks amino acid sequences expressed by exon 26A.     

Molecular and supramolecular structural studies on human tropoelastin sequences

One of the unusual properties of elastin is its ability to coacervate, which has been proposed to play an important role in the alignment of monomeric elastin for cross-linking into the polymeric elastin matrix. The temperature at which this transition takes place depends on several factors including protein concentration, ionic strength, and pH. Previously, polypeptide sequences encoded by different exons of the human tropoelastin gene have been analyzed for their ability to coacervate and to self-assemble. Few of them were indeed able to coacervate and only one, that encoded by exon 30 (EX30), gave amyloid fibers. In this article, we report on two chemically synthesized peptides-a decapeptide and an octadecapeptide-whose sequences are contained in the longer EX30 peptide and on a polypeptide (EX1-7) of 125 amino-acid residues corresponding to the sequence coded by the exons 1-7 and on a polypeptide (EX2-7) of 99 amino-acid residues encoded by exons 2-7 of human tropoelastin obtained by recombinant DNA techniques. Molecular and supramolecular structural characterization of these peptides showed that a minimum sequence of approximately 20 amino acids is needed to form amyloid fibers in the exon 30-derived peptides. The N-terminal region of mature tropoelastin (EX2-7) gives rise to a coacervate and forms elastinlike fibers, whereas the polypeptide sequence containing the signal peptide (EX1-7) forms mainly amyloid fibers. Circular dichroism spectra show that beta-structure is ubiquitous in all the sequences studied, suggesting that the presence of a beta-structure is a necessary, although not sufficient, requirement for the appearance of amyloid fibers.     

The action of neutrophil serine proteases on elastin and its precursor

This study aimed to investigate the degradation of the natural substrates tropoelastin and elastin by the neutrophil-derived serine proteases human leukocyte elastase (HLE), proteinase 3 (PR3) and cathepsin G (CG). Focus was placed on determining their cleavage site specificities using mass spectrometric techniques. Moreover, the release of bioactive peptides from elastin by the three proteases was studied. Tropoelastin was comprehensively degraded by all three proteases, whereas less cleavage occurred in mature cross-linked elastin. An analysis of the cleavage site specificities of the three proteases in tropoelastin and elastin revealed that HLE and PR3 similarly tolerate hydrophobic and/or aliphatic amino acids such as Ala, Gly and Val at P₁, which are also preferred by CG. In addition, CG prefers the bulky hydrophobic amino acid Leu and accepts the bulky aromatic amino acids Phe and Tyr. CG shows a strong preference for the charged amino acid Lys at P₁ in tropoelastin, whereas Lys was not identified at P₁ in CG digests of elastin due to extensive cross-linking at Lys residues in mature elastin. All three serine proteases showed a clear preference for Pro at P₂ and P₄′. With respect to the liberation of potentially bioactive peptides from elastin, the study revealed that all three serine proteases have a similar ability to release bioactive sequences, with CG producing the highest number of these peptides. In bioactivity studies, potentially bioactive peptides that have not been investigated on their bioactivity to date, were tested. Three new bioactive GxxPG motifs were identified; GVYPG, GFGPG and GVLPG.     

Site-specific mutagenesis identifies amino acid residues critical in prohormone processing.

Peptide hormones are generally synthesized as inactive higher mol. wt precursors. Processing of the prohormone into biologically active peptides by specific proteolytic cleavages occurs most often at pairs of basic amino acids but also at single arginine residues. To study the role of protein secondary structure in this process, we used site-directed mutagenesis to modify the predicted secondary structure around the cleavage sites of human prosomatostatin and monitored the processing of the precursor after introduction of the mutated cDNAs in Neuro2A cells. Amino acid substitutions were introduced that affected the possibility of forming beta-turn structures in the immediate vicinity of the somatostatin-28 (S-28) and somatostatin-14 (S-14) cleavage sites. Infection of Neuro2A cells with a retrovirus carrying a human somatostatin cDNA resulted in the expression of prosomatostatin and its processing into S-28 and S-14, indicating that these cells have the necessary enzymes to process prohormone at both single and paired amino acid residues. Disruption of the different beta-turns had various effects on prosomatostatin processing: substitution of Ala for Pro-5 drastically decreased prosomatostatin processing and replacement of Pro-9 by Ala led to the accumulation of the intermediate maturation product [Arg-2Lys-1]-S-14. In contrast, substitution of Ala for Asn-12, Gly+2 and Cys+3 respectively had only very little effect on the proteolytic processing of prosomatostatin. Our results show that amino acids other than the basic amino acid residues are required to define the cleavage sites for prohormone proteolytic processing and suggest that higher orders of protein structure are involved in substrate recognition by the endoproteases.     

Rational design of tropoelastin peptide-based inhibitors of metalloproteinases

Abnormal production of matrix metalloproteinases (MMPs) has been observed in a variety of diseases, such as emphysema, atherosclerosis, and cancer metastasis. Destruction of connective tissue ensues and elastin is often a key target. Three of the main elastolytic MMPs are the gelatinases MMP-2 and MMP-9 and the metalloelastase MMP-12. To investigate the possibility of using peptides to inhibit the elastolytic activity of these enzymes, we mapped the sites within tropoelastin recognized by MMP-9 and MMP-12. Peptides that correspond to regions overlapping these sites were then tested for their ability to inhibit these MMPs. These included an unmodified peptide directed against MMP-9 (peptide PP), cysteine-containing peptides that mimicked either the MMP-9 (peptide NCP) or the MMP-12 (peptide lin24) cleavage sites in tropoelastin and their cyclized forms (CP and cyc24, respectively), and a peptide containing a zinc-chelating hydroxamate group directed against MMP-9 (HP). The presence of a free sulfhydryl or hydroxamate group capable of chelating the zinc ion in the active site of the MMPs was generally found to increase the inhibitory activity of the peptides. The specificity of the inhibitors varied, with some of the inhibitors showing activity against all of the MMPs examined. None of the inhibitors had any significant effect on the activity of the unrelated serine protease, plasmin. K(i) values for the inhibitors were in the micromolar range. Our results suggest ways of developing other MMP inhibitors based on substrate recognition sites that may provide greater levels of inhibition.     

Expression of elastolytic cathepsins in human skin and their involvement in age-dependent elastin degradation

BackgroundSkin ageing is associated with structure-functional changes in the extracellular matrix, which is in part caused by proteolytic degradation. Since cysteine cathepsins are major matrix protein-degrading proteases, we investigated the age-dependent expression of elastolytic cathepsins K, S, and V in human skin, their in vitro impact on the integrity of the elastic fibre network, their cleavage specificities, and the release of bioactive peptides.MethodsCathepsin-mediated degradation of human skin elastin samples was assessed from young to very old human donors using immunohistochemical and biochemical assays, scanning electron microscopy, and mass spectrometry.ResultsElastin samples derived from patients between 10 and 86 years of age were analysed and showed an age-dependent deterioration of the fibre structure from a dense network of thinner fibrils into a beaded and porous mesh. Reduced levels of cathepsins K, S, and V were observed in aged skin with a predominant epidermal expression. Cathepsin V was the most potent elastase followed by cathepsin K and S. Biomechanical analysis of degraded elastin fibres corroborated the destructive activity of cathepsins. Mass spectrometric determination of the cleavage sites in elastin revealed that all three cathepsins predominantly cleaved in hydrophobic domains. The degradation of elastin was efficiently inhibited by an ectosteric inhibitor. Furthermore, the degradation of elastin fibres resulted in the release of bioactive peptides, which have previously been associated with various pathologies.ConclusionCathepsins are powerful elastin-degrading enzymes and capable of generating a multitude of elastokines. They may represent a viable target for intervention strategies to reduce skin ageing.     

Consensus sequence for processing of peptide precursors at monobasic sites

Many regulatory peptide precursors undergo post-translational processing at mono- and/or dibasic residues. Comparison of amino acids around the monobasic cleavage sites suggests that these cleavages follow certain sequence motifs and can be described as the rules that govern monobasic cleavages: (i) a basic amino acid it present at either 3, 5, or 7 amino acids N-terminal to the cleavage site, (ii) hydrophobic aliphatic amino acids (leucine, isoleucine, valine, or methionine) are never present in the position C-terminal to the monobasic amino acid at the cleavage site, (iii) a cysteine is never present in the vicinity of the cleavage site, and (iv) an aromatic amino acid is never present at the position N-terminal to the monobasic amino acid at the cleavage site. In addition to these rules, the monobasic cleavages follow certain tendencies: (i) the amino acid at the cleavage site tends to be predominantly arginine, (ii) the amino acid at the position C-terminal to the cleavage site tends to be serine, alanine or glycine in more than 60% of the cases, (iii) the amino acid at either 3, 5, or 7 position N-terminal to the cleavage site tends to be arginine, (iv) aromatic amino acids are rare at the position C-terminal to the monobasic amino acid at the cleavage site, and (v) aliphatic amino acids tend to be in the two positions N-terminal to and the two positions C-terminal to the cleavage site, except as noted above. When compared with a large number of sequence containing single basic amino acids, these rules and tendencies are capable of not only correctly predicting the processing sites, but also are capable of excluding most of the single basic sequences that are known to be uncleaved. Many or these rules can also be applied to correctly predict the dibasic and multibasic cleavage sites suggesting that the rules and tendencies could govern endoproteolytic processing at the monobasic, dibasic and multibasic sites.     

Neuropeptide-processing carboxypeptidases

Neuropeptides are generally produced from precursor proteins by selective cleavage at specific sites, usually involving basic amino acids. Enzymes such as the prohormone convertases and carboxypeptidase E are highly specific for these basic amino acid-containing sites. In addition to this "traditional" pathway, several neuropeptides are known to be cleaved at non-basic sites, and the enzymes responsible for these cleavages have not been conclusively identified. In a recent search for novel members of the metallocarboxypeptidase family, we found three human genes. One of these, named "CPA-5," has a specificity for C-terminal hydrophobic amino acids and mRNA expression in brain, pituitary, and testis. To test whether CPA-5 protein has a distribution pattern in pituitary that is consistent with a role for this enzyme in the non-basic processing of proopiomelanocortin-derived peptides such as beta-endorphin and adrenocorticotropin, we examined the distribution of CPA-5 using immunocytochemistry. In the pituitary, CPA-5 is detected in the neurointermediate lobe and in scattered cells in the anterior lobe. In the AtT-20 corticotroph cell line, CPA-5 has a perinuclear distribution. Taken together, these results are consistent with a role for CPA-5 in the intracellular processing of proopiomelanocortin-derived peptides at non-basic sites.     

Characterization of endothelin-converting enzyme-2. Implication for a role in the nonclassical processing of regulatory peptides

Most neuroendocrine peptides are generated by proteolysis of the precursors at basic residue cleavage sites. Prohormone convertases belonging to the subtilisin family of serine proteases are primarily responsible for processing at these "classical sites." In addition to the classical cleavages, a subset of bioactive peptides is generated by processing at "nonclassical" sites. The proteases responsible for these cleavages have not been well explored. Members of several metalloprotease families have been proposed to be involved in nonclassical processing. Among them, endothelin-converting enzyme-2 (ECE-2) is a good candidate because it exhibits a neuroendocrine distribution and an acidic pH optimum. To examine the involvement of this protease in neuropeptide processing, we purified the recombinant enzyme and characterized its catalytic activity. Purified ECE-2 efficiently processes big endothelin-1 to endothelin-1 by cleavage between Trp(21) and Val(22) at acidic pH. To characterize the substrate specificity of ECE-2, we used mass spectrometry with a panel of 42 peptides as substrates to identify the products. Only 10 of these 42 peptides were processed by ECE-2. A comparison of residues around the cleavage site revealed that ECE-2 exhibits a unique cleavage site selectivity that is related to but distinct from that of ECE-1. ECE-2 tolerates a wide range of amino acids in the P1-position and prefers aliphatic/aromatic residues in the P1'-position. However, only a small fraction of the aliphatic/aromatic amino acid-containing sites were cleaved, indicating that there are additional constraints beyond the P1- and P1'-positions. The enzyme is able to generate a number of biologically active peptides from peptide intermediates, suggesting an important role for this enzyme in the biosynthesis of regulatory peptides. Also, ECE-2 processes proenkephalin-derived bovine adrenal medulla peptides, and this processing leads to peptide products known to have differential receptor selectivity. Finally, ECE-2 processes PEN-LEN, an endogenous inhibitor of prohormone convertase 1, into products that do not inhibit the enzyme. Taken together, these results are consistent with an important role for ECE-2 in the processing of regulatory peptides at nonclassical sites.     

Evidence that differentiates between precursor cleavages at dibasic and Arg-X-Lys/Arg-Arg sites.

It is well known that precursor cleavage at paired basic amino acids (e.g., Lys-Arg, Arg-Arg) within the regulated secretory pathway is one of the key steps to produce bioactive peptides. On the other hand, we have recently shown that precursors with an Arg residue at the fourth residue upstream of the cleavage site besides the basic pair, i.e. with the Arg-X-Lys/Arg-Arg (RXK/RR) motif, are cleaved within the constitutive secretory pathway. To discriminate between the precursor cleavage at RXK/RR sites within the constitutive pathway and that at dibasic sites within the regulated pathway, we examined the effects of drugs affecting the secretory process, intracellular Ca2+ depletion, and a protease inhibitor on these cleavages. Chloroquine (a weak base), depletion of intracellular Ca2+ by A23187 (a Ca2+ ionophore), and the Pittsburgh-type mutant of alpha 1-protease inhibitor differentially affected these two cleavages. Brefeldin A, which impedes protein transport from the endoplasmic reticulum to the Golgi complex, inhibited both cleavages. Colchicine (an anti-microtubular drug) had no discernible effect on either cleavage. These observations support the notion that the precursor cleavages at dibasic and RXK/RR sites occur in different subcellular compartments, and are catalyzed by different processing endoproteases.     

Comparative analysis of neuropeptide cleavage sites in human, mouse, rat, and cattle

Neuropeptides are an important class of signaling molecules that result from complex and variable posttranslational processing of precursor proteins and thus are difficult to identify based solely on genomic information. Bioinformatics prediction of precursor cleavage sites can support effective biochemical characterization of neuropeptides. Neuropeptide cleavage models were developed using comprehensive human, mouse, rat, and cattle precursor data sets and used to compare predicted neuropeptide processing across these species. Logistic regression and artificial neural network models were used to predict cleavages based on amino acid and physiochemical properties of amino acids at precursor sequence locations proximal to cleavage. Correct cleavage classification rates across species and models ranged from 85% to 100%, suggesting that amino acid and amino acid properties have major impact on the probability of cleavage and that these factors have comparable effects in human, mouse, rat, and cattle. The variable accuracy of each species-specific model to predict cleavage sites indicated that there are species- and precursor-specific processing patterns. Prediction of mouse cleavages using rat models was highly accurate, yet the reverse was not observed. Sensitivity and specificity revealed that logistic models are well suited to maximize the rate of true noncleavage predictions with moderate rates of true cleavage predictions; meanwhile, artificial neural networks maximize the rate of true cleavage predictions with moderate to low true noncleavage predictions. Logistic models also provided insights into the strength of the amino acid associations with cleavage. Prediction of neuropeptide cleavage sites using human, mouse, rat, and cattle models are available at . Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Allison Tegge and Bruce Southey contributed equally to this work.     

Dentin sialophosphoprotein is processed by MMP-2 and MMP-20 in vitro and in vivo

Dentin sialophosphoprotein (DSPP) is a major secretory product of odontoblasts and is critical for proper tooth dentin formation. During dentinogenesis, DSPP is proteolytically cleaved into smaller subunits. These cleavages are proposed activation steps, and failure to make these cleavages is a potential cause of developmental tooth defects. We tested the hypothesis that dentin-resident matrix metalloproteinases catalyze the cleavages that process DSPP. We defined the exact DSPP cleavages that are catalyzed by proteases during crown formation by isolating DSPP-derived proteins from developing porcine molars and characterizing their N-terminal sequences and apparent size on SDS-PAGE and Western blots. The in vivo DSPP cleavage sites were on the N-terminal sides of Thr(200), Ser(330), Val(353), Leu(360), Ile(362), Ser(377), Ser(408), and Asp(458). The initial DSPP cleavage is between dentin glycoprotein (DGP) and dentin phosphoprotein (DPP), generating dentin sialoprotein (DSP)/DGP and DPP. Gelatin and casein zymograms identified MMP-2, MMP-20, and KLK4 in the dentin extracts. MMP-2 and MMP-20 were purified from over 150 g of porcine dentin powder and incubated with DSP-DGP and DPP. These enzymes show no activity in further cleaving DPP. MMP-20 cleaves DSP-DGP to generate DSP and DGP. MMP-20 also cleaves DSP at multiple sites, releasing N-terminal DSP cleavage products ranging in size from 25 to 38 kDa. MMP-2 makes multiple cleavages near the DSP C terminus, releasing larger forms of DGP, or "extended DGPs." Exact correspondence between DSPP cleavage sites that occur in vivo and those generated in vitro demonstrates that MMP-2 and MMP-20 process DSPP into smaller subunits in the dentin matrix during odontogenesis.     

A model two-component system for studying the architecture of elastin assembly in vitro

Tropoelastin is encoded by a single human gene that spans 36 exons and is oxidized in vivo by mammalian lysyl oxidase at the epsilon amino group of available lysines to give the adipic semialdehyde, which then facilitates covalent cross-link formation in an enzyme-free process involving tropoelastin association. We demonstrate here that this process is effectively modeled by a two protein component system using purified lysyl oxidase from the yeast Pichia pastoris to facilitate the oxidation and subsequent cross-linking of recombinant human tropoelastin. The oxidized human tropoelastin forms an elastin-like polymer (EL) that is elastic, shows hydrogel behavior and contains typical elastin cross-links including lysinonorleucine, allysine aldol, and desmosine. Protease digestion and subsequent mass-spectrometry analysis of multiple ELs allowed for the identification of specific intra- and inter-molecular cross-links, leading to a model of the molecular architecture of elastin assembly in vitro. Specific intra-molecular cross-links were confined to the region of tropoelastin encoded by exons 6-15. Inter-molecular cross-links were prevalent between the regions encoded by exons 19-25. We find that assembly of tropoelastin molecules in ELs are highly enriched for a defined subset of cross-links.     

Schistosome invasion of human skin and degradation of dermal elastin are mediated by a single serine protease

Aquatic larvae (cercariae) of the trematode parasite Schistosoma mansoni rapidly penetrate human skin by degrading host proteins including elastin. Two serine proteases, one chymotrypsin-like and the second trypsin-like, have been proposed to be involved. To evaluate the relative roles of these two proteases in larval invasion, both were purified, identified by sequence, and then biochemically characterized. The trypsin-like activity was resolved into two distinct serine proteases 76% similar in predicted amino acid sequence. Southern blot analysis, genomic polymerase chain reaction, and immunolocalization demonstrated that the trypsin-like proteases are in fact not from the schistosome, but are released with larvae from the snail host Biomphalaria glabrata. Invasion inhibition assays using selective inhibitors confirmed that the chymotrypsin-like protease is the enzyme involved in skin penetration. Its ability to degrade skin elastin was confirmed, and the three sites of cleavage within elastin help define a new family of elastases.     

Characterization and expression of the gene encoding membralin, an evolutionary conserved protein expressed in the central nervous system

We have identified a novel gene that encodes an evolutionary conserved protein that we name membralin since it contains multiple transmembrane regions. The human gene C19orf6 localizes to chromosome 19p13.3. Splice variant membralin-1 is encoded by 11 exons, translating into 620 amino acids. In addition, we found evidence for two additional splice variants in the human. The mouse gene ORF61 localizes to chromosome 10. We cloned two splice variants in mouse: membralin-1, which is encoded by 12 exons, translating into 574 amino acids, and membralin-2, which translates into 598 amino acids. The existence of rat membralin-1 (574 amino acids long) is, so far, only supported by in situ hybridization result, whereas the existence of rat membralin-2 (598 amino acids long) is strongly supported by overlapping ESTs. Gene homologues were also identified in fruit-fly (CG8405, chromosome 2R 52; two splice variants), nematode (chromosome III), and Arabidopsis thaliana (chromosome 1). Sequence analysis revealed no closely related genes, suggesting that membralin represents the sole member of a unique protein family.     

Lysozyme degradation by the bovine multicatalytic proteinase complex (proteasome): evidence for a nonprocessive mode of degradation.

The multicatalytic proteinase complex (MPC, proteasome) is composed of 28 subunits organized into four rings surrounding a water-filled canal. The catalytic centers face the inner canal confining protein substrates to an enclosed space. Experimental findings obtained with MPC from archaebacteria suggest that degradation of proteins by the complex is processive and have led to the proposal that the lengths of the peptides formed during degradation depend on the distances between active sites in the catalytic chamber. To test whether these postulates are valid for the MPC from a higher organism, we examined the size distributions of products formed early versus late in the course of protein degradation using reduced carboxamidomethylated lysozyme (RCM-lysozyme) and MPC from bovine spleen and pituitary. The majority of final degradation products ranged in length from 6 to 20 amino acids without a clear predilection for peptides of a particular, uniform size. Our observations suggest that selection of cleavage sites is governed by the amino acid sequence specificity of the MPC catalytic sites rather than the distances between the active sites. Early in the course of degradation, peptides with masses between 5 and 10 kDa accumulated in more than 80-fold molar excess over the MPC, indicating dissociation of large, partially degraded intermediates. Initial cleavages occurred at distances between 10 and 44 amino acids from the N- or C-terminus of the molecule and often involved removal of a fragment from both the N- and C-termini of RCM-lysozyme. Our data indicate that degradation of proteins by MPCs from higher organisms involves a nonprocessive mechanism comprised of multiple, independent cleavages with dissociation of degradation intermediates. A general model for protein degradation by the MPC is discussed.     

Photolysis and ozonolysis of (iso)desmosine-containing crosslinked peptides from porcine aorta elastin

This report describes the use of photolysis and ozonolysis as a means of achieving complete cleavage of the pyridinium ring of (iso)desmosine in crosslinked elastin peptides. Although photolysis leads to the opening of the ring with concomitant formation of lysine, the peptide chains remain attached. Subsequent ozonolysis is able to completely achieve the cleavage of the rest of the ring skeleton, thus leading to the separation of the peptide chains. Formation of new amino acids, i.e. alpha-aminoadipic and glutamic acids, is emphasized. Localization of these amino acids within the released peptides should be of help in structural investigations on the crosslinking zones involving either isodesmosine or desmosine. However, other amino acids such as tyrosine and phenylalanine are sensitive to this procedure and side reactions occur which are responsible for peptide bond cleavage with the formation of breakdown products.