Intermediate labile intermolecular crosslinks in collagen fibres

A Mathematical Model for Postimplant Collagen Remodeling in an Autologous Engineered Pulmonary Arterial Conduit

This study was undertaken to develop a mathematical model of the long-term in vivo remodeling processes in postimplanted pulmonary artery (PA) conduits. Experimental results from two extant ovine in vivo studies, wherein polyglycolic-acid (PGA)/poly-L-lactic acid tubular conduits were constructed, cell seeded, incubated for 4 weeks, and then implanted in mature sheep to obtain the remodeling data for up to two years. Explanted conduit analysis included detailed novel structural and mechanical studies. Results in both studies indicated that the in vivo conduits remained dimensionally stable up to 80 weeks, so that the conduits maintained a constant in vivo stress and deformation state. In contrast, continued remodeling of the constituent collagen fiber network as evidenced by an increase in effective tissue uniaxial tangent modulus, which then stabilized by one year postimplant. A mesostructural constitute model was then applied to extant planar biaxial mechanical data and revealed several interesting features, including an initial pronounced increase in effective collagen fiber modulus, paralleled by a simultaneous shift toward longer, more uniformly length-distributed collagen fibers. Thus, while the conduit remained dimensionally stable, its internal collagen fibrous structure and resultant mechanical behaviors underwent continued remodeling that stabilized by one year. A time-evolving structural mixture-based mathematical model specialized for this unique form of tissue remodeling was developed, with a focus on time-evolving collagen fiber stiffness as the driver for tissue-level remodeling. The remodeling model was able to fully reproduce (1) the observed tissue-level increases in stiffness by time-evolving simultaneous increases in collagen fiber modulus and lengths, (2) maintenance of the constant collagen fiber angular dispersion, and (3) stabilization of the remodeling processes at one year. Collagen fiber remodeling geometry was directly verified experimentally by histological analysis of the time-evolving collagen fiber crimp, which matches model predictions very closely. Interestingly, the remodeling model indicated that the basis for tissue homeostasis was maintenance of the collagen fiber ensemble stress for all orientations, and not individual collagen fiber stresses. Unlike other growth and remodeling models that traditionally treat changes in the external boundary conditions (e.g., changes in blood pressure) as the primary input stimuli, the driver herein is changes to the internal constituent collagen fiber themselves due to cellular mediated cross-linking.

Characterization of the layer, direction and time-dependent mechanical behaviour of the human oesophagus and the effects of formalin preservation

The mechanical characterization of the oesophagus is essential for applications such as medical device design, surgical simulations and tissue engineering, as well as for investigating the organ's pathophysiology. However, the material response of the oesophagus has not been established ex vivo in regard to the more complex aspects of its mechanical behaviour using fresh, human tissue: as of yet, in the literature, only the hyperelastic response of the intact wall has been studied. Therefore, in this study, the layer-dependent, anisotropic, visco-hyperelastic behaviour of the human oesophagus was investigated through various mechanical tests. For this, cyclic tests, with increasing stretch levels, were conducted on the layers of the human oesophagus in the longitudinal and circumferential directions and at two different strain rates. Additionally, stress-relaxation tests on the oesophageal layers were carried out in both directions. Overall, the results show discrete properties in each layer and direction, highlighting the importance of treating the oesophagus as a multi-layered composite material with direction-dependent behaviour. Previously, the authors conducted layer-dependent cyclic experimentation on formalin-embalmed human oesophagi. A comparison between the fresh and embalmed tissue response was carried out and revealed surprising similarities in terms of anisotropy, strain-rate dependency, stress-softening and hysteresis, with the main difference between the two preservation states being the magnitude of these properties. As formalin fixation is known to notably affect the formation of cross-links between the collagen of biological materials, the differences may reveal the influence of cross-links on the mechanical behaviour of soft tissues.

Validation of Fourier Transform Infrared Microspectroscopy for the Evaluation of Enzymatic Cross-Linking of Bone Collagen

Enzymatic cross-linking of the bone collagen is important to resist to crack growth and to increased flexural strength. In the present study, we proposed a new method for assessment of enzymatic cross-link based on Fourier transform infrared (FTIR) microspectroscopy that takes into account secondary structure of type I collagen. Briefly, femurs were collected from sham or ovariectomized mice and subjected either to high-performance liquid chromatography-mass spectrometry or embedded in polymethylmethacrylate, cut and analyzed by FTIR microspectroscopy. FTIR acquisition was recorded before and after ultraviolet (UV) exposure or acid treatment. In addition, femurs from a second animal study were used to compare gene expression of Plod2 and Lox enzymes and enzymatic cross-links determined by FTIR microspectroscopy. We evidenced here that intensities and areas of subbands located at ~1660, ~1680, and ~1690 cm-1 were positively and significantly associated with the concentration of pyridinoline (PYD), deoxypyridinoline, or immature dihydroxylysinonorleucine/hydroxylysinonorleucine cross-links. Seventy-two hours exposure to UV light significantly reduced by ~86% and ~89% the intensity and area of the ~1660 cm-1 subband. Similarly, 24 h of acid treatment significantly reduced by 78% and 76% the intensity and area of the ~1690 cm-1 subband. Plod2 and Lox expression were also positively associated to the signal of the ~1660 and ~1690 cm-1 subbands. In conclusion, our study provided a new method for decomposing the amide I envelope of bone section that positively correlates with PYD and immature collagen cross-links. This method allows for investigation of tissue distribution of enzymatic cross-links in bone section.

Effect of testing temperature on the nanostructural response of tendon to tensile mechanical overload

Despite many in vitro mechanical experiments of tendon being conducted at room temperature, few assessments have been made to determine how the structural response of tendon to mechanical overload may vary with ambient temperature. We explored whether damage to the collagen nanostructure of tendon resulting from tensile rupture varies with temperature. Use of bovine tail tendons in combination with NaBH₄ crosslink stabilization treatment allowed us to probe the mechanisms underlying the observed changes. Untreated tendons and NaBH₄-stabilized tendons were pulled to rupture at temperatures of 24, 37, and 55 °C. Of nine mechanical parameters measured from the resulting stress-strain curves, only yield stress differed between the tendons tested at 37 and 24 °C. When tested at 55 °C, untreated tendons showed large reductions in ultimate strength and toughness, while NaBH₄-stabilized tendons showed smaller reductions. Differential scanning calorimetry was used to assess damage to the collagen fibril nanostructure of tendons resulting from rupture, with samples from the ruptured tendons compared to samples from the same tendons removed prior to loading. While there was indication that overload-induced molecular packing disruption to collagen fibrils may be heightened at 37 °C, statistical increases in damage compared to that occurring at 24 °C were only seen when testing was conducted at 55 °C. The results show that the temperature sensitivity of tendon to ramp loading depends on crosslinking within the tissue. In poorly crosslinked tissues, collagen may be more susceptible to mechanical damage when tested at physiologic temperature compared to room temperature. For tendons with a high density of thermally stable crosslinks, such as the human Achilles or patellar tendons, testing at room temperature should produce comparable results to testing at physiologic temperature.

Collagen fibrils in functionally distinct tendons have differing structural responses to tendon rupture and fatigue loading

In this study we investigate relationships between the nanoscale structure of collagen fibrils and the macroscale functional response of collagenous tissues. To do so, we study two functionally distinct classes of tendons, positional tendons and energy storing tendons, using a bovine forelimb model. Molecular-level assessment using differential scanning calorimetry (DSC), functional crosslink assessment using hydrothermal isometric tension (HIT) analysis, and ultrastructural assessment using scanning electron microscopy (SEM) were used to study undamaged, ruptured, and cyclically loaded samples from the two tendon types. HIT indicated differences in both crosslink type and crosslink density, with flexor tendons having more thermally stable crosslinks than the extensor tendons (higher TFmax of >90 vs. 75.1±2.7°C), and greater total crosslink density than the extensor tendons (higher t1/2 of 11.5±1.9 vs. 3.5±1.0h after NaBH4 treatment). Despite having a lower crosslink density than flexor tendons, extensor tendons were significantly stronger (37.6±8.1 vs. 23.1±7.7MPa) and tougher (14.3±3.6 vs. 6.8±3.4MJ/m(3)). SEM showed that collagen fibrils in the tougher, stronger extensor tendons were able to undergo remarkable levels of plastic deformation in the form of discrete plasticity, while those in the flexor tendons were not able to plastically deform. When cyclically loaded, collagen fibrils in extensor tendons accumulated fatigue damage rapidly in the form of kink bands, while those in flexor tendons did not accumulate significant fatigue damage. The results demonstrate that collagen fibrils in functionally distinct tendons respond differently to mechanical loading, and suggests that fibrillar collagens may be subject to a strength vs. fatigue resistance tradeoff.

STATEMENT OF SIGNIFICANCE

Collagen fibrils-nanoscale biological cables-are the fundamental load-bearing elements of all structural human tissues. While all collagen fibrils share common features, such as being composed of a precise quarter-staggered polymeric arrangement of triple-helical collagen molecules, their structure can vary significantly between tissue types, and even between different anatomical structures of the same tissue type. To understand normal function, homeostasis, and disease of collagenous tissues requires detailed knowledge of collagen fibril structure-function. Using anatomically proximate but structurally distinct tendons, we show that collagen fibrils in functionally distinct tendons have differing susceptibilities to damage under both tensile overload and cyclic fatigue loading. Our results suggest that the structure of collagen fibrils may lead to a strength versus fatigue resistance tradeoff, where high strength is gained at the expense of fatigue resistance, and vice versa.

Fracture mechanics of collagen fibrils: influence of natural cross-links

Tendons are important load-bearing structures, which are frequently injured in both sports and work. Type I collagen fibrils are the primary components of tendons and carry most of the mechanical loads experienced by the tissue, however, knowledge of how load is transmitted between and within fibrils is limited. The presence of covalent enzymatic cross-links between collagen molecules is an important factor that has been shown to influence mechanical behavior of the tendons. To improve our understanding of how molecular bonds translate into tendon mechanics, we used an atomic force microscopy technique to measure the mechanical behavior of individual collagen fibrils loaded to failure. Fibrils from human patellar tendons, rat-tail tendons (RTTs), NaBH₄ reduced RTTs, and tail tendons of Zucker diabetic fat rats were tested. We found a characteristic three-phase stress-strain behavior in the human collagen fibrils. There was an initial rise in modulus followed by a plateau with reduced modulus, which was finally followed by an even greater increase in stress and modulus before failure. The RTTs also displayed the initial increase and plateau phase, but the third region was virtually absent and the plateau continued until failure. The importance of cross-link lability was investigated by NaBH₄ reduction of the rat-tail fibrils, which did not alter their behavior. These findings shed light on the function of cross-links at the fibril level, but further studies will be required to establish the underlying mechanisms.

Cross-link stabilization does not affect the response of collagen molecules, fibrils, or tendons to tensile overload

We investigated whether immature allysine-derived cross-links provide mechanically labile linkages by exploring the effects of immature cross-link stabilization at three levels of collagen hierarchy: damaged fibril morphology, whole tendon mechanics, and molecular stability. Tendons from the tails of young adult steers were either treated with sodium borohydride (NaBH₄) to stabilize labile cross-links, exposed only to the buffer used during stabilization treatment, or maintained as untreated controls. One-half of each tendon was then subjected to five cycles of subrupture overload. Morphologic changes to collagen fibrils resulting from overload were investigated using scanning electron microscopy, and changes in the hydrothermal stability of collagen molecules were assessed using hydrothermal isometric tension testing. NaBH4 cross-link stabilization did not affect the response of tendon collagen to tensile overload at any of the three levels of hierarchy studied. Cross-link stabilization did not prevent the characteristic overload-induced mode of fibril damage that we term discrete plasticity. Similarly, stabilization did not alter the mechanical response of whole tendons to overload, and did not prevent an overload-induced thermal destabilization of collagen molecules. Our results indicate that hydrothermally labile cross-links may not be as mechanically labile as was previously thought.

Repeated subrupture overload causes progression of nanoscaled discrete plasticity damage in tendon collagen fibrils

A critical feature of tendons and ligaments is their ability to resist rupture when overloaded, resulting in strains or sprains instead of ruptures. To treat these injuries more effectively, it is necessary to understand how overload affects the primary load-bearing elements of these tissues: collagen fibrils. We have investigated how repeated subrupture overload alters the collagen of tendons at the nanoscale. Using scanning electron microscopy to examine fibril morphology and hydrothermal isometric tension testing to look at molecular stability, we demonstrated that tendon collagen undergoes a progressive cascade of discrete plasticity damage when repeatedly overloaded. With successive overload cycles, fibrils develop an increasing number of kinks along their length. These kinks-discrete zones of plastic deformation known to contain denatured collagen molecules-are accompanied by a progressive and eventual total loss of D-banding along the surface of fibrils, indicating a loss of native molecular packing and further molecular denaturation. Thermal analysis of molecular stability showed that the destabilization of collagen molecules within fibrils is strongly related to the amount of strain energy dissipated by the tendon after yielding during tensile overload. These novel findings raise new questions about load transmission within tendons and their fibrils and about the interplay between crosslinking, strain-energy dissipation ability, and molecular denaturation within these structures.

Designed to fail: a novel mode of collagen fibril disruption and its relevance to tissue toughness

Collagen fibrils are nanostructured biological cables essential to the structural integrity of many of our tissues. Consequently, understanding the structural basis of their robust mechanical properties is of great interest. Here we present what to our knowledge is a novel mode of collagen fibril disruption that provides new insights into both the structure and mechanics of native collagen fibrils. Using enzyme probes for denatured collagen and scanning electron microscopy, we show that mechanically overloading collagen fibrils from bovine tail tendons causes them to undergo a sequential, two-stage, selective molecular failure process. Denatured collagen molecules-meaning molecules with a reduced degree of time-averaged helicity compared to those packed in undamaged fibrils-were first created within kinks that developed at discrete, repeating locations along the length of fibrils. There, collagen denaturation within the kinks was concentrated within certain subfibrils. Additional denatured molecules were then created along the surface of some disrupted fibrils. The heterogeneity of the disruption within fibrils suggests that either mechanical load is not carried equally by a fibril's subcomponents or that the subcomponents do not possess homogenous mechanical properties. Meanwhile, the creation of denatured collagen molecules, which necessarily involves the energy intensive breaking of intramolecular hydrogen bonds, provides a physical basis for the toughness of collagen fibrils.

Mechanical strain stabilizes reconstituted collagen fibrils against enzymatic degradation by mammalian collagenase matrix metalloproteinase 8 (MMP-8)

BACKGROUND

Collagen, a triple-helical, self-organizing protein, is the predominant structural protein in mammals. It is found in bone, ligament, tendon, cartilage, intervertebral disc, skin, blood vessel, and cornea. We have recently postulated that fibrillar collagens (and their complementary enzymes) comprise the basis of a smart structural system which appears to support the retention of molecules in fibrils which are under tensile mechanical strain. The theory suggests that the mechanisms which drive the preferential accumulation of collagen in loaded tissue operate at the molecular level and are not solely cell-driven. The concept reduces control of matrix morphology to an interaction between molecules and the most relevant, physical, and persistent signal: mechanical strain.

METHODOLOGY/PRINCIPAL FINDINGS

The investigation was carried out in an environmentally-controlled microbioreactor in which reconstituted type I collagen micronetworks were gently strained between micropipettes. The strained micronetworks were exposed to active matrix metalloproteinase 8 (MMP-8) and relative degradation rates for loaded and unloaded fibrils were tracked simultaneously using label-free differential interference contrast (DIC) imaging. It was found that applied tensile mechanical strain significantly increased degradation time of loaded fibrils compared to unloaded, paired controls. In many cases, strained fibrils were detectable long after unstrained fibrils were degraded.

CONCLUSIONS/SIGNIFICANCE

In this investigation we demonstrate for the first time that applied mechanical strain preferentially preserves collagen fibrils in the presence of a physiologically-important mammalian enzyme: MMP-8. These results have the potential to contribute to our understanding of many collagen matrix phenomena including development, adaptation, remodeling and disease. Additionally, tissue engineering could benefit from the ability to sculpt desired structures from physiologically compatible and mutable collagen.

The classic: Bone morphogenetic protein

This Classic Article is a reprint of the original work by Marshall R. Urist and Basil S. Strates, Bone Morphogenetic Protein. An accompanying biographical sketch of Marshall R. Urist, MD is available at DOI 10.1007/s11999-009-1067-4; a second Classic Article is available at DOI 10.1007/s11999-009-1069-2; and a third Classic Article is available at DOI 10.1007/s11999-009-1070-9. The Classic Article is copyright 1971 by Sage Publications Inc. Journals and is reprinted with permission from Urist MR, Strates BS. Bone morphogenetic protein. J Dent Res. 1971;50:1392-1406.

Differences in collagen cross-linking between the four valves of the bovine heart: a possible role in adaptation to mechanical fatigue

Hydrothermal isometric tension (HIT) testing and high-performance liquid chromatography were used to assess the molecular stability and cross-link population of collagen in the four valves of the adult bovine heart. Untreated and NaBH4-treated tissues under isometric tension were heated in a water bath to a 90°C isotherm that was sustained for 5 h. The denaturation temperature (Td), associated with hydrogen bond rupture and molecular stability, and the half-time of load decay ( t1/2), associated with peptide bond hydrolysis and intermolecular cross-linking, were calculated from acquired load/temperature/time data. An unpaired group of samples of the same population was biochemically assayed for the types and quantities of enzymatic cross-links present. Tissues known to endure higher in vivo transvalvular pressures had lower Td values, suggesting that molecular stability is inversely related to in vivo loading. The treated inflow valves (mitral and tricuspid) had significantly lower t1/2 values than did treated outflow valves (aortic and pulmonary), suggesting lower overall cross-linking in the inflow valves. Inflow valves were also found to fail during HIT testing significantly more often than outflow valves, also suggestive of a decreased cross-link population. Inflow valves may be remodeling at a faster rate and may be at an earlier state of molecular “maturity” than outflow valves. At the molecular level, the thermal stability of collagen is associated with in vivo loading and may be influenced by the mature, aldimine-derived cross-link, histidinohydroxylysinonorleucine. We conclude that the valves of the heart utilize differing, location-specific strategies to resist biomechanical fatigue loading.

Structure and function of tuna tail tendons

The caudal tendons in tunas and other scombrid fish link myotomal muscle directly to the caudal fin rays, and thus serve to transfer muscle power to the hydrofoil-like tail during swimming. These robust collagenous tendons have structural and mechanical similarity to tendons found in other vertebrates, notably the leg tendons of terrestrial mammals. Biochemical studies indicate that tuna tendon collagen is composed of the (alpha1)(2),alpha2 heterotrimer that is typical of vertebrate Type I collagen, while tuna skin collagen has the unusual alpha1,alpha2,alpha3 trimer previously described in the skin of some other teleost species. Tuna collagen, like that of other fish, has high solubility due to the presence of an acid-labile intermolecular cross-link. Unlike collagen in mammalian tendons, no differences related to cross-link maturation were detected among tendons in tuna ranging from 0.05 to 72 kg (approx. 0.25-6 years). Tendons excised post-mortem were subjected to load cycling to determine the modulus of elasticity and resilience (mean of 1.3 GPa and 90%, respectively). These material properties compare closely to those of leg tendons from adult mammals that can function as effective biological springs in terrestrial locomotion, but the breaking strength is substantially lower. Peak tendon forces recorded during steady swimming appear to impose strains of much less than 1% of tendon length, and no more than 1.5% during bursts. Thus, the caudal tendons in tunas do not appear to function as elastic storage elements, even at maximal swimming effort.

Separation methods for the study of collagen and treatment of collagen disorders

Liquid chromatographic and electrophoretic methods applicable to the separation of collagen and its fragments are reviewed. Special attention is paid to the separation of both stabile and labile crosslinking elements. Identification procedures exploiting the mapping of either collagen alpha-chains or of cyanogen bromide fragments are discussed. These methods can be used for diagnosing inborn errors of collagen metabolism using bioptic or necroptic samples. Analysis of urinary hydroxyproline-containing peptides or the determination of peptidically bound pyridinoline is suitable for measuring the intensity of collagen metabolism.

The inhibition of protein-lysine 6-oxidase by various lathyrogens. Evidence for two different mechanisms

Lathyrogens decrease collagen and elastin cross-linking by inhibiting lysine oxidase. The lathyrogens isoniazid and semicarbazide decrease liver pyridoxal phosphate and are teratogenic; all their effects are reversed by pyridoxal. beta-Aminopropionitrile, another lathyrogen, does not affect liver pyridoxal phosphate, and its lathyrogenic and teratogenic effects are not reversed by pyridoxal. Time courses of these effects differ greatly, suggesting enzyme inhibition by different mechanisms.

Growth changes of collagen cross-linking, calcium, and water content in bone

It has been claimed that the increase in the strength of growing bone is due to increased mineral content. The strength of collagen is based on intermolecular covalent cross-links, and it has also been proposed that cross-link changes increase bone strength. Measurements of the content of calcium, collagen, and water, as well as cross-link analyses, were performed on the tibial cortex of growing dogs. Within the age range studied (8-44 weeks), no changes in calcium content expressed as a percentage of dry bone weight were seen. Collagen content expressed as weight of hydroxyproline per dry bone weight showed a minor reduction during growth. However, water content decreased considerably up to an age of about 25 weeks, which implies a concomitant increase in the amount of bone material. Of the two cross-link main groups, reducible and nonreducible, it is only possible chemically to analyze the reducible. During the final part of the period of growth and mechanical maturation of the bones, the number of reducible cross-links decreases. This indicates a concomitant increase in the more stable nonreducible forms. The possible mechanical relevance of the chemical changes found during growth is discussed.

Mechanical properties of the tibia from chickens with idiopathic scoliosis

The tibia from six-week old chickens that develop idiopathic scoliosis were studied with stress relaxation experiments and torsional strength testing. Most parameters observed did not show any significant differences between tibias obtained from chickens with scoliosis and tibias from the control birds; however, the rate of stress relaxation of the tibia from the birds with scoliosis was minimally increased over the controls. There were no significant differences noted in ultimate torsional strength, maximum angular deformity or modulae of torsional rigidity of the tibias from scoliotic chickens when compared to tibias from control chickens.

Structural properties of Type I collagen isolated from chickens with scoliosis

This study examines biochemically the Type I collagen isolated from skin of chickens that develop idiopathic scoliosis. Previous studies indicate a defect in collagen exists in these chickens. Alpha 1 (I) and alpha 2 chains were separated by gel filtration and carboxymethyl cellulose column chromatography and were then subjected to the analytical techniques of sodium dodecyl sulfate gel electrophoresis, Staphylococcus aureus V8 protease digestion, cyanogen bromide peptide mapping and amino acid analyses. In all categories, the scoliotic alpha 1 (I) and alpha 2 chains were identical to alpha chains isolated from normal chickens. These data suggest that the altered properties of collagen solubility and connective tissue stress relaxation seen in these scoliotic chickens are not a manifestation of an altered primary structure of the alpha chains or post-translational modification affecting chromatographic elution profiles or electrophoretic migration patterns.

Thermal stability of reconstituted collagen fibrils. Shrinkage characteristics upon in vitro maturation

Thermal stability measured as area shrinkage without tension during heating was determined for membranes of collagen fibrils reconstituted from solutions of highly purified rat skin collagens. Shrinkage in per cent of area at 25 degrees C and shrinkage temperature were quantitated in a standardized way and determined as a function of in vitro maturation time for 11 to 104 days after aggregation for the collagen membranes. Similar to reports on intact rat skin, shrinkage temperature remained constant and shrinkage per cent declined with a rate decreasing with time during maturation. Solubility in water at 80 degrees C for 2 hours was 95-96% and remained unchanged for the maturation time (about 2 months) studied. The decreased shrinkage reflecting a lower degree of collapse is ascribed to an increasing thermal stability of the membranes during maturation. Development of heat-stable bonds in the reconstituted collagen fibrils is taken up to be amenable to this increased stability. Similarity in changes of shrinkage characteristics during in vivo and in vitro maturation indicates that maturation changes in reconstituted collagen fibrils reflect those occurring in intact collagen during in vivo aging.

A major intermolecular cross-linking site in bovine dentine collagen involving the alpha 2 chain and stabilizing the 4D overlap

Approximately 20% of the radioactivity incorporated into the dentine collagen of unerupted bovine molars after reduction with tritiated sodium borohydride was recovered in a cyanogen bromide peptide fraction of Mr 61 000 following chromatography on agarose A5m. After rechromatography on agarose A1.5m, this fraction was resolved into ten components by gel isoelectric focusing. Of these components, nine (the most acidic) were tritiated and contained the reduced cross-links dihydroxylysinonorleucine and hydroxylysinonorleucine. The amino acid compositions were consistent with the identification of each of these components as alpha 2CB3,5 linked to one or two small peptides. By limited Edman degradation, with and without prior digestion with pyroglutamate aminopeptidase (EC 3.4.11.8), these small peptides were identified as alpha 1CB0,1 and alpha 2CB1, occurring in a ratio of approximately 2:1. Specific cleavage with cathepsin D revealed that all the cross-link was associated with the C-terminal one-third of the alpha 2 chain, thus fixing the displacement of the participating molecules at 4D. The content of the known reducible cross-links present in these peptides, calculated from the specific activity of the reductant, was sufficient to account for only 10--20% of the cross-linking actually found, suggesting that stabilization is mainly through nonreducible cross-links of as yet undetermined structure. By quantitative analysis of homoserine content and semiquantitative amino-terminal analyses, it was determined that virtually all of the alpha 2 chain of bovine dentine collagen is cross-linked in this manner. One cross-link per molecule in this location could made a major contribution to the mechanical stability of the insoluble collagen fibrils in this tissue.

Some observations on the ageing in vitro of reprecipitated collagen fibres

The changes in solubility and amounts of reducible cross-links have been studied during "ageing" in vitro of reprecipitated rat skin collagen fibres by incubation at 37 degrees C. Fibres from pre-reduced collagen devoid of aldehyde precursors became insoluble at the same rate as that of normal fibres during "ageing". Insolubilization occurred at a much faster rate in the presence of oxygen than in air and was almost completely inhibited when oxygen was excluded. The rate of decline of the reducible cross-links was, however, unaffected by oxygen tension. The results indicate that, during "ageing" in vitro, conversion of the lysine-derived cross-links to a non-reducible form is not associated with solubility changes. The relationship of these in vitro changes to those ocurring in vivo is unknown.

Simulation of lung tissue properties in age and irreversible obstructive syndromes using an aldehyde

Weak solutions of CHOH alter tissue properties, probably by forming intermolecular cross-linkages. The maximum length (Lmax) to which alveolar wall can be extended is reduced. If exposed to CHOH while extended, the resting length (LO) of alveolar wall increases. Maximum extensibility (Lambdamax equal to Lmax/LO) decreases. Similar changes are found in the alveolar wall of man with aging and are significantly more marked in patients with irreversible obstructive pulmonary syndromes. A reduction in the energy loss of the length-tension cycle (hysteresis) was seen after exposure to CHOH, however, that does not occur with age or in obstructive syndromes. Because an exposure of alveolar wall to elastase increases LO and hysteresis, we used a staged exposure to CHOH followed by elastase. Tissue suitably prepared by exposure to CHOH followed by elastolysis better simulates the tissue changes of age and irreversible obstructive syndromes.

Evidence for the local denaturation of collagen fibrils during the mechanical rupture of human tendons

The ends of ruptured human tendons have been examined by scanning electron microscopy. The collagen fibres taper markedly, often leading up to a coiled segment or a knot of collagen at the point of rupture. This tapering of the collagen fibres was shown to be typical of denatured collagen by its selective removal by trypsin digestion. This denaturation caused by mechanical rupture was shown to be localized at the point of rupture, the rest of the collagen fibre remaining in the native state. The possible significance of these observations to the healing of ruptured tendons is discussed.

Denaturation of collagen fibers in NaI, NaCl and water of different pH values as studied by differential scanning calorimetric measurements

Microcalorimetric measurements of rat tail tendon collagen in different aqueous media show the following behavior of denaturation temperature (Td) and of denaturation enthalpy (deltaHd): H+ and OH- ions lower Td significantly beyond pH 4...11. Addition of salts (0.15 M) generally lowers Td in the range between pH 5 and 10, I- being more effective than Cl- at pH greater than i.p. However, in the region of pH less than i.p., I- raises Td to values above those of Cl- and even those of water. The value of deltaHd was found to be insensitive to pH over the pH range of 4...11. Hence the lowering of Td is assumed firstly to be due to a (ionic) disorganization of the water contacting the polypeptide chains and secondly to their mutual electrostatic repulsion. The appearance of deltaHd is thought mainly to be due to a rupture of interchain bonds.

The chemistry of the collagen cross-links. The characterization of fraction C, a possible artifact produced during the reduction of collagen fibres with borohydride

The present paper describes the isolation and identification of a major radioactive component of borotritide-reduced collagen, previously designated Fraction C. The derived structure for the compound confirms that it is identical with the ;post-histidine' component described by Tanzer et al. (1973) and given the trivial name histidino-hydroxymerodesmosine. Detailed studies of the effects of acid pH on the formation of Fraction C after borohydride reduction demonstrated the apparent lability of the non-reduced form, thus confirming our previous findings (Bailey & Lister, 1968). Inhibition of the formation of this component by the acid treatment appears to be due to protonation of the histidine imidazole group. Since the only new component formed on reduction of the acid-treated fibres was the reduced aldol condensation product, these results indicate that neither the histidine nor the hydroxylysine residues can be involved in covalent linkage with the aldol condensation product in the native fibre. It is suggested therefore that the proposed non-reduced aldimine form of Fraction C does not exist as an intermolecular cross-link in vivo. Thus the presence of histidino-hydroxymerodesmosine as a tetrafunctional cross-link in reduced collagen fibres is a result of a base-catalysed reaction promoted by the borohydride-reduction procedure and this component must therefore be considered as an artifact.