An Ehrlich chromogen in collagen cross-links

Relationship between meat quality and intramuscular collagen characteristics of muscles from calf-fed, yearling-fed and mature crossbred beef cattle

Intramuscular Ehrlich Chromogen (EC) and pyridinoline (Pyr) concentrations in the gluteus medius (GM) and semitendinosus (ST) from crossbred Angus calf- (n = 14) and yearling-fed (n = 14) steer and mature cow (MC, n = 12) carcasses were related to collagen and intramuscular connective tissue (IMCT) thermal stability and peak Warner-Bratzler shear force (WBSF). In both muscles, Pyr density was greater in MC, while EC concentrations were comparable in calf- and yearling-fed steer muscles and lowest in MC muscles. Thermal denaturation temperature and enthalpy of IMCT were highest in both muscles when from MC, although only total collagen was correlated with WBSF in calf fed-yearling fed steer data. Results confirmed that EC concentration contributed to collagen thermal stability in steer muscles, but decreased it in MC muscles, while Pyr was consistently associated with collagen thermal stability.

Enzymatic and non-enzymatic crosslinks found in collagen and elastin and their chemical synthesis

This review summarized the enzymatic and non-enzymatic crosslinks found in collagen and elastin and their organic synthesis.

The sulfilimine cross-link of collagen IV contributes to kidney tubular basement membrane stiffness

Basement membranes (BMs), a specialized form of extracellular matrix, underlie nearly all cell layers and provide structural support for tissues and interact with cell surface receptors to determine cell behavior. Both macromolecular composition and stiffness of the BM influence cell-BM interactions. Collagen IV is a major constituent of the BM that forms an extensively cross-linked oligomeric network. Its deficiency leads to BM mechanical instability, as observed with glomerular BM in Alport syndrome. These findings have led to the hypothesis that collagen IV and its cross-links determine BM stiffness. A sulfilimine bond (S = N) between a methionine sulfur and a lysine nitrogen cross-links collagen IV and is formed by the matrix enzyme peroxidasin. In peroxidasin knockout mice with reduced collagen IV sulfilimine cross-links, we find a reduction in renal tubular BM stiffness. Thus this work provides the first direct experimental evidence that collagen IV sulfilimine cross-links contribute to BM mechanical properties and provides a foundation for future work on the relationship of BM mechanics to cell function in renal disease.

A rapid and specific method for the detection of indole in complex biological samples

Indole, a bacterial product of tryptophan degradation, has a variety of important applications in the pharmaceutical industry and is a biomarker in biological and clinical specimens. Yet, specific assays to quantitate indole are complex and require expensive equipment and a high level of training. Thus, indole in biological samples is often estimated using the simple and rapid Kovács assay, which nonspecifically detects a variety of commonly occurring indole analogs. We demonstrate here a sensitive, specific, and rapid method for measuring indole in complex biological samples using a specific reaction between unsubstituted indole and hydroxylamine. We compared the hydroxylamine-based indole assay (HIA) to the Kovács assay and confirmed that the two assays are capable of detecting microgram amounts of indole. However, the HIA is specific to indole and does not detect other naturally occurring indole analogs. We further demonstrated the utility of the HIA in measuring indole levels in clinically relevant biological materials, such as fecal samples and bacterial cultures. Mean and median fecal indole concentrations from 53 healthy adults were 2.59 mM and 2.73 mM, respectively, but varied widely (0.30 mM to 6.64 mM) among individuals. We also determined that enterotoxigenic Escherichia coli strain H10407 produces 3.3 ± 0.22 mM indole during a 24-h period in the presence of 5 mM tryptophan. The sensitive and specific HIA should be of value in a variety of settings, such as the evaluation of various clinical samples and the study of indole-producing bacterial species in the gut microbiota.

Chemical structure, biosynthesis and synthesis of free and glycosylated pyridinolines formed by cross-link of bone and synovium collagen

This review focuses on the chemical structure, biosynthesis and synthesis of free and glycosylated pyridinolines (Pyds), fluorescent collagen cross-links, with a pyridinium salt structure. Pyds derive from the degradation of bone collagen and have attracted attention for their use as biochemical markers of bone resorption and to assess fracture risk prediction in persons suffering from osteoporosis, bone cancer and other bone or collagen diseases. We consider and critically discuss all reported syntheses of free and glycosylated Pyds evidencing an unrevised chemistry, original and of general utility, analysis of which allows us to also support a previously suggested non-enzymatic formation of Pyds in collagen better rationalizing and justifying the chemical events.

Bone collagen cross-links: a convergent synthesis of (+)-deoxypyrrololine

A convergent total synthesis of (+)-deoxypyrrololine (Dpl, 4), a putative cross-link of bone collagen, is described starting from a commercially available L-glutamic acid derivative, (4S)-5-(tert-butoxy)-4-[(tert-butoxycarbonyl)amino]-5- oxopentanoic acid (16). Condensation of aldehyde (S)-(-)-17 with nitro compound (S)-(-)-27, both of which were prepared from a common precursor (S)-16, gave the alpha-hydroxynitro compound 28, which upon acetylation afforded alpha-acetoxynitro compound 14 in good yield. Subsequent condensation and cyclization of alpha-acetoxynitro compound 14 with benzyl isocyanoacetate (15) in the presence of DBU in THF gave the key pyrrole intermediate (S,S)-(-)-12 in 57% yield. N-Alkylation of pyrrole (S,S)-(-)-12 with iodide (S)-(-)-13 using t-BuOK in THF afforded the 2-benzyloxycarbonyl-1,3,4-substituted pyrrole derivative (-)-29 in 42% yield. Removal of the protective groups in (-)-29 followed by hydrogenolysis and decarboxylation afforded the cross-link (+)-Dpl (4) in good overall yield. The synthesis of an analogue (S)-(+)-24 and formation of a novel tetrahydroindole derivative (-)-31 are also described.

p-Dimethylaminobenzaldehyde-reactive substances in tail tendon collagen of streptozotocin-diabetic rats: temporal relation to biomechanical properties and advanced glycation endproduct (AGE)-related fluorescence

In the present work, pepsin digests of tail tendons from streptozotocin-diabetic rats were found to contain material that reacted rapidly at room temperature with p-dimethylaminobenzaldehyde (Ehrlich's reagent) to give an adduct with an absorbance spectrum characteristic of the Ehrlich chromogen of pyrrolic nature determined in ageing collagens. A significant correlation of the Ehrlich adduct with tendon mechanical strength and collagen fluorescence characteristic of advanced glycation endproducts was observed. Collagen content of the Ehrlich-positive material was found to be significantly elevated in tendons of diabetic rats compared with age-matched healthy controls. The results indicate that the p-dimethylaminobenzaldehyde-reactive pyrrole moieties may contribute to the increased cross-linking of diabetic matrix collagen. Profound inhibitory effect of aminoguanidine was observed, underlining the role of non-enzymatic mechanisms of advanced glycation in pyrrolisation and cross-linking of collagen exposed to hyperglycaemia. It is hypothesised that quantification of the p-dimethylaminobenzaldehyde-reactive material in matrix collagen may provide a tissue measure of integrated hyperglycaemia over prolonged periods of time. Further research is to assess the significance of p-dimethylaminobenzaldehyde-reactive substances in diabetic collagen tissues and to reveal their relationship to enzyme-mediated physiological pyrrolisation of ageing collagens.

Molecular site specificity of pyridinoline and pyrrole cross-links in type I collagen of human bone

Compared with soft tissue collagens, bone type I collagen displays a distinctive pattern of covalent cross-linking, with evidence of preferred sites of molecular interaction and a prominence of both immature, divalent cross-links and mature, trivalent cross-links in the adult tissue. In this study the site-specificity of the mature cross-links in human bone collagen was examined. Peptides containing fluorescent pyridinoline cross-links and Ehrlich's-reactive pyrrole cross-links were isolated from a bacterial collagenase digest of demineralized bone matrix. The digest was fractionated by molecular sieve chromatography, monitoring for peptide absorbance, pyridinoline fluorescence, pyrroles by Ehrlich's reagent, and immunoassay for cross-linked N-telopeptides. Individual cross-linked peptides were resolved by ion-exchange and reverse-phase HPLC. Structures were established by NH2-terminal microsequencing, cross-link analysis, electrospray mass spectrometry, and immunoassay. Two, about equally occupied, sites of pyridinoline cross-linking were identified, N-telopeptide to helix and C-telopeptide to helix. Pyrroles were alternative cross-linking products at the same sites, but concentrated (85%) at the N-telopeptide end. Only one combination of chains was cross-linked by pyridinolines at the C-telopeptide to helix site, [alpha1(I)C]2alpha1(I)helix. Several peptide combinations arose from the N-telopeptide to helix site, but the main source of pyridinolines was from the locus, alpha1(I)Nalpha2(I)Nalpha1(I)helix. Pyridinolines linking two alpha1(N) telopeptides were a minor component. Pyrroles were concentrated at the locus, alpha1(I)Nalpha2(I)Nalpha2(I)helix. The cross-link ratio of hydroxylysylpyridinoline to lysylpyridinoline differed between N-telopeptide and C-telopeptide sites, and between the individual interchain combinations. Cross-linked N-telopeptides accounted for two-thirds of the total lysylpyridinoline in bone. N-telopeptide pyridinoline fluorescence was quenched on chromatography, so that reliance on peptide fluorescence alone can underestimate the level of N-telopeptide pyridinoline cross-linking.

Protein modification by the degradation products of ascorbate: formation of a novel pyrrole from the Maillard reaction of L-threose with proteins

Ascorbate (vitamin C) degradation products can undergo non-enzymatic glycation (Maillard reaction) with proteins to form highly crosslinked structures with brown pigmentation and characteristic fluorescence. Proteins in the body, especially the long-lived proteins develop similar changes during aging and diabetes. Several studies have shown excessive degradation of ascorbate in plasma in diabetes, and in ocular lens during aging and cataract formation. Recent studies have suggested that ascorbate degradation products-mediated glycation plays a role in lens pigmentation and cataract formation. However, the precise chemical nature of ascorbate-specific advanced glycation end-products are not known. Here, we report the purification and characterization of a glycation end-product derived from one of the major degradation products of ascorbate, L-threose. This compound was characterized to be 2-acetamido-6-(3-(1,2-dihydroxyethyl)-2-formyl-4-hydroxymethyl-1- pyrrolyl)hexanoic acid (formyl threosyl pyrrole or FTP) formed by the condensation of epsilon-amino group of lysine with two molecules of threose. Formation of FTP occurred rapidly in the incubation of threose and lysine and reached plateau level within a day. We have developed a sensitive assay for its quantification in proteins based on enzyme digestion followed by HPLC. Ribonuclease A and human lens crystallins incubated with L-threose showed time- and sugar concentration-dependent increases in FTP, reaching 8.2 and 2.48 nmol per mg protein, respectively after one week of incubation. Human plasma proteins showed a peak with identical retention time as that of purified FTP under two different HPLC conditions. FTP may be used as a sensitive marker to assess ascorbate-mediated protein glycation and modifications in aging and diabetes.

Comparative studies on the extractability of collagen from aortas of stroke-prone spontaneously hypertensive and normotensive rats

The molecular states of collagen in the aortas of age-matched stroke-prone spontaneously hypertensive (SHRSP) and normotensive Wistar Kyoto rats (WKY) were studied by analyzing its extractability under defined conditions. The monomeric and oligomeric collagen extractable with 0.5 M acetic acid/6 M urea from aortic homogenates of 9-month-old SHRSP and WKY comprised approx. 0.6 and 2.0%, respectively, of the total collagen. On incubation of the acetic acid/urea-extracted residues with pepsin at 4 degrees C, the levels of the collagen alpha 1(I) and alpha 2(I) chains solubilized from the SHRSP residues were both less than 50% of those from the WKY residues. When the residues were incubated with pepsin at 15 or 25 degrees C, the differences became smaller. When the acetic acid/urea residues were hydrolyzed with cyanogen bromide, nearly identical peptide maps were obtained for SHRSP and WKY. The aortas from 2-month-old SHRSP and WKY contained much larger proportions of acid/urea-extractable collagen than those of the older rats (8.2 and 13% of the respective total collagen). The levels of the alpha 1(I) and alpha 2(I) chains solubilizable from the respective residues by pepsin at 4 degrees C were similar to each other. These results indicate that aortic collagen fibrils in SHRSP are stiffened more prominently than those in WKY.

Identification of the loci of the collagen-associated Ehrlich chromogen in type I collagen confirms its role as a trivalent cross-link

Collagenous peptides containing the Ehrlich chromogen (EC) were selectively isolated from a tryptic digest of bovine tendon by coupling to a diazotized polyacrylamide support. The isolated p-phenol-azo-EC peptides were purified and characterized by amino acid and sequence analyses. EC occurred in stoichiometric amounts in trimeric cross-linked chains originating from the known cross-link regions of type-I collagen. The major locus of the EC was alpha 2(I)Hyl-933 x alpha 1(I)Lys(Hyl)-9N x alpha 2(I)Lys(Hyl)-5N but it was also shown to occur at the loci alpha 1(I)Hyl-87 x alpha 1(I)Lys(Hyl)-16C x alpha 1(I)Lys(Hyl)-16C and alpha 1(I)Hyl-930 x alpha 1(I)Lys(Hyl)-9N x alpha 2(I)Lys(Hyl)-5N. After sequence analyses of the C-terminal helical cross-link region alpha 2(I)928-963, corrections are presented for residues 927, 930, 932 and 933 of the bovine alpha 2(I) chain. The collagen-associated EC is postulated to be a trisubstituted pyrrole formed by the reaction of the aldehyde form of a telopeptidyl lysine residue with a bifunctional keto amino cross-link. It is also proposed that when the telopeptidyl lysine residue is hydroxylated the above reaction will result in pyridinoline formation.

Collagen crosslinks and their relationship to the thermal properties of calf tendons

The hydrothermal isometric tension and thermal transition temperature of collagen were determined in tendons from three different calf muscles. The levels of the nonreducible collagen crosslink, pyridinoline, and the collagen-associated Ehrlich chromogen were also measured in the three tendons. The reducible collagen crosslinks, hydroxylysinonorleucine, dihydroxylysinonorleucine, and histidinohydroxymerodesmosine were measured in two tendons. The thermal properties and levels of crosslinks were found to vary considerably between the different tendons, and also at different sites in two of the tendons. A strong correlation was observed between the thermal transition temperatures and the hydrothermal isometric tensions of the nine tendon sites examined. Both thermal properties correlated with the concentration of both pyridinoline and Ehrlich chromogen. The analogous behavior of the collagen-associated Ehrlich chromogen and the pyridinoline crosslink supports the role of the Ehrlich chromogen as a nonreducible crosslink.

Ehrlich chromogens, probable cross-links in elastin and collagen

(1) Proteolytic digests of tissue elastin contain material which reacts with dimethylaminobenzaldehyde in acid solution (Ehrlich's reagent) to give a cherry-pink colour. This Ehrlich chromogen(s) [EC(s)] is similar to but not identical with EC(s) previously demonstrated in tissue collagens [Scott, Hughes & Shuttleworth (1979) Biosci. Rep. 1, 611-618]. Both ECs react with diazonium salts in acid to give coloured products. (2) Diazobenzene linked via a phenolic ester to polyacrylamide beads (Biogel P10) has been used to absorb ECs specifically and almost quantitatively from proteolytic digests. The coupled deeply coloured azo-EC-peptides were then recovered after mild alkaline cleavage from the support and purified by gel chromatography. (3) Using 15N-labelled NaNO2, the collagen azo-EC-peptides were prepared, and 15N abundance measured therein. The molar absorption coefficient of the azo-EC group was calculated (18,700) based on the assumption that each azo-EC group contained one 15N atom. (4) Collagen azo-EC-peptides contained glucose and galactose, whereas elastin azo-EC peptides did not. The amino acid patterns of the two peptides were quite different, the former being rich in polar amino acids, the latter containing much alanine. The patterns were compatible with an origin from the cross-linking regions of collagen and elastin respectively. (5) Quantitative (molar) comparisons of the azo-EC group content with amino acid, amino end-group and sugar contents, and azo-EC peptide molecular mass, suggest that a structure is present in the collagen azo-EC-peptides containing two EC groups shared between four peptide chains. Three peptide chains probably meet at each (cross-linking) EC group. (6) Based on this structure, about 15% of adult bovine skin collagen contains EC groups.

Construction of a model for the aggregation and cross-linking region (7S domain) of type IV collagen based upon an evaluation of the primary structure of the alpha 1 and alpha 2 chains in this region

The amino acid sequence of the 212-residues-long N-terminal aggregation and cross-linking region of the alpha 2(IV) chain of human basement membrane collagen is presented. Comparing this with the primary structure of alpha 1(IV)7S [Glanville et al. (1985) Eur. J. Biochem. 152, 213-219] revealed a high degree of similar subdivisions in three functional regions. These are the 21-residue-long N-terminal non-triple-helical regions (NH1) containing cysteine and lysine residues which are putative cross-linking sites, a 117-residue-long triple-helical region (TH1) responsible for the aggregation of four molecules to form the 7S domain and which also possess cross-linking sites, and finally a 10-residue-long non-triple-helical region (NH2) which introduces the first of many flexible areas into the triple helical body of the molecule [Hofmann et al. (1984) J. Mol. Biol. 172, 325-343]. Computer calculations of interaction scores between parallel and antiparallelly aligned triple-helical regions (TH1) of the 7S domain allowed the prediction of a detailed model for the structure of the 7S complex which agreed well with models based primarily on electron micrographs of rotary shadowed type IV collagen tetramers. The results indicated that the assembly of the 7S domain is directed by hydrophobic interactions and is self-limiting to a tetramer. The most favourable chain configuration is alpha 2-alpha 1-alpha 1.

Detection of D-glucose-derived pyrrole compounds during Maillard reaction under physiological conditions

5-Hydroxyl-1-neopentylpyrrole-2-carbaldehyde, 2-(2-hydroxyacetyl)-1-neopentylpyrrole, in decreasing order or abundance, have been isolated and the structures characterized. These compounds were obtained from the reaction of a mixture of D-glucose and neopentylamine under physiological conditions of pH and temperature. In addition, 4H-dihydropyran-4-one, a known intermediate product of the Maillard reaction, was detected. The neopentylamine adducts were already detectable after one week of incubation, but rapid acid and alkaline degradation explains the lack of detection in body proteins.

Characterisation of a type-I collagen trimeric cross-linked peptide from calf aorta and its cross-linked structure. Detection of pyridinoline by time-of-flight secondary ion-mass spectroscopy and evidence for a new cross-link

A collagenous trimeric cross-linked peptide has been isolated from the insoluble matrix of calf aorta, using trypsin solubilisation, and purified by gel filtration, cation-exchange chromatography and reversed-phase HPLC. Molecular mass and amino acid composition indicated that the C-terminal, non-helical region of type I collagen in its dimer form, designated as [ColC(I)]2, is cross-linked to a tryptic peptide TN(I) from the N-terminal helical cross-link region of an adjacent type I molecule, forming the cross-linked peptide [ColC(I)]2 X TN(I). Amino acid sequence analysis of the peptide yielded a series of sequences corresponding to the cross-linking domains ColC(I) and TN(I) and furnished the first direct chemical evidence for the 4D staggered arrangement of type I molecules within native fibers. The trifunctional cross-linking amino acid pyridinoline was shown to occur in the peptide, confirming the peptides three-chain structure. Pyridinoline was isolated from the cross-linked peptide by preparative amino acid analysis and reversed-phase HPLC and identified by its ultraviolet absorption spectra, its fluorescence excitation and emission spectra and, for the first time, its time-of-flight secondary ion-mass spectrum. The high sensitivity of the latter method, exceeding that of fast-atom-bombardment mass spectroscopy by three orders of magnitude, allowed detection of pyridinoline in the picomole range. The occurrence of pyridinoline in non-stoichiometric amounts, the presence of hydroxylysine in hydrolysates of all cross-linked peptides and the finding that hydrolysates also contained an unidentified component indicated that there is at least one cross-link form that is different from pyridinoline and is hydrolysable.

Amino acid sequence of the N-terminal aggregation and cross-linking region (7S domain) of the alpha 1 (IV) chain of human basement membrane collagen

The amino acid sequence of the 216-residue-long N-terminal aggregation and cross-linking 7S domain of the alpha 1 (IV) chain of human placental basement membrane collagen is presented. The N terminus of the alpha 1 (IV) chain starts with a non-triple-helical region, which is at least 15 residues long and contains four cysteine and two lysine residues as putative cross-linking sites. This segment is followed by a 120-residue-long triple helical region, which contains the unusual occurrence of a cysteine residue in the Xaa position of a Gly-Xaa-Yaa triplet. Since individual molecules in the 7S domain are associated in an antiparallel manner, this cysteine probably aligns with one of the four cysteines in the amino-terminal end of an adjacent molecule, forming an intermolecular disulfide bridge. The length of the overlap of two adjacent molecules is estimated to be about 110 residues. The triple helix adjacent to the overlap zone is interrupted by a 10-residue-long non-helical area, which is probably responsible for the flexible region of the molecules in the neighbourhood of the overlap zone observed in the electron microscope. The mode of aggregation of the 7S domain, the formation of intermolecular cross-links as well as the relatively high stability of this region against proteolytic attack are discussed in the light of the elucidated amino acid sequence.

Separation and characterization of two polypeptide chains from the 7S cross-linking domain of basement-membrane (type IV) collagen

The long-form 7S domain of human placental type IV collagen was prepared and after reduction, denaturation and aminoethylation, was separated into its subunits. The monomer subunit was further separated into two polypeptide chains of Mr about 25 000. From compositional data and CNBr peptide patterns it was shown that the two chains were different. Furthermore, all subunits contained both chains, thus supporting a proposed subunit structure for the 7S domain and a chain composition [alpha 1(IV)]2 alpha 2(IV) for the type IV molecule.