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D T Cheung

Publications and source records attributed to D T Cheung.

34 records · Page 2Linked to original sources
PubMed1992

Correlation of structure and viscoelastic properties in the pericardia of four mammalian species.

Although the pericardium is recognized as having important contributions to ventricular function, the relationship between its functional role and structural composition remains poorly understood. Conflicting evidence from low strain rate experiments has shown that differences exist between the mechanical properties of canine, human, and bovine pericardium but with no structural explanation for these differences. This paper examines the pericardia of calves, dogs, pigs, and sheep using a structural/mechanical approach with techniques novel to the pericardial literature. High strain rate mechanical testing for stress-strain response, stress relaxation, and forced vibration response has shown the pericardium to be much more viscoelastic than previously believed under large deformations, but to be quite elastic in small vibrations. The thinner canine and porcine pericardia were found to be significantly stiffer than the thicker bovine and ovine tissues, but equivalently viscoelastic. Biochemical analysis shows these thinner tissues to have significantly higher levels of type III collagen combined with a higher degree of cross-linking. This is the first structural explanation for differences in mechanical properties between the pericardia of different species.

Animals↗
PubMed1992

Collagen in the developing larynx. Preliminary study.

The primary purpose of this study was to determine the types of collagen in the developing human larynx that contribute to the structural framework and function of various components of this organ. The infant larynx is much more than a mere miniature of the adult "voice box." There are many age-related differences that occur in the larynx from the newborn period to the adult period of life. While collagen has been studied in numerous tissues, both normal and diseased, there have been no studies of the whole organ content, types, and/or changes of collagen in the developing human larynx that may account for many of the clinical findings. This study may at least in part explain whether collagen differences may account for the structural changes and responses that are seen in clinical practice.

Child, Preschool↗
PubMed1991

Effects of indomethacin on demineralized bone-induced heterotopic ossification in the rat.

Indomethacin inhibits bone formation when treatment is initiated before the implantation of demineralized bone matrix (DBM). For the inhibition of bone induction to occur, indomethacin treatment had to be initiated 6 h or more before implantation of DBM. Initiating the drug treatment at or after the time of DBM implantation had no effects on the amounts of new bone formed. The inhibition by indomethacin is dose related over a range between 0.04 and 4 mg/kg body weight. Recovered day-1 DBM implants, transplanted into indomethacin pre- and posttreated syngeneic rats, formed bone at the same rate as controls did. However, recovered day-1 DBM implants lyophilized before transplantation showed decreased bone formation but significant dystrophic calcification as judged by a lower alkaline phosphatase activity and an elevated calcium content.

Alkaline Phosphatase↗
PubMed1990

The effect of gamma-irradiation on collagen molecules, isolated alpha-chains, and crosslinked native fibers.

This study shows how collagen molecules are readily damaged by gamma-radiation at dosages commonly used for sterilizing biomedical products. At 1 Mrad, while the reported effectiveness of the radiation at such a low dosage to completely sterilize a material is questionable, less damage was caused to the collagen peptide backbone. Above such dosage, however, significant damage was clearly demonstrated with collagen alone and collagen in a chemically crosslinked tissue matrix. The enzyme digestion study showed that the material exposed to a very high dosage of radiation resisted degradation by pronase. However, molecular weight analysis showed a significant number of peptide bonds being cleaved by the radiation which could cause considerable changes in the long-term characteristics of the material. Therefore, tissues exposed to high dosages of gamma-radiation should be tested for long term functional changes. We want to caution against the usage of the enzyme degradation assay as a universal test for all bioprosthetic derived from biological tissues.

Animals↗
PubMed1990

Biochemical changes and cytotoxicity associated with the degradation of polymeric glutaraldehyde derived crosslinks.

The reversibility of glutaraldehyde crosslinks has been suggested as a reason for failure of long-term bioprosthetic implants. The stability of such crosslinks was investigated in tendons and model compounds. Small but cytotoxic levels of glutaraldehyde were still released from crosslinked tendons even after these tendons were extensively rinsed for up to 6 months. The toxic effect was evidenced by the death of fibroblasts surrounding a midsection piece of rinsed crosslinked tendon, while the end section pieces did not show toxic effects. The formation and stability of glutaraldehyde modified [14C]-L-lysine derivatives were investigated. The polymerization of glutaraldehyde with amino compounds was initially fast but continued to proceed slowly for months. Degradation of high-molecular-weight soluble polymers was detected by gel filtration chromatography. Low-molecular-weight soluble materials were also released from insoluble products which were formed when high concentrations of glutaraldehyde and radioactive lysine were reacted. These chemical and biological studies suggest that local cytotoxicity of glutaraldehyde crosslinked bioprostheses may be due to unstable glutaraldehyde polymers that persist in the interstices of crosslinked tissues.

Animals↗
PubMed1990

A highly specific and quantitative method for determining type III/I collagen ratios in tissues.

The distribution of type I and type III collagens in rat, bovine and human skin were examined by a quantitative 2-D CNBr peptide mapping method. The procedure involved the solubilization of tissues by digestion with CNBr, radioactive labeling in vitro by [3H]-NaBH4 in dimethylformamide, reduction by mercaptoethanol, a second CNBr digestion and 2-D (isoelectric focusing and NaDodSO4 electrophoresis) mapping. The amounts of type I and type III collagen peptide spots in the fluorographs of 2-D maps were analyzed by 2-D scanning densitometer/analyzer. Mixtures containing various ratios of purified type I and type III collagen were used to obtain a standard curve. Using this procedure we were able to determine that in adult human skin (age range 35-65 years) 22% (+1.3%) of the labelled collagen is type III. This value is significantly higher than that was previously estimated by less accurate methods.

Adult↗
PubMed1990

Alteration of collagen composition and cross-linking in keloid tissues.

Collagen composition and cross-linking in human keloid and normal skin tissues were analyzed biochemically. CNBr peptides were separated by 2-dimensional (2-D) mapping and high performance liquid chromatography (HPLC). The amounts of type I and type III collagen was quantified by 2-D scanning densitometry of fluorographs of 2-D maps derived from samples radioactively labelled in vitro by [3H]-NaBH4 in dimethylformamide. Keloid tissues contained 31.6 +/- 2.2 percent type III collagen as compared to 21.4 +/- 2.7 percent type III present in normal human skin dermis. HPLC profiles of CNBr peptides showed that approximately 5 percent of the high molecular weight material in keloids is mercaptoethanol reducible, compared to insignificant amounts in normal skin. 2-D maps derived from CNBr peptides of keloid collagen demonstrated thiol reduction sensitive alpha 1(III)-CB9 dimer as well as 24,000- and 32,000-dalton CNBr peptides, which were not mercaptoethanol reduction sensitive in normal skin due to cross-linking via the lysyl oxidase pathway. Also, a group of 20,000- to 25,000-dalton CNBr peptides, in the alpha 1(I)-CB6 cross-linking region were prominent in keloid tissues.

Adult↗
PubMed1990

Mechanism of crosslinking of proteins by glutaraldehyde. IV: In vitro and in vivo stability of a crosslinked collagen matrix.

The use of native or reconstituted collagen as a bioprothesis for tissue augmentation requires the introduction of exogenous synthetic crosslinks. The degree of crosslinking determines the rate of resorption or replacement of the implanted materials by the host. Since biophysical and chemical methods to quantify these crosslinks have in general been difficult to evaluate, we have developed in vitro enzymatic approaches which enable us to correlate the degree of crosslinking with the rates of enzymatic degradation. When the number of stable crosslinks formed is large it is essential to partially unfold the collagen fibrils by heating or by exposure to denaturing agents to enhance their susceptibility to hydrolysis. In the present study we demonstrate that increasing the number of reactive amino groups on collagen by coupling 1,6-diaminohexane to carboxyl groups using a water soluble carbodiimide can significantly enhance the number of crosslinks introduced by glutaraldehyde. We also show that the enzymatic method developed correlates well with the biodegradation of radiolabeled crosslinked collagenous tissues implanted subcutaneously in rats.

Acetylation↗
PubMed1988

Biochemical differences between dystrophic calcification of cross-linked collagen implants and mineralization during bone induction.

Ectopic calcification of diseased tissues or around prosthetic implants can lead to serious disability. Therefore, calcification of implants of glutaraldehyde-cross-linked collagenous tissues and reconstituted collagen was compared with mineralization induced by demineralized bone matrix (DBM). Whereas implants of DBM accumulated large amounts of calcium and a bone-specific gamma-carboxyglutamic acid protein (BGP or osteocalcin) following implantation in both young and older rats, implants of cross-linked pericardium calcified with only traces of BGP. Glutaraldehyde-cross-linked DBM failed to calcify after implantation in 8-month-old rats for 2-16 weeks. Implants of cross-linked type I collagen exhibited small calcific deposits 2 weeks postimplantation but calcium content eventually dropped to levels equal to those of soft tissues as the implants were resorbed. The calcium content of DBM implanted in 1- and 8-month-old rats reached comparable levels after 4 weeks, but the BGP content was approximately twice as high in the younger animals than in the older ones. Glutaraldehyde-cross-linked implants of DBM, tendon, and cartilage calcified significantly in young but not in old animals. This form of dystrophic calcification was associated with only trace amounts of BGP. Alkaline phosphatase activity was high in implants of DBM and undetectable in implants of cross-linked collagenous tissues. These results show that implants of glutaraldehyde-cross-linked collagenous tissues and reconstituted collagen calcify to different extents depending upon their origin and the age of the host, and that the mechanism of dystrophic calcification differs significantly from the process of mineralization associated with bone induction as reflected by alkaline phosphatase activity and BGP accumulation.

Alkaline Phosphatase↗
PubMed1985

Mechanism of crosslinking of proteins by glutaraldehyde III. Reaction with collagen in tissues.

Bovine pericardium, a dense collagenous connective tissue, was crosslinked with glutaraldehyde using different modalities of fixation. The degree of crosslinking was evaluated as a function of the ability of CNBr and pronase to solubilize collagen. Our results suggest that glutaraldehyde fixes primarily the surface of the fibers and creates a polymeric network which hinders the further crosslinking of the interstitium of the fiber. When a low concentration of glutaraldehyde was used, a slow time-dependent crosslinking process was observed. This slow process is maintained over a long period of time, greatly beyond that required for the actual penetration of glutaraldehyde to occur.

Aldehydes↗
PubMed1983

The presence of intermolecular disulfide cross-links in type III collagen.

Bovine and lathyritic rat type III collagen preparations were analyzed for the presence and in vitro formation of intermolecular disulfide cross-links. Type III collagen from fetal bovine skin was extracted with the aid of pepsin and purified by differential salt precipitation, guanidine denaturation, and renaturation. Nearly all of the type III collagen was present as reduction-sensitive gamma-components and higher molecular weight aggregates. After cleavage with CNBr, the peptides were analyzed by two-dimensional mapping. The presence of intermolecular disulfide bonds was demonstrated by the existence of a hexamer of the COOH-terminal CNBr peptide, CB9B. This cross-linked peptide was completely converted to the CB9B monomer by reduction. Type III collagen from the skins of beta-aminoproprionitrile-treated rats was used to test for the in vitro formation of intermolecular disulfide cross-links. This was prepared by salt extraction, differential salt precipitation, and pepsin treatment. Sodium dodecyl sulfate-gel electrophoresis of this partially purified type III collagen before reconstitution into fibers detected primarily gamma-chains. After reconstitution into fibers, the majority of the material was present as higher molecular weight aggregates. Upon reduction, these aggregates generated predominantly alpha-chains. These data demonstrate the existence of intermolecular disulfide bonds in native type III collagen and their formation during in vitro fibrillogenesis.

Animals↗
PubMed1982

Mechanism of crosslinking of proteins by glutaraldehyde I: reaction with model compounds.

3H-Glycine and 6-aminohexanoic acid were used as model amine compounds and reacted with glutaraldehyde. Based on the spectral characteristics and the molecular weights obtained from the reaction products, it is concluded that glutaraldehyde can modify amines to form an intermediate which absorbs at 300 nm and has a molecular weight of about 200. In the presence of excess glutaraldehyde, this intermediate is quickly converted to a much larger intermediate which absorb strongly at 265 nm. The larger intermediates are finally altered to yield a strong absorption peak at 325 nm with no apparent change in the molecular weight. These results suggest that a process of polymerization is induced by the initial reaction of glutaraldehyde with amines. The glutaraldehyde-polymer amine complex is self-limiting in size and can undergo internal rearrangement to become chemically inert.

Aldehydes↗
PubMed1982

Mechanism of crosslinking of proteins by glutaraldehyde II. Reaction with monomeric and polymeric collagen.

Collagen in three different states, i.e. native soluble molecules, denatured molecules and reconstituted fibers, was exposed to various concentrations of glutaraldehyde. The degree of intramolecular and intermolecular crosslink formation was evaluated by measuring collagen solubility, beta and gamma chain formation, resistance towards cleavage by CNBr or collagenase digestion. Modification of lysyl residues was measured by amino acid analysis. When dilute collagen solutions were reacted with low concentrations of glutaraldehyde, intramolecular crosslinks were observed as the predominant crosslinks. When the glutaraldehyde concentration was increased, the collagen became more insoluble, indicating the formation of intermolecular crosslinks. When reconstituted collagen fibers were reacted with low concentrations of glutaraldehyde, intermolecular crosslinks were formed, which prevented the material from being solubilized by CNBr. However, these materials could still be solubilized by collagenase. When the glutaraldehyde concentration was increased, the materials became resistant to collagenase, while there was only a small increase in the number of lysyl residues modified. This reflects an increase in the molecular length of the glutaraldehyde polymers extending from the initial glutaraldehyde and lysyl residue reaction sites rather than an increase in the actual number of crosslinking sites.

Aldehydes↗
PubMed1981

The use of 2-dimensional CNBr peptide maps for the analysis of crosslinked peptides in bone collagen.

CNBr peptides from insoluble bovine cortical bone collagen were analyzed using a 2-D mapping technique. The major type 1 collagen CNBr peptides were detected by fluorography after general-labelling with 3H-NaBH4 in dimethyl-formamide. These maps were similar to those visualized by coomassie blue staining and demonstrated a proportional decrease of alpha 1CB6. New groups of peptides, different from those normally present in soluble type I collagen were detected. Some of these peptides were slightly larger and more acidic than alpha 1CB6 and were highly labelled when the demineralized bone was specifically labelled for the presence of aldehydes and crosslinks with 3H-NaBH4 in a phosphate buffer, pH 7.4. Based on the size and charge characteristics of these specifically labelled peptides, they were tentatively identified as crosslinking peptides containing different combinations of alpha 1CB6, alpha 1CB0,1 and alpha 1CB5. The specificity of the labelling method using 3H-NaBH4 in phosphate buffer was demonstrated by the detection of other known crosslinked peptides and by the virtual absence of label in alpha 1CB7, CB8, and CB3. We feel that this simple methodological approach developed in these experiments will prove to be very useful in the analysis of collagen crosslinks present in insoluble collagens derived from normal tissues of various ages as well as from pathological states.

Animals↗