Polar-apolar characteristics and fibrillogenesis of glycosylated collagen

To find the effect of carbohydrate on collagen fibrillogenesis, type I skin collagen was glycosylated by glycosyltransferase, and type II cartilage collagen was deglycosylated by glycosidase. The secondary structures remained unchanged, but the tertiary structures were altered, as shown by increased TNS fluorescence of the bound probe in the glycosylated form. Since TNS binds preferentially to the hydrophobic region of a protein molecule, glycosylation caused an apparent increase in the available hydrophobic sites for the dye. Glycosylation also resulted in a longer lag time and a slower growth rate of fibrillogenesis, although the amount of fibrils formed was unchanged. Deglycosylation resulted in a shorter lag time and an increased rate of fibrillogenesis. Neither glycosylation nor deglycosylation changed the stability of the molecule, as was evident from the melting temperature.

Structure and stability of gamma-crystallins. Denaturation and proteolysis behavior

The denaturation behavior of bovine lens gamma-crystallin fractions II, III, and IV and their susceptibility to proteolysis in vitro was compared to determine whether differences in their stability could play a role in cataract formation. Tertiary and secondary structure changes induced by increasing concentrations of urea, guanidine hydrochloride, and sodium dodecyl sulfate and by increasingly alkaline pH were followed by near-UV and far-UV circular dichroism, Trp fluorescence emission, and exposure of sulfhydryl groups. Major differences were found in the denaturation and proteolysis behavior of the three gamma-crystallin fractions. In general, the unfolding of gamma-II and gamma-III crystallins is rather gradual, suggesting the presence of intermediate unfolding states; in contrast, the order-disorder transition of gamma-IV crystallin is abrupt. The gamma-IV crystallin fraction is the most stable in urea and guanidine hydrochloride, but is most susceptible to nonspecific proteolysis and alkaline pH denaturation. Differences in denaturation and proteolysis behavior are attributed to the inherent differences in the tertiary structures of these crystallins.

Effects of visible-light irradiation on vitreous structure in the presence of a photosensitizer

Sensitized photo-induced changes of vitreous structure were investigated using both in vivo and in vitro model systems. In the former, rabbit eyes were injected with the photosensitizer riboflavin, and in the latter, calf vitreous samples were treated with riboflavin or Methylene Blue prior to irradiation with white light. The active species of oxygen, i.e. singlet oxygen, superoxide anion, hydroxyl radical and hydrogen peroxide, generated by the photodynamic action of the sensitizer, caused significant liquefaction of the calf vitreous in vitro. There was little liquefaction of the rabbit vitreous in vivo, suggesting the presence of a protective mechanism in vivo. hyaluronidase induced significantly greater liquefaction in vitro than either Methylene Blue or riboflavin. This study suggests that loss of gel vitreous structure can result from extensive depolymerization of hyaluronidase by hyaluronidase and less drastic conformation and molecular weight changes in the photosensitized reactions. Although light-induced liquefaction was less marked than enzyme-induced liquefaction, the mechanism of the former is more pertinent to age-related vitreous synchysis.

Conformational differences between hyaluronates of gel and liquid human vitreous: fractionation and circular dichroism studies

Post-mortem human vitreous samples (liquid and gel) of comparable intrinsic viscosity values (n approximately equal to 3000 cc/g) were chromatographed on DEAE-Sephacel columns at 4 degrees C using a linear salt gradient ranging from 0----0.4 M NaCl. All samples examined produced numerous discrete hyaluronic acid (HA) fractions. The HA fractions from liquid vitreous were eluted at lower salt concentrations than those from gel vitreous. Several HA fractions from vitreous analyzed by circular dichroism (CD) displayed CD minima at 210 nm, with an ellipticity value of 14 to 16 X 10(3) deg X cm2d mole-1. However, all HA fractions from liquid vitreous showed lower ellipticity values and a weak positive signal above 240 nm. This signal was absent in HA fractions from gel vitreous. Results suggest that subtle but definite conformational differences involving carboxylic groups exist between gel and liquid vitreous hyaluronate.

Structure and stability of gamma-crystallins. II. Differences in microenvironments and spatial arrangements of cysteine residues

The gamma-crystallin fractions II, III and IV from calf eye lens were treated with the thiol-specific fluorescent probe 2-(4'-maleimidylanilino)naphthalene-6-sulfonate (MIANS), in order to determine the reactivity of the seven (gamma-II) or six (gamma-III, gamma-IV) cysteine residues. Two classes of reactive cysteines were distinguished by variations in fluorescence intensity with increasing molar excess of the probe, and approximately three cysteines were nonreactive in each gamma-crystallin. From the position of the emission maximum, it is apparent that MIANS-labeled cysteines of gamma-IV are in the least hydrophobic environment. Fluorescence energy transfer was observed from tryptophan to MIANS-labeled cysteines in both gamma-II and gamma-III crystallins, with efficiencies of 86% and 89%, respectively, but not in gamma-IV crystallin. We suggest that the spatial arrangements and microenvironments of cysteine residues of gamma-crystallins are sufficiently different from each other to account for the variations in fluorescence characteristics of the MIANS-labeled proteins and the lack of energy transfer in gamma-IV crystallins.

Structure and stability of gamma-crystallins. I. Spectroscopic evaluation of secondary and tertiary structure in solution

The three major bovine gamma-crystallin fractions (gamma-II, gamma-III and gamma-IV) are known to have closely related (80-90%) amino acid sequences and three-dimensional folding of the polypeptide backbone. Their chiroptical and emission properties, as measured by circular dichroism (CD) and fluorescence, are now shown to differ distinctly. The far-ultraviolet CD spectra indicate that all three gamma-crystallins have predominantly beta-sheet conformation (45-60%) with only subtle differences in secondary structure. The fluorescence emission maxima of gamma-II, gamma-III and gamma-IV, due to the four tryptophan residues, appear at 324, 329 and 334 nm, respectively, suggesting that tryptophan residues are buried in environments of decreasing hydrophobicity. Corresponding differences in quantum yield may be due to fluorescence quenching by neighboring sulfur-containing residues. Titratable tyrosines are maximal for gamma-III, as manifested from difference absorption spectra at alkaline pH. The near-ultraviolet CD spectra differ in position, magnitude and sign of tryptophan and tyrosine transitions. In addition, a characteristic CD maximum at 235 nm, presumably due to tyrosine-tyrosine exciton interactions, differs in magnitude for each gamma-crystallin. This study shows that the environment and interactions of the aromatic residues of the individual gamma-crystallin fractions are quite different. These variations in tertiary structure may be significant, in terms of stability of gamma-crystallins towards aggregation and denaturation, for understanding lens transparency and cataract formation in general.

Sensitized photo-oxidation of bovine alpha-crystallin: change in microenvironments of thiol groups

Methylene blue- or riboflavin-sensitized photo-oxidation of alpha-crystallin caused a major change in the environment of thiol groups of the protein which could not be significantly prevented by inhibitors specific for active species of oxygen, 1O2, .OH, O.2, and H2O2. In contrast, the sensitizer N-formylkynurenine had no such effect. Sensitizer-induced change in the microenvironments of thiol groups was attributed to a conformational alteration, probably an unfolding of the protein.

Age-related changes in protein conformation in bovine lens crystallins

In order to investigate the conformational changes associated with the aging process, circular dichroism (CD), absorption and fluorescence measurements of bovine lens crystallins isolated from the nucleus of old (cow) and young (calf) animals are reported. Results show considerable differences in spectroscopic parameters between the young and old alpha-crystallin; however, no such changes were observed for beta- and gamma-crystallins. Age-related changes include an increased absorption in near-u.v. and decreased intensity in the far-u.v. region; near-u.v. circular dichroism shows a considerable difference, whereas the dichroism in far-u.v. remains the same. The decrease in tryptophan fluorescence of old alpha-crystallin is of the same magnitude as is the increase in non-tryptophan fluorescence. The fluorescence of the sulfhydryl (SH) specific probe, 2-(4'-maleimidylanilino) naphthalene-6-sulfonate, indicates that accessible (to the probe) SH groups of cow alpha-crystallin are fewer than those of calf, and they are also in a more polar environment. This study demonstrates that, with aging, alpha-crystallin undergoes a change in the tertiary structure involving tryptophan, tyrosine and cysteine residues. This conformational change has been explained by the suggestion that a large portion of the protein unfolds during the aging process, resulting in a change in interaction properties between the aromatic amino acid residues and between the residues and the peptide backbone. The unfolding is also associated with the accessibility, reactivity and spatial arrangement of these residues, including the cysteine by which aggregation or cross-linking of the protein is likely to occur.

Ascorbic acid therapy in a thermal burn model of corneal ulceration in rabbits

We studied the effect of topical and systemic ascorbic acid on a rabbit model of corneal ulceration produced by thermal burn. Central stromal ulceration occurred in 77% to 100% of eyes in all treatment groups. The rate of ulceration was accelerated in animals treated with systemic ascorbic acid. Ulceration began after 5.80 +/- 2.14 days in control eyes and 2.75 +/- 1.26 days in the eyes of rabbits treated with both topical and systemic ascorbic acid (P less than .02). Subconjunctival ascorbic acid alone did not affect the rate of ulceration (average onset, 6 +/- 1 days). Once ulceration occurred in animals treated with both topical and systemic ascorbic acid, the incidence of perforation and descemetocele formation was increased and that of healing was decreased. The time required for healing was not significantly altered by treatment (9.50 +/- 2.28 days for control eyes vs 5.75 +/- 2.78 days for treated eyes) (P greater than .05).

Role of singlet oxygen in the degradation of hyaluronic acid

To investigate the effect of singlet oxygen on the molecular properties of hyaluronic acid, the polymer was irradiated in the presence of a dye sensitizer for singlet oxygen. Viscosity and circular dichroism techniques were used to monitor these changes. The relative viscosity of the polymer solution decreased steadily with increasing duration of irradiation, indicating an apparent decrease in molecular weight of hyaluronic acid. Circular dichroism measurements of the irradiated sample, however, did not show any appreciable change in the secondary structure, but do suggest that the generated singlet oxygen changes the tertiary structure and that this change is followed by a minor depolymerization.

Spectroscopic studies on pepsin-solubilized vitreous and cartilage collagens

Circular dichroism and the fluorescent probe, 2-p-toluidinyl-naphthalene-6-sulfonate, were used to compare the molecular properties of pepsin-solubilized vitreous collagen with cartilage and calfskin collagens. Type II vitreous and cartilage collagens have more hydrophobic regions along their molecular domain than does type I calfskin collagen. The rate of fibril growth is faster in type II collagens than in type I. The increased hydrophobicity of type II collagens is attributed to high carbohydrate content and compositional variations. Although the secondary structures of the three collagens do not differ significantly, differences in carbohydrate content, composition, and hydrophobic character may cause some variations in the tertiary structures. It is suggested that the tertiary structure plays an important role in the nature and rate of fibril growth. Differences between cartilage and vitreous collagen in fluorescence behavior, fibril growth, and melting temperature indicate that vitreous collagen may be a "special type II collagen."

Glycosaminoglycans: structure and interaction

In the last few years, there has been considerable progress in the studies on glycosaminoglycans, a group of acidic polysaccharides present in the intercellular matrix of connective tissue. X-ray diffraction studies have indicated that these polymers can exist in the condensed phase in some helical form. Chiroptical and hydrodynamic measurements have provided significant information regarding the molecular conformation in solution and other physicochemical properties of the polymers. Studies related to the interaction properties of glycosaminoglycans with polypeptides, metal ions, and other molecules are numerous. This review covers mainly the results and their interpretations of both published and as yet unpublished material of the 1970s, but certain previous data are also included. A present-day concept regarding the structure and interaction properties of these molecules on the basis of various physicochemical measurements is presented. The biosynthesis and metabolism of glycosaminoglycans, and the structure of proteoglycans and glycoproteins, are not discussed.

Optical characteristics of carboxyl group in relation to the circular dichroic properties and dissociation constants of glycosaminoglycans

Circular dichroism studies of glycosaminoglycans including chemically transformed heparins at various pH values reveal that carboxyl chromophore plays an important role in the dichroic behavior of the polymers. With decreasing pH, iduronic acid-containing glycosaminoglycans show increased negative ellipticity near 220 nm whereas the polymers containing glucuronic acid display enhanced negative dichroism near 230 nm and decreased negative dichroism around 210 nm. The pH-dependent optical properties have been utilized to determine the pKa values of uronic acid moieties. The acid strengths of the iduronic acid-containing glycosaminoglycans are inherently smaller than those of corresponding glucuronic acid-containing polymers. Glycosaminoglycans in which the amino sugars are linked with iduronic acid display a very weak n leads to pi* amide transition, or none. The rotational strength at 210 nm of these polymers is largely due to iduronic acid moieties. The CD variations above 200 nm with change in pH do not indicate any major conformational transition of the molecules but the difference between dermatan sulfate and heparin can be attributed to difference either in iduronic acid conformation or in intersaccharide linkages.