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

Electronic circular dichroism for the detection of microalbuminuria

Over the past decades, chiroptical spectroscopy has proved its incomparable ability to elucidate the structure and spatial arrangement of chiral molecules. Systematic analysis of biomolecules in the natural environment of biofluids, however, remains challenging. In this study, we used chiroptical spectroscopy to monitor urinary levels of human serum albumin. Not only severe proteinuria but even just a slightly increased urinary excretion of albumin (microalbuminuria) may indicate serious health complications, especially for diabetic individuals. Given the chiral nature of albumin and its typical spectral pattern, it may be easily observable by chiroptical spectroscopy, particularly electronic circular dichroism. The performed chiroptical analysis of urine not only allowed the detection of clinically confirmed microalbuminuria but was also able to reveal this pathological condition in cases beyond the diagnostic capability of common clinical procedures. Thus, our approach suggests that electronic circular dichroism is a useful tool for the fast and reliable qualitative monitoring of microalbuminuria with the potential for a quantitative analysis in the future.

Effects of Advanced Glycation End Products on Hyaluronan Photolysis: A New Mechanism of Diabetic Vitreopathy

PURPOSE

To test the effects of advanced glycation end products (AGEs), which are increased in vitreous of diabetic patients, on photolysis of hyaluronan.

METHODS

Pullulan standards were used as molecular weight (MW) markers to obtain a calibration curve. 0.02% hyaluronan solutions were divided into AGE-added and AGE-free samples; each sample was irradiated using a xenon lamp or kept in the dark. Retention time (RT) was measured for each sample using high-performance liquid chromatography.

RESULTS

RTs and logarithm of MW of pullulan standards were negatively correlated. In hyaluronan samples exposed to light, RT increased significantly for both AGE-added and AGE-free samples compared with samples kept in the dark. RT in AGE-added samples was greater by 3% than that in AGE-free samples (p = 0.02).

CONCLUSIONS

Exposure to light decreases MW of hyaluronan; addition of AGEs promotes this change. The photosensitizer activity of AGEs may be associated with accelerated depolymerization of hyaluronan in diabetic patients.

Free radical depolymerization of hyaluronan by Maillard reaction products: role in liquefaction of aging vitreous

The degradation of hyaluronan was followed by viscosimetry and by HPLC in order to study the possible role of Maillard products (lysine-glucose) on the alteration of the vitreous gel in aging and diabetes. Lysine-glucose generated Maillard products produced a decrease of viscosity and of the number average molecular weight (Mn) of hyaluronan during a 1 h incubation at 37 degrees C. This effect was comparable to that produced by 1 U/ml of testicular hyaluronidase but was weaker than the effect of a Fenton-type reagent (Udenfriend's reagent). The polydispersity of hyaluronan incubated with Maillard products appeared higher than with hyaluronidase suggesting a more random reaction. Antioxydant enzymes (SOD, catalase), the iron chelators (desferrioxamine, transferrin) and the free radical scavengers (uric acid, carnosine) inhibited the degradation by Maillard products confirming its free radical nature and the intervention of trace metals. Maillard products have been detected in diabetic vitreous and may play a role in its accelerated modifications (liquefaction) in diabetes as compared to normal aging.

Human vitreous hyaluronidase: isolation and characterization

PURPOSE

Hyaluronic acid (HA) is the predominant glycosaminoglycan (GAG) of the human vitreous. Interaction of this HA with vitreous collagen is important for maintaining gel structure. The mechanism of HA homeostasis in the vitreous is incompletely understood. The aim of this study was to determine whether hyaluronidase, an endoglycosidase that degrades HA, was present in human vitreous.

METHODS

Vitreous samples were collected from post-mortem eye bank specimens and from non-hemorrhagic, non-inflamed biopsy specimens. Vitreous hyaluronidase was purified by a series of column chromatographic steps, and its activity was measured by an ELISA-like assay and by substrate gel electrophoresis through and HA-impregnated gel. The purified hyaluronidase was also analyzed by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) and by Western blotting.

RESULTS

Hyaluronidase activity was detected in vitreous samples from both post-mortem and biopsy specimens. The enzyme was most active at acid pH, but demonstrated significant activity at neutral pH. The partially purified enzyme migrated as a 59 kDa protein on SDS-PAGE, and a single band on Western blots.

CONCLUSIONS

Hyaluronidase is present in the human vitreous. Thus, hyaluronidase may be involved in HA catabolism in the vitreous and may play a role in determining its gel structure.

Mechanisms of photo-induced vitreous liquefaction

To understand the mechanisms of photo-induced vitreous liquefaction, this study investigated the effects of free radicals on collagen and hyaluronic acid (HA). Bovine vitreous collagen or HA was irradiated by visible light in the presence of riboflavin (RF) as a photosensitizer. The changes in the molecular weight of collagen and HA were monitored by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and high-performance liquid chromatography, respectively. Free radicals were shown to cause an increase in the high-molecular-weight components and insolubilization of the vitreous collagen and a decrease in the molecular weight of HA. The change in molecular properties of the vitreous collagen could be attributed to extensive crosslinks of the molecules. Since RF is present in the vitreous, which is irradiated by visible light over a lifetime, both cross-links of vitreous collagen and degradation of HA may contribute to age-related vitreous liquefaction.

Monitoring in situ circular dichroism of the intact vitreous: a new approach

For the first time, an attempt has been made to study the vitreous humor in situ by circular dichroism (CD). The vitreous, an avascular and acellular gel-like tissue, is optically transparent and homogeneous, and, thus, light scattering is minimal. The macromolecular components of this tissue, hyaluronate (HA), collagen and noncollagenous protein (NC-P), appear to exist in the matrix in a nonoriented fashion. As a result, no linear dichroism was observed. A typical CD of the vitreous shows a minimum at 206 nm with a shoulder at 220 nm and one small positive peak at approximately 252 nm. Gaussian analysis resolves this spectrum into four component bands. CD analysis of individual components reveals that NC-P makes the major contribution to the dichroic strength of the vitreous; contributions of HA and collagen, on the other hand, are small. The positive peak arises largely from ascorbic acid in the vitreous. CD measurement of the intact vitreous appears to be a useful technique for assessing the structure and changes of the constituent molecules in the normal and diseased vitreous.

Age-related changes in human vitreous structure

Changes in vitreous structure that occur with aging are important in the pathogenesis of vitreous liquefaction (synchisis senilis), vitreous detachment, and retinal disease. Vitreous morphology was studied in 59 human eyes post-mortem using dark-field horizontal slit illumination of the entire dissected vitreous. In many individuals younger than 30 years, the vitreous was homogeneous in structure. Middle-aged individuals had macroscopic fibers in the central vitreous, which coursed anteroposteriorly and inserted into the vitreous base and the vitreous cortex, posteriorly. During senescence, the vitreous volume was reduced, the vitreous body was collapsed (syneresis), and the fibers were thickened, tortuous, and surrounded by liquid vitreous. This sequence of age-related changes probably results from a progressive reorganization of the hyaluronic acid and collagen molecular networks. Characterization of the molecular events underlying these changes will elucidate the mechanisms of the phenomena of synchisis, syneresis, and detachment, and may provide methods with which to prevent or induce vitreous detachment prophylactically.

Ageing of the vitreous

Changes that occur in the vitreous during ageing contribute to a variety of vitreo-retinal disorders. These age-related changes are rheologic, biochemical, and structural in nature. Our current knowledge of these ageing changes is reviewed. Hypotheses for the mechanisms of liquefaction (synchisis senilis) and posterior vitreous detachment are proposed.