Age-related changes in the composition of proteins in the trabecular meshwork of the human eye

The composition of the trabecular meshwork proteins of human eyes ranging in age from 36 days to 84 years was examined by polyacrylamide gel electrophoresis and amino acid analysis. Proteins of different molecular weights could be extracted from the tissue with acetic acid. Although their electrophoretic patterns became less distinct with increasing age, proteins of molecular weights ranging from 50 000 to 69 000 always prevailed. The amino acid compositions of the acetic acid-insoluble trabecular meshwork residues revealed the prevalence of collagenous proteins. The peptide maps produced by treatment with cyanogen bromide indicate that most of the fragments solubilized from the trabecular meshwork of younger eyes are derived from type I collagen. Beyond 40 years of age, the trabecular meshwork was resistant to cyanogen bromide and pepsin digestion. A rough estimate of the distribution of collagen types in the trabecular meshwork was based on 3-hydroxyproline/4-hydroxyproline ratios, indicating an age-related increase of type I collagen from about 55 to 70 per cent, and of type IV collagen from about 2 to 5 per cent of the total protein present. During ageing, some of the protein-bound methionine is oxidized to methionine sulfoxide, reaching about 35 per cent of the total methionine content at the age of 20 years and, with a slower rate of oxidation, a mean value of 40 per cent at 80 years of age. Electron-microscopic analysis of specimens remaining undissolved after cyanogen bromide cleavage and pepsin treatment no longer revealed regular collagenous fibrils but rather elastic-like fibers surrounded by wide sheaths consisting of fine fibrils with a regular cross-banding periodicity of 40-50 nm. In addition, clusters of so-called curly (lattice) collagen were found. The amino acid composition of this insoluble material suggests that altered collagen-like molecules prevail among the proteins of the residues.