Lens crystallins, glycosylated

Although nonenzymatic glycosylation may affect practically every protein in vivo, it is likely that nonenzymatic browning will occur only in proteins that have a slow turnover or none at all, such as lens crystallins, collagen, elastin and proteoglycans. In some tissues, these proteins are, in effect, "stored" for a lifetime and undergo some characteristic changes, many of which have been observed in stored and processed foodstuffs (Table II). 

Stevens, V. J., Rouzer, C. A., Monnier, V. M., and Cerami, A., Diabetic cataract formation Potential role of glycosylation of lens crystallins. Proc. Natl. Acad. Sci U.SA. 75,2918-2922 (1978). 

Following the study of hemoglobin (B41), glycosylation has been demonstrated for many proteins, ranging from those with short (e.g., albumin) to very long (viz., lens crystallins, elastin, collagens) biological half-lives. 

Glycosylation of the lens crystallin proteins (All, C8, C9, H6a, M5, M31, P4, S45) is of considerable interest because of the long-lived nature of the a, P, and 7 crystallins, the molecular aggregation and decreased solubility of the glycosylated products, the possibility of further reactions leading to browning effects (M28, M29), and their relevance to cataract formation and other complications in diabetes and aging. 

The TBA method has been used not only to assay glycosylated hemoglobin, but also almost exclusively for the following glycosylated proteins plasma albumin (D22, El, NIO, M12), plasma proteins and albumin (K6), capillary whole-blood protein collected on filter paper (L14), erythrocyte spectrin (M13), skin collagen (P2), lens basement membrane (M5), and crystallins (M28). In assays on hair and epidermal keratin, cyclohexanone extraction of the color developed with TBA and reading the absorbance of the cyclohexanone layer at 433 nm has provided increased sensitivity and accuracy (T4). A possible objection to the procedure is the use of oxalic and thiobarbituric acids, which are potentially toxic chemicals. 

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