Alpha-crystallin can act as a chaperone under conditions of oxidative stress

Previous studies have shown that alpha-crystallin, a major lens protein, acts as a chaperone preventing the thermal denaturation of other lens crystallins. However, there has not been an examination of the alpha-crystallin chaperone ability with respect to the types of insult thought to cause human cataract. Therefore, an examination of the chaperone potential of alpha-crystallin under conditions of oxidative stress was undertaken.

Oxidation of alpha-, beta low (beta L)-, and gamma-crystallins was performed with an ascorbate FeCl3-EDTA-H2O2 system. Thermal denaturation was carried out by heating preparations at 62 degrees C or 72 degrees C. After protein denaturation, 360 nm scatter was measured. Protein-complex formation was measured with a TSK gel G4000 SW 600 x 7.5 mm exclusion column.

This study indicates that: (1) alpha-crystallin markedly reduces the 360-nm light scatter of gamma-crystallin caused by oxidation at 37 degrees C. (2) alpha-crystallin appears to protect the gamma-crystallin thiol groups from extensive oxidation. (3) Oxidation of alpha-crystallin causes only a small change in its ability to prevent heat-induced scattering of either gamma- or beta L-crystallin. (4) Oxidation of both alpha- and gamma-crystallin does not significantly affect the ability of alpha-crystallin to inhibit 360-nm light scattering of gamma-crystallin at 72 degrees C. (5) Oxidation of beta L-crystallin decreases its susceptibility to thermally induced scattering, but, conversely, oxidation of gamma-crystallin increases such susceptibility. (6) Oxidation of beta L-crystallin at 37 degrees C produces only a slight increase in light scatter, in contrast to observations obtained with gamma-crystallin. (7) alpha-crystallin provides long-term protection against thermally induced scatter of beta L-crystallin but not of gamma-crystallin. High-performance liquid chromatography (HPLC) analysis suggests that the alpha-gamma-crystallin complex gradually becomes insoluble at 72 degrees C, in contrast to the alpha-beta L-crystallin complex. Differing from thermal insult, alpha-crystallin causes a marked decrease in gamma-crystallin light scattering under long-term oxidation. (8) The alpha-gamma-crystallin complex that results from oxidation represents a weak interaction because it cannot be isolated with procedures used to obtain the thermally induced complex. (9) This work confirms a previous study demonstrating that each alpha monomer (alpha m) contains a binding site for a partially denatured crystallin.

The overall results indicate that alpha-crystallin can act as a chaperone under conditions of oxidative stress, decreasing the light scatter and thiol oxidation of other crystallins. Because oxidative stress is thought to be present under normal physiological conditions, it is probable that alpha-crystallin contributes to the mechanisms that maintain the lens in a transparent state.