Photosensitized structural modifications of the lens protein alpha-crystallin: do all modifications impair chaperone-like activity?
Among chaperone-like functioning proteins, the lens alpha-crystallins are of particular interest because they are not renewed, and even minor alterations can hurt their function of maintaining the proper refractive index and avoiding cataract formation in the lens. Several reports have suggested the occurrence of remarkable structural modifications in lens proteins in the presence of endogenous and exogenous sensitizers upon exposure to light. In particular, it has been shown in vitro that hypericin, the active ingredient of Hypericum, can bind to and, in the presence of light, cause the photopolymerization of alpha-crystallin. On the basis of these results it has also been suggested that a subsequent significant impairment of the protein function can occur. Using absorption and emission spectroscopic techniques, as well as circular dichroism, we have studied the structural modifications of alpha-crystallin resulting from its interaction with hypericin after irradiation with visible light. To investigate the chaperone-like function of alpha-crystallin, the heat-induced aggregation kinetics of another lens protein, betaLow-crystallin, was monitored by measuring the apparent absorption due to scattering at 360 nm as a function of time, and no apparent damage to its functional role was observed. Spectroscopic results, on the contrary, show a prominent reduction in both tryptophan and hypericin fluorescence emission intensity after light irradiation, suggesting an alteration in the tryptophan microenvironment and a high degree of packing of the chromophore due to photoinduced modification of the molecular framework. Control experiments on alpha-crystallin structurally modified by light in the presence of hypericin indicated that the protein still retains its ability to chaperone both lens crystallins and insulin.
Sgarbossa
Antonella
A
Istituto Biofisica CNR, Via Moruzzi 1, 56100 Pisa, Italy.
Youssef
Tareq
T
Lenci
Francesco
F
D000818:Animals; D000873:Anthracenes; D006801:Humans; D018832:Molecular Chaperones; D010569:Perylene / Q000031:analogs & derivatives* / Q000494:pharmacology; D017319:Photosensitizing Agents / Q000494:pharmacology; D011487:Protein Conformation / Q000187:drug effects / Q000528:radiation effects; D013053:Spectrophotometry; D038201:alpha-Crystallins / Q000737:chemistry* / Q000528:radiation effects*
D000873:Anthracenes; D018832:Molecular Chaperones; D017319:Photosensitizing Agents; D038201:alpha-Crystallins; D010569:Perylene; C004965:hypericin