Structural changes in the lens proteins of hereditary cataracts monitored by Raman spectroscopy

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Abstract
Raman spectra were measured for the lenses from cac-strain mice. These mice possess a hereditary defect and provide lenses at various stages of opacification. The Raman spectra of normal mouse lenses were obtained also for comparison purposes. The amide I and III bands appear in very similar positions in the Raman spectra of cataractous and normal lenses, suggesting that the peptide backbone of main lens proteins does not undergo a major conformational change upon lens opacification. However, lens opacification causes significant changes in the intensity ratio of the tyrosine doublet near 840 cm-1 and in that of the Raman bands at 881 and 760 cm-1 due to tryptophan residues. These changes could be observed even in the incipient stage of hereditary cataract and became more pronounced with cataract development. In the course of lens opacification some tyrosine residues undergo a change in their H-bonding environment and some buried tryptophan residues became exposed. The present Raman spectroscopic study provides insight into the 2SH .fwdarw. S.sbd.S conversion in lens proteins. It was found that the conversion proceeded at a faster rate in a hereditary cataractous lens than in normal lens; this difference was fairly small at the early stage of cataract development. The 2SH .fwdarw. S.sbd.S conversion was accelerated after nuclear cataract formation. These observations support the hypothesis that the formation of S.sbd.S linkages is not a predominant factor for initiating lens opacification. Probably the formation of S.sbd.S linkages plays an important role in stabilizing the protein aggregates which are the cause of lens opacification. The intensity of the SH stretching mode (2579 cm-1) was very weak or absent in the Raman spectrum of a well-developed cataractous lens, suggesting that most sulfhydryl groups form disulfide bonds. The fact that this occurs without major conformational changes of peptide backbones implies that most cysteine residues in lens crystallins are accessible to solvent or are clustered closely together.