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

Abstract

Raman spectra have been 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 have been 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. These observations indicate that in the course of lens opacification some tyrosine residues undergo a change in their hydrogen-bonding environment and some buried tryptophan residues became exposed. In addition, the present Raman spectroscopic study provides insight into the 2SH leads to S-S conversion in lens proteins. It was found that the conversion proceeded at a faster rate in a hereditary cataractous lens than in a normal lens; however, this difference was fairly small at the early stage of cataract development. Importantly, the 2SH leads to S-S conversion was accelerated after nuclear cataract formation. These observations support the hypothesis that the formation of S-S linkages is not a predominant factor for initiating lens opacification. Probably the formation of S-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. Moreover, 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.

Ageing and changes in protein conformation in the human lens: A Raman microspectroscopic study

“…de Mu1 and Greve, 1987). The intensity of the 1450 cm-' band depends only on the number of protein CH, and CH, groups and is insensitive to protein secondary structure (Fasman et al, 1978;Tu, 1982;Ozaki et al. 1983).…”

“…Almost all Raman studies devoted to age-related or cataractous changes in the eye lens have used peak ratios of certain specific and relevant assignments to analyse possible alterations in the amino acid composition and/or the secondary and tertiary conformation of the lens proteins (Iriyama et al, 1982Itoh et al, 1983;Ozaki et al, 1983Ozaki et al, , 1987Parker, 1983;Yu et al, 1985;De Nagel et al, 1988;Nie et al, 1990;Schyns et al, 1990;Yu, Bando and Kuck, 1990). The analysis of age and cataract-related post-translational changes and of age-related transcriptional/translational changes using this peakratio approach may be biased by local variations in protein content (Ozaki et al, 1983;Otto, de Mu1 and Greve, 198 7).…”

“…The analysis of age and cataract-related post-translational changes and of age-related transcriptional/translational changes using this peakratio approach may be biased by local variations in protein content (Ozaki et al, 1983;Otto, de Mu1 and Greve, 198 7). Local variations in absolute protein content have recently been described Siebinga et al, 1991) and the variations proved to be within very short distances.…”

“…It does not include the Raman signals of disulphide bonds and sulphydryl groups. However, with respect to age-related changes in these bonds and groups numerous papers have already appeared (Itoh et al, 1983;Ozaki et al, 1983Ozaki et al, , 1987Parker, 1983;Yu et al, 1985Yu et al, , 1987Yu, Bando and Kuck, 1990;Nie et al, 1990). …”

“…As, for example, suggested by biochemical and Raman studies the rise in disulphide bridges and concomitant decrease in sulphydryl groups are age-related in many species, leading to the formation of high-molecular-weight proteins and possibly preceding nuclear cataract formation (Harding, 1981;Bloemendal, 1981, 1983;Itoh et al, 1983;Ozaki et al, 1983;Spector, 1985;Yu et al, 1985;Ozaki et al, 1987;Nie et al, 1990). However, human and guinea-pig lenses seem to be an exception as indicated by the Raman study of Yu et al (1987).…”