Inter- and intramolecular disulfide bond formation and related structural changes in the lens proteins. A Raman spectroscopic study in vivo of lens aging.

Abstract

Raman spectra have been measured for intact rat lens nuclei at various stages of aging in an attempt to gain further insight into age-related structural changes in the lens proteins, especially changes concerning protein sulfhydryl groups. Two Raman bands at 2579 and 2561 cm-1 were observed to be assignable to SH stretching modes of the cysteine residues. These bands have been attributed to "exposed" and "buried" sulfhydryl groups of the lens proteins, respectively, on the basis of a model compound study. The relative intensities of both SH stretching modes decreased with lens aging, and concurrently the intensity of a S-S stretching mode at 509 cm-1 due to disulfide bridges increased, suggesting that not only exposed but also buried protein sulfhydryl groups are converted to disulfide groups as a result of aging. The rate of the intensity decrease in the 2561 cm-1 band was similar to that in the 2579 cm-1 band. Therefore, it seems likely that the sulfhydryl groups in the two distinct environments are nearly equally subjected to the oxidation. Cysteine and cystine residues of the lens proteins gave their C-S stretching modes at 708 cm-1, indicating that they predominantly assume PC and/or PN conformers. The intensity ratio of a tyrosine doublet near 840 cm-1 (I832/I855) changed from approximately 0.86 to approximately 0.81 with the aging of the rat lens. This result implies that some tyrosine residues undergo a change in their hydrogen bonding environments during the course of aging. Of particular importance is that the relative intensity change of the tyrosine doublet with normal aging and that with cataract formation are in opposite directions.