Alteration in the ubiquitin structure and function in the human lens: a possible mechanism of senile cataractogenesis

Abstract

High-performance liquid chromatography puri¢ca-tion followed by mass spectrometry analyses highlighted that human senile cataractous lens includes a 8182 Da species which is absent in the normal lens, whereas a 8566/8583 Da species is present in both lenses. Western blot analysis identi¢ed both species as ubiquitin. The species at lower molecular weight is a shorter form due to the cleavage of the C-terminal residues 73^76. As it is the last amino acid of ubiquitin which is involved in the protein degradation mechanism, we suggest that this structure modi¢cation compromises the function of ubiquitin and consequently the physiologically occurring degradation of the lens proteins.

Separation and characterization of superoxide dismutase 1 (SOD‐1) from human erythrocytes by capillary electrophoresis time‐of‐flight mass spectrometry

“…The deconvoluted zero‐charge spectrum of Fig. B‐i shows a main molecular mass of 8565.1 Da that could correspond to ubiquitin (UBI, M theo = 8564.7 Da, without posttranslational modifications), a small protein of 76 amino acids present in almost all tissues of eukaryotic organisms, which is involved in protein degradation . The minor peak of 8451.0 Da was in good agreement with a truncated ubiquitin that resulted after the cleavage of the C‐terminus segment 75–76 (Gly‐Gly) (M theo = 8450.6 Da).…”

The proteasomal system

“…Apart from the mentioned proteases the role of a strictly regulated proteasomal activity may be more important. In an age-dependent manner the activity of ubiquitin conjugating enzymes has been found to decrease in the lens (Wride et al, 2006), while even structural changes in the ubiquitin itself have been revealed (Stiuso et al, 2002). Whether the activity of the proteasome does change is unclear, since according to some studies it remained unchanged (Zetterberg et al, 2003), whereas according to others it did decline (Viteri et al, 2004).…”

Proteases in eye development and disease

“…It has also been shown that oxidative stress (a contributing factor towards age‐related cataract) results in disruption of the UPP (Hosler et al, 2003), which is itself involved in the removal of oxidatively damaged proteins (Shang et al, 2001). Finally, alterations in the structure of ubiquitin have been observed during senile cataractogenesis (Stiuso et al, 2002). Thus, further investigation should be undertaken to identify the function of the UPP pathway in lens development and cataract.…”