Disulfide-linked membrane proteins in X-ray-induced cataract

The formation of membrane protein disulfide was investigated at various stages of development of X-ray-induced cataract in the rabbit. Intermolecular disulfide bonding of lens membrane proteins was detected not only in the mature cataract (occurring 8-9 weeks after the X-ray dose) but also at 1 week prior to maturation, in which no significant increase in lens hydration occurs and where opacification is confined mainly to the posterior subcapsular region. Two-dimensional diagonal electrophoresis revealed that polypeptides with apparent molecular weights of 21, 23, 25, 31, 35, 46 and 53 kilodaltons were involved in cross-linking. The MP26 membrane polypeptide was not significantly involved in the disulfide bonding. The oxidation of membrane proteins in stages other than mature was evident only in the lens nucleus (which remained clear) and not in the cortex. The results of this study indicate that an intermolecular disulfide linkage of cytosolic proteins to membranes occurs prior to formation of mature cataract, and may be a precursor to protein aggregation and insolubilization in the mature nuclear cataract.