Biochemical evidence for membrane disintegration in human

Biochemical evidence is presented for the dis- integration of the lens fiber plasma membrane in human cata- racts. The intrinsic membrane proteins are found in both the water-soluble and water-insoluble nonmembrane fractions of the cataract lens but not in the normal tissue.. Furthermore, in con- trast to the normal lens, not all of the lipid found;in the cataractous lens is isolated with the membrane fraction. In cataracts, both the membrane and membrane fragments are involved in covalent high molecular weight aggregates with an extrinsic membrane protein (43,000 daltons) and a.cytoplasmic protein (y-crystallin). Studies of the structure of human senile cataracts have dem- onstrated a disappearance of normal fiber structure in the opaque region of the tissue. Both by electron microscopy and light microscopy, with the fluorescent antibody technique, re- gions of the lens are found in which the plasma membrane has disintegrated. In these regions of disruption, there' are amor- phous pools of material, globular structures, and.multilaminar structures (1-8). Some of these structures can be stained with an antiserum to the 43,000-dalton extrinsic membrane protein (9). These disorganized areas probably contribute to the light- scattering characteristics observed in cataractogenesis. Al- though the morphological studies clearly suggest a breakdown of fiber structure, no convincing biochemical data to support such observations have been reported. Investigation of the biochemical changes in senile cataracts indicates a rapid increase of water-insoluble material at the ex- pense of the water-soluble fraction (10-12). The acceleration in the generation of the insoluble fraction appears to be pri- marily due to the formation of new types of protein aggregates. A major component of the water-insoluble fraction of cataracts is the high! molecular weight disulfide-linked (high Mr S-S) ag- gregates (13). These aggregates are present only in the catarac- tous lens and reflect the oxidative stress to which the tissue.is subjected. Starting with oxidation at the membrane of the older normal lens, the oxidation spreads to,cytoplasmic components with the formation of opacity (14). Methionine and other amino acids besides cysteine are involved. The high Mr S-S aggregates have been shown to contain the 43,000-dalton extrinsic fiem- brane protein (15). It has been suggested that such polypeptides may act as nucleation sites on the membrane for formation of high M, S-S aggregates. Immunochemical and biochemical evidence is presented in- dicating that the plasma membrane disintegrates in senile cat- aracts. This disrupted plasma membrane is shown' by immu- nochemical means to be associated, with the high Mr S-S aggregates and with aggregates that cannot be dissociated' by reduction and alkylation. These unusual aggregates, which are