Effect of calcium on the calf lens cytoplasm.

Opacification was induced in calf lens cytoplasmic extracts by addition of calcium. The sample turbidity was shown to increase with calcium molarity, incubation time and temperature and to decrease with the protein cytoplasmic concentration. Although this turbidity was enhanced when membrane fragments were left over in the cytoplasmic extracts, it did show up in the absence of any detectable vesicular fragment. Scattering techniques (X-ray and light) showed that the calcium-induced opacification is linked to enhanced light scattering, which results from the formation of additional scatterers, a few tens of nm in diameter. Additional structures were indeed visualized by freeze-fracture electron microscopy (FFEM): they appear as molecular clusters with diameters ranging from 20 to 90 nm, made of densely packed particles, with heterogeneous sizes. The turbidity expected from these clusters, as well as the expected variation of turbidity with cytoplasmic concentration, was calculated to be in agreement with the measurements. When compared with cold cataract, these results illustrate that similar opacities may result from completely different biophysical mechanisms.