Opacification of gamma-crystallin solutions from calf lens in relation to cold cataract formation

To determine the molecular mechanisms for cold cataract formation in the nucleus of the young mammalian lens, we have investigated the thermally reversible opacification of gamma-crystallin solutions isolated from calf lens. Coexistence curves (plots of opacification temperature Tc versus protein concentration) were determined for the individual gamma-crystallin fractions II, III, and IV as well as for the unfractionated gamma-crystallin mixtures isolated from the nucleus and cortex. The coexistence curve of gamma IV-crystallin is remarkably elevated above those of gamma II- and gamma III-crystallin and the gamma-crystallin mixtures. The gamma IV-crystallin fraction is the major determinant of the opacification temperature within the whole lens or isolated cytoplasm. Quasielastic light-scattering spectroscopy of gamma IV-crystallin solutions indicates that above Tc there are two populations of protein aggregates of distinctly different mean size. As the temperature is lowered towards Tc, both populations increase in size. Opacification occurs when the population of large scatterers, which is composed of less than 0.1% protein by weight, reaches an average radius of about 20,000 A.