A differential scanning calorimetric study of the bovine lens crystallins

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Abstract
Differential scanning calorimetry was performed on the five major lens crystallin fractions [HM-.alpha., .alpha., .beta.H, .beta.L, and (.beta.s + .gamma.)] of the bovine lens as well as on more purified forms of .alpha.- and .gamma.-crystallins. All were found to be relatively thermally stable although the .alpha.-crystallin fractions were found to at least partially unfold at an approximately 10.degree. C lower temperature than the .beta. and .gamma. fractions. Increasing protein concentration had little effect on .gamma.-crystallin thermograms but had marked effects on those of the .alpha.- and .beta.-crystallins. Increases in the thermal stability with increasing protein concentration for the .beta.-crystallins can be explained most simply by the known .beta.L/.beta.H equilibrium, but, in the case of the .alpha.-crystallins, excluded volume effects may be an important factor. In both cases, the increased stability at high concentrations could be of physiological relevance. As well as the expected endothermic unfolding transitions, all of the lens crystallins revealed exothermic peaks that correlate with protein precipitation. Interestingly, this phenomenon occurs only after extensive structural alteration in the case of the .alpha.-crystallins but is present very early in the initial stages of structural perturbation of the .beta.- and .gamma.-crystallins.