High-molecular-weight protein aggregates of calf and cow lens: spectroscopic evaluation

To gain insight into the molecular features of the high-molecular-weight (HMW) fraction of soluble lens proteins and their changes in aging, we isolated this fraction from the nucleus of calf and cow lenses and measured fluorescence and circular dichroism (CD) properties of the samples. Not only was there an increase in the HMW fraction in the older lens, but there was also an age-related difference in tertiary structure that was clearly manifested in the fluorescence and CD parameters. The far-u.v. CD of low- and high-molecular-weight proteins do not differ significantly in band position and magnitude, but the near-u.v. CD of HMW protein does differ distinctly from that of all other crystallins (alpha, beta and gamma); the entire CD spectrum of this protein is displayed in the negative region. Millipore filtration further revealed that HMW aggregates are essentially a polydisperse population of different conformation (tertiary structure) and that these aggregates are associated by non-convalent interactions. This association is caused mainly by the apolar (hydrophobic) nature of the constituent protein. alpha-Crystallin has more hydrophobic domain along the peptide chain that do other crystallins and thus is likely to be the predominant protein in HMW aggregates.