Covalent changes at the N- and C-terminal regions of gamma crystallin during aging of the normal human lens

Degradation of gamma D- and gamma s-crystallins in human lenses

The aim of this study was to determine age-related degradation of gammaD-crystallin and the cleavage sites in the connecting peptide regions of the two domains of gammaD- and gammas-crystallins. The water-soluble (WS) proteins from lenses of donors of different ages and a purified gamma-crystallin fraction were analyzed for the fragments of gammaD-crystallin by the Western blot method. Four site-specific antibodies (Ab) raised to the four regions of human gammaD-crystallin, i.e., anti-gammaD-N-Ab to the N-terminal end (residue nos. 1-9), anti-gammaD-C-Ab to the C-terminal end (residue nos. 165-173), and two to the middle regions, anti-gammaD-M1 Ab (residue nos. 78-86) and anti-gammaD-M2 Ab (residue nos. 87-95), were used. The gamma-crystallin fragments were also separated by a preparative SDS-PAGE method prior to Western blot analysis. The two-dimensional gel electrophoretic method (first dimension of isoelectric focusing followed by the second dimension of SDS-PAGE) was used to separate crystallin fragments and desired fragments were analyzed for their partial N-terminal sequences. The Western blot results showed seven major gammaD-crystallin fragments of about 4, 5, 11, 14, 15, and 17 kDa with intact N-termini but cleaved C-termini. In contrast, only three fragments with Mrs of about 5, 9, and 11 kDa were observed with intact C-termini but cleaved N-termini. Similar analysis also identified fragments with Mrs of about 5, 9, 11, and 14 kDa that originated via cleavage in the middle region of the molecule. The partial N-terminal sequencing results of the 9- to 10-kDa fragments showed cleavage in the connecting peptide region, i. e., two cleavage sites at D73-S74 and G86-S87 in gammaD-crystallin whereas four such sites at R83-A84, A84-V85, H85-L86 and G90-G91 in gammas-crystallin. Together, the results suggest that the degradation in the gammaD-crystallin mostly occurs at the C-terminal region with repeated cleavage of certain sites during aging. In addition, the major fragments with Mr of 9-10 kDa were produced via cleavages within or close to the connecting peptide regions of gammaD- and gammas-crystallins at the two and four cleavage sites, respectively, as described above.

Deamidation and disulfide bonding in human lens gamma-crystallins

Detailed analysis of the three gamma-crystallins present in the water-soluble portion of human lenses, gammaS, gammaD and gammaC, has identified disulfide bonding and deamidation as the major post-translational modifications of these crystallins. Chromatographic and mass spectrometric techniques were used to isolate and identify water-soluble gamma-crystallins from normal lenses, ages 32 week gestation, 0 day old, 4 day old, 19, 31, 45 and 55 year old. The amino acid sequences of the gamma-crystallins were confirmed and/or corrected by mass spectrometric analysis of peptides produced by enzymatic digestion or chemical fragmentation of the isolated crystallins. The molecular weight of peptides were also used to identify, locate and quantify modifications. Each of the gamma-crystallins had two disulfide bonds as well as several deamidated glutamine and asparagine residues. The extent of disulfide bond formation and deamidation appeared to increase with the age of the lens. This examination of normal human lens gamma-crystallins, the first detailed characterization of the gamma-crystallins, will provide a basis for comparison with modifications found in the water-insoluble portion and in cataractous lenses.

Age-related changes in human lens crystallins identified by HPLC and mass spectrometry

Analysis of water-soluble crystallins from human lenses, ages 32 week fetal to 55 years has led to identification of the major modifications of the proteins comprising the lens. These modifications were identified by the masses of the proteins determined by electrospray ionization mass spectrometry after the proteins were separated by gel filtration and reversed phase high performance liquid chromatography. Examination of all the proteins isolated from the water soluble portion demonstrated that the major age-related modifications causing significant alteration in the molecular weights of the lens crystallins include truncation of the N-termini of beta B1, beta A3 and beta A1, and partial phosphorylation and C-terminal degradation of alpha-crystallins. N-terminal degradation of beta B1, beta A3 and beta A1 was evident in human lenses less than one year old, and the proportion of these truncated proteins became greater with age. Phosphorylation of alpha A- and alpha B-crystallins increased from the fetal to the 3 year old lens, but did not change with further aging. Minor components indicating truncation of the C-termini of alpha-crystallins were found in older lenses. In contrast to beta B1, beta A3 and beta A1, the masses of the major species of alpha A, alpha B, beta B2, beta A4, gamma S, gamma C, and gamma D did not change with aging. This suggested that the major modifications to these crystallins are limited to deamidation and possibly intra-molecular disulfide bonds. These data, in conjunction with the data in the accompanying manuscript, established deamidation as a common modification, since deamidation, which causes only a one dalton change in mass, is the only modification that is consistent with the absence of a detectable change in molecular weight and the observed increased acidity demonstrated in the two-dimensional gels of the accompanying paper. Other age related changes included a decrease in beta B3 (M(r) 24224), a major component of the fetal lens, which was not detected in lenses older than 3 years, and increases in the ratios of alpha B:alpha A and gamma S:gamma C.

Near-infrared Fourier transform Raman and conventional Raman studies of calf gamma-crystallins in the lyophilized state and in solution

We present in this report a detailed structural study of calf gamma-crystallins both in the solid state and in solution by the newly developed technique of near-infrared (IR) Fourier transform (FT)-Raman spectroscopy as well as by the conventional Raman method. In comparison with conventional laser Raman spectroscopy, the near-IR FT-Raman approach exhibits several attractive features such as fluorescence rejection capability, frequency accuracy, and the FT's multiplex and throughput advantages. These distinct characteristics combined form the basis for the particular suitability of FT-Raman in crystallin structural analysis and elucidation. We have thus obtained evidence in support of the view that native calf gamma-II crystallin does not contain a disulfide bond either in the lyophilized state or in solution. In addition, conventional Raman spectra are examined for all four gamma-crystallin fractions. gamma-S, gamma-II, gamma-III, and gamma-IV, and the results indicate a high degree of structural similarities among them. It is also found that the sulfhydryl groups in all four gamma-crystallins are highly resistant to air oxidation and are capable of maintaining their reduced state during isolation in the absence of added reductants or such chelating agents as EDTA.

The conformational status of a protein influences the aerobic photolysis of its tryptophan residues: melittin, beta-lactoglobulin and the crystallins

We have studied the aerobic photolysis of the tryptophan residues of the proteins melittin and beta-lactoglobulin when the proteins are in ordered conformations and when they are in randomly coiled states. The results suggest that the conformational status of the protein is a factor that influences the photolysis of the constituent tryptophan residues. This point appears to be of relevance to the photo-oxidation of the tryptophan residues of the eye lens proteins crystallins.

Spectroscopic studies on the photooxidation of calf-lens gamma-crystallin

The photooxidation of calf gamma-crystallin has been investigated in (a) riboflavin-sensitized reaction and (b) direct photolysis of the tryptophan residues of the protein at 300 nm. Partial insolubilization of the protein is evident by the increase in turbidity of the photolyzed protein solution. The turbidity is diminished when anaerobic conditions are used for irradiation. Spectroscopic studies on the soluble phase of the photolyzed protein reveal significant changes in the near-UV circular dichroism spectrum, suggesting that changes in the tertiary structure of the protein precede insolubilization. Isoelectric focusing analyses reveal that the gamma-crystallin polypeptides have more acidic pI's after photooxidation under both conditions. The insoluble protein is highly crosslinked into dimers and higher oligomers via covalent, non-disulfide crosslinks, but inter-subunit crosslinking is negligible in the soluble phase of the photolyzed protein. The mechanism of photooxidation involves the generation of H2O2 in the riboflavin-sensitized reaction. In the direct photolysis of tryptophan residues by 300 nm irradiation, there is a rapid disruption of the protein structure, apparently by the oxidation of the hydrophobic tryptophan residues.