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The review considers the problem of hydrogen peroxide decomposition and hydroxyl radical formation in the presence of iron in vivo and in vitro. Analysis of the literature data allows us to conclude that, under physiological conditions, transport of iron, carried out with the help of carrier proteins, minimizes the possibility of appearance of free iron ions in cytoplasm of the cell. Under pathological conditions, when the process of transferring an iron ion from a donor protein to an acceptor protein can be disrupted due to modifications of the carrier proteins, iron ions can enter cytosol. However, at pH values close to neutral, which is typical for cytosol, iron ions are converted into water-insoluble hydroxides. This makes it impossible to decompose hydrogen peroxide according to the mechanism of the classical Fenton reaction. A similar situation is observed in vitro, since buffers with pH close to neutral are used to simulate free radical oxidation. At the same time, iron hydroxides are able to catalyze decomposition of hydrogen peroxide with formation of a hydroxyl radical. Decomposition of hydrogen peroxide with iron hydroxides is called Fenton-like reaction. Studying the features of Fenton-like reaction in biological systems is the subject of future research.

The absence of cellular organelles in fiber cells and very high cytoplasmic protein concentration (up to 900 mg/ml) minimize light scattering in the lens and ensure its transparency. Low oxygen concentration, powerful defense systems (antioxidants, antioxidant enzymes, chaperone-like protein alpha-crystallin, etc.) maintain lens transparency. On the other hand, the ability of crystallins to accumulate age-associated post-translational modifications, which reduce the resistance of lens proteins to oxidative stress, is an important factor contributing to the cataract formation. Here, we suggest a mechanism of cataractogenesis common for the action of different cataractogenic factors, such as age, radiation, ultraviolet light, diabetes, etc. Exposure to these factors leads to the damage and death of lens epithelium, which allows oxygen to penetrate into the lens through the gaps in the epithelial layer and cause oxidative damage to crystallins, resulting in protein denaturation, aggregation, and formation of multilamellar bodies (the main cause of lens opacification). The review discusses various approaches to the inhibition of lens opacification (cataract development), in particular, a combined use of antioxidants and compounds enhancing the chaperone-like properties of alpha-crystallin. We also discuss the paradox of high efficiency of anti-cataract drugs in laboratory settings with the lack of their clinical effect, which might be due to the late use of the drugs at the stage, when the opacification has already formed. A probable solution to this situation will be development of new diagnostic methods that will allow to predict the emergence of cataract long before the manifestation of its clinical signs and to start early preventive treatment.

The structure of the oligomeric protein α-crystallin from bovine eye lens was investigated by small-angle neutron scattering (SANS) with contrast variation. Based on the SANS curves, the match point for α-crystallin (43% D2O) and its average scattering length density at this point (2.4·10(10) cm(-2)) were evaluated. The radius of gyration and the distance distribution functions for α-crystallin were calculated. On the basis of these calculations, it was concluded that α-crystallin is characterized by homogeneous distribution of scattering density in the domains inaccessible for water penetration, and all polypeptide subunits in α-crystallin oligomers undergo equal deuteration. The latter indicates that all α-crystallin subunits are equally accessible for water and presumably for some other low molecular weight substances. These conclusions on the α-crystallin structure (homogeneous distribution of scattering density and equal accessibility of all subunits for low molecular weight substances) should be taken into account when elaborating α-crystallin quaternary structure models.

The damaging effect of UV radiation (lambda > 260 nm) on bovine alpha-crystallin in solution was studied by small-angle X-ray scattering, gel permeation chromatography, electrophoresis, absorption and fluorescence spectroscopy, and differential scanning calorimetry. The results obtained show that damage to even a large number of subunits within an alpha-crystallin oligomer does not cause significant rearrangement of its quaternary structure, aggregation of oligomers, or the loss of their solubility. Due to the high resistance of its quaternary structure, alpha-crystallin is able to prevent aggregation of destabilized proteins (especially of gamma- and beta-crystallins) and so to maintain lens transparency throughout the life of an animal (the chaperone-like function of alpha-crystallin).

Thermal aggregation of rabbit skeletal muscle glycogen phosphorylase b (Phb) has been investigated using dynamic light scattering under conditions of a constant rate of temperature increase (1 K/min). The linear behavior of the dependence of the hydrodynamic radius on temperature for Phb aggregation is consistent with the idea that thermal aggregation of proteins proceeds in the kinetic regime wherein the rate of aggregation is limited by diffusion of the interacting particles (the regime of "diffusion-limited cluster-cluster aggregation"). In the presence of alpha-crystallin, a protein exhibiting chaperone-like activity, the dependence of the hydrodynamic radius on temperature follows the exponential law; this suggests that the aggregation process proceeds in the kinetic regime where the sticking probability for colliding particles becomes lower than unity (the regime of "reaction-limited cluster-cluster aggregation"). Based on analysis of the ratio between the light scattering intensity and the hydrodynamic radius of Phb aggregates, it has been concluded that the addition of alpha-crystallin results in formation of smaller size starting aggregates. The data on differential scanning calorimetry indicate that alpha-crystallin interacts with the intermediates of the unfolding process of the Phb molecule. The proposed scheme of thermal denaturation and aggregation of Phb includes the stage of reversible dissociation of dimers of Phb into monomers, the stage of the formation of the starting aggregates from the denatured monomers of Phb, and the stage of the sticking of the starting aggregates and higher order aggregates. Dissociation of Phb dimer into monomers at elevated temperatures has been confirmed by analytical ultracentrifugation.