Calcium binding properties of gamma-crystallin: calcium ion binds at the Greek key beta gamma-crystallin fold.

The beta- and gamma-crystallins are closely related lens proteins that are members of the betagamma-crystallin superfamily, which also include many non-lens members. Although beta-crystallin is known to be a calcium-binding protein, this property has not been reported in gamma-crystallin. We have studied the calcium binding properties of gamma-crystallin, and we show that it binds 4 mol eq of calcium with a dissociation constant of 90 microm. It also binds the calcium-mimic spectral probes, terbium and Stains-all. Calcium binding does not significantly influence protein secondary and tertiary structures. We present evidence that the Greek key crystallin fold is the site for calcium ion binding in gamma-crystallin. Peptides corresponding to Greek key motif of gamma-crystallin (42 residues) and their mutants were synthesized and studied for calcium binding. These peptides adopt beta-sheet conformation and form aggregates producing beta-sandwich. Our results with peptides show that, in Greek key motif, the amino acid adjacent to the conserved aromatic corner in the "a" strand and three amino acids of the "d" strand participate in calcium binding. We suggest that the betagamma superfamily represents a novel class of calcium-binding proteins with the Greek key betagamma-crystallin fold as potential calcium-binding sites. These results are of significance in understanding the mechanism of calcium homeostasis in the lens.

Antioxidant properties of green and black tea, and their potential ability to retard the progression of eye lens cataract.

Aqueous extracts of green and black tea are shown to quench reactive oxygen species such as singlet oxygen, superoxide and hydroxyl radicals, prevent the oxidative cross-linking of test proteins and inhibit single strand breakage of DNA in whole cells. They are also seen to be able to counteract the oxidative insult mounted by cigarette smoke. In rats in which cataract was induced by subcutaneous injection of selenite, administration of green or black tea extracts led to a retardation of the progression of lens opacity, suggesting the potential cataracto-static ability of tea.

beta-carbolines that accumulate in human tissues may serve a protective role against oxidative stress.

beta-Carbolines are tricyclic nitrogen heterocycles formed in plants and animals as Maillard reaction products between amino acids and reducing sugars or aldehydes. They are being detected increasingly in human tissues, and their physiological roles need to be understood. Two beta-carboline carboxylates have been reported to accumulate in the human eye lens. We report here on the identification of another beta-carboline, namely 1-methyl-1-vinyl -2, 3,4-trihydro-beta-carboline-3-carboxylic acid, in the lenses of some cataract patients from India. Analysis of these three lenticular beta-carbolines using photodynamic and antioxidant assays shows all of them to be inert as sensitizers and effective as antioxidants; they quench singlet oxygen, superoxide and hydroxyl radicals and inhibit the oxidative formation of higher molecular weight aggregates of the test protein, eye lens gamma-crystallin. Such antioxidative ability of beta-carbolines is of particular relevance to the lens, which faces continual photic and oxidative stress. The beta-carboline diacid IV is also seen to display an unexpected ability of inhibiting the thermal coagulation of gamma-crystallin and the dithiothreitol-induced precipitation of insulin. These results offer experimental support to earlier suggestions that one of the roles that the beta-carbolines have is to offer protection against oxidative stress to the human tissues where they accumulate.

The bacterial pigment xanthomonadin offers protection against photodamage.

Xanthomonas oryzae pv. oryzae is a bacterial pathogen that causes leaf blight, a serious disease of rice. Most members of the genus Xanthomonas produce yellow, membrane bound, brominated aryl polyene pigments called xanthomonadins whose functional role is unclear. We find that pigment-deficient mutants of X. oryzae pv. oryzae exhibit hypersensitivity to photobiological damage. A clone containing the xanthomonadin biosynthetic gene cluster alleviates the hypersensitivity of the pigment-deficient mutant. Extracts containing xanthomonadin provide protection against photodynamic lipid peroxidation in liposomes. These results lead us to suggest a role for the pigment, namely protection against photodamage.

Amphiphilic properties of polysaccharides: dextrin chains as example.

Polysaccharide chains are usually considered to be highly hydrophilic, since they contain no obvious apolar moieties. However, it is possible for even these chains to display hydrophobic character, arising out of stereochemical constraints in the chain. We had earlier shown that linear dextrin chains display amphiphilic properties, since all the hydroxyl groups are disposed on one side or face of the chain and the hydrogens disposed on the other. We provide further evidence here for this conclusion that dextrins are amphiphilic chains. In contrast, dextrans and cellulosic chains do not display amphiphilicity. Oligosaccharides that can adopt incipient helical structures might display amphiphilicity. This property might be relevant to intermolecular recognition on cell surfaces, lectin-sugar binding, antigen-antibody interactions and the like, and might be manifested more in a heteromolecular recognition process than as homomolecular self-aggregation.

Hydrophobic surfaces in oligosaccharides: linear dextrins are amphiphilic chains.

Polysaccharide chains are usually considered to be highly hydrophilic, since they have no obvious nonpolar moieties in them. Yet, it is possible to realise conformations in these chains wherein all the hydroxy groups are disposed in one side or face of the chain and the hydrogens disposed in the other. We experimentally demonstrate that such an amphiphilic surface is present in linear oligomeric dextrins, i.e., alpha-1,4-linked D-glucosides, but not in alpha-1,6-D-glucosides (dextrans) or in beta-1,4-D-glucosides (cellulose). This amphiphilicity is generated as a consequence of the stereochemical constraints, which vary with the structure of the sugar and with the type of linkage. Oligosaccharide chains that can adopt incipient helical structures might display amphiphilicity. This property might be relevant to intermolecular recognition on cell surfaces, lectin-sugar binding, antigen-antibody interactions and the like, and might be manifested more in heteromolecular recognition process than as homomolecular self-aggregation.