Coagulation of bovine oxyhemoglobin in the presence of mercuric acetate in concentrations within a range including concentrations exceeding those required to block the single pair of thiol groups of the protein has been investigated in Tris-acetate buffer. The values of initial coagulation rate plotted against mercury-to-hemoglobin molar ratio give curves exhibiting a clear break points at ratios corresponding to full blocking of the mentioned thiol groups. Larger amounts of mercury reagents producing enhanced protein coagulation effect depend approximately quadratically on the mercury concentration. Interaction of the excess mercuric ions with some mercury-binding sites located on or near the dimer-dimer contact surfaces of the protein producing stronger coagulation effect is suggested.

Silver(I) ion has been shown to produce aggregation effect on bovine oxyhemoglobin (HbO(2)) in Tris buffer even when taken in amounts corresponding to only two or less silver ions per one HbO(2) tetramer. The extent of produced effect is comparable to those previously observed for Hg(II), Cd, Zn, and Ni in spite of significantly different electronic configurations of the ions in question. Aggregation effect of the silver is ascribed to an interaction of the reactive thiol group sulfur-bound silver atom with the carboxylate residues surrounding the reactive thiol group-bearing cysteine beta93 group of hemoglobin. Mercury ligands, in particular, Tris molecules and OH(-) anions markedly suppress the protein coagulation, thereby supporting the proposed protein aggregation mechanism.

At a sufficiently high concentration of bovine oxyhemoglobin, the effect Zn(II) ions exert on its coagulation compares with that of Hg(II) ions. It is suggested that the center of preferential stoichiometric binding with Zn(II) ions in the protein is the sulfur atom of reactive SH groups. However, the binding of Zn(II) ions with groups other than thiolic equally affects the protein aggregation. Analysis of the pH dependence of the protein aggregation rate showed that the most likely alternative binding centers are histidine residues of the protein.

Histidin has been shown to effectively inhibit coagulation of horse oxyhemoglobin (HbO(2)) modified by mercury(II) ion bound to reactive thiol groups of protein. Kinetic parameters were measured and the histidin-to-mercury binding constant was kinetically estimated. Histidin, as other pharmaceutically acceptable compounds with some mercury-binding capacity, has been suggested to alleviate mercury intoxication conditions.

The coagulation kinetics of hemoglobin (mainly horse oxyhemoglobin) in the presence of cadmium acetate in amounts comparable with the concentration of reactive thiol groups of hemoglobin was studied. At hemoglobin concentrations of about 10(-4) M (calculated for the tetramer) the coagulation rate of the cadmium-modified protein is close to that of mercurated hemoglobin. Presumably, at cadmium concentrations less than or equal to those required for binding one reactive thiol group per hemoglobin tetramer, the tetrameric form of hemoglobin mainly participates in aggregation, whereas at higher concentrations of the cadmium salt the tetrameric form of the protein loses its stability.

Coordination by external ligands of mercury bound to bull methemoglobin at reactive thiol groups hinders coagulation of this mercurated protein, as it was observed previously with mercurated horse oxyhemoglobin. This fact confirms the previously proposed mechanism of the coagulation effect of mercury, involving additional coordination of S-bound mercury to other centers of the protein.

A review of the authors' papers on the problem published over a period from 1993 to 1995. The similarity of the mechanism of hemoglobin coagulation in the presence of organic compounds including hydrophobic moieties with that in the presence of mercurials is one of the key authors' conclusions. It is supposed that in both cases the chemical attack on the protein results in an alteration of the hemoglobin dimers interaction leading to weakening the tendency of the hemoglobin tetrameric form formation and simultaneously increasing the tendency of formation of one-dimensional higher oligomers-presursors of the three-dimensional aggregates.

The dependence of the human oxyhemoglobin autooxidation in a neutral medium at 20 degrees C versus the cetyltrimethylammonium bromide (CTAB) concentration is characterized by the presence of an initial threshold and also a break point corresponding to the first critical micelle concentration of the surfactant. At higher temperatures the threshold becomes diffuse and entirely disappears in the presence of 1 M sodium chloride. The surfactant molecules being free in solution were shown to affect the autooxidation kinetics stronger than those bound in spheric micelles, whereas with increasing temperature this difference lowers.

We have shown that ascorbic acid in neutral aqueous media rather rapidly interacts with nitrite ions to form dehydroascorbic acid. If the acidity of solution is sufficiently high (pH value does not exceed much above similar to 7) and the nitrite concentration is of order of 0.05 M or more, the only mechanism of the reaction under aerobic conditions includes, as the first, rate-limiting reaction step, the nitrosation of ascorbic acid. A second-order reaction kinetics was observed in respect to protons, which determined a sharp acceleration of the ascorbic acid conversion when the acidity of the medium was raised. Hence, a suggestion can be drawn that the presence of nitrites in vegetables under acidic storage strongly increases the decay of vitamin C.