Structural model for the trialkyltin binding site on cat hemoglobin

Reversible and potent uncoupling of hog gastric (H(+)+K(+))-ATPase by prodigiosins

Prodigiosin, prodigiosin 25-C, and metacycloprodigiosin all strongly inhibited the acidification activity of (H(+)+K(+))-ATPase on membrane vesicles from hog gastric mucosa (IC(50) = 32 to 103 pmol/mg protein). But, the prodigiosins, unlike omeprazole, showed little inhibitory effect on K(+)-dependent ATPase (K(+)-ATPase) activity, although at higher concentrations they inhibited K(+)-ATPase activity with an IC(50) of 1.5 to 3.0 microM. Furthermore, the inhibitory effect of the prodigiosins was rapid and completely reversible unlike that of omeprazole, and the mode of inhibition was non-competitive with respect to ATP. Hog gastric (H(+)+K(+))-ATPase itself showed an absolute requirement of halide (effectively, chloride) for acidification activity. Prodigiosins also showed a chloride requirement for inhibition of vesicular acidification, and quickly reversed the acidification of vesicular pH to neutrality even in the presence of N, N'-dicyclohexylcarbodiimide (DCCD), showing their ionophoric nature of acidification inhibitory activity. In fact, tributyltin chloride (TBT, an OH(-)/Cl(-) exchange ionophore) also inhibited vesicular acidification, but it inhibited K(+)-ATPase activity too. Finally, the prodigiosins inhibited the acid secretion from parietal cells isolated from rabbit gastric mucosa. These results suggest that prodigiosins are potent reversible uncouplers of (H(+)+K(+))-ATPase that inhibit gastric acid secretion.

The interaction of native DNA with dimethyltin(IV) species

The reaction of aqueous native DNA (calf thymus) with the solvated organotin(IV) species [(CH3)2SnCl2(C2H5OH)n], as well as with [(CH3)2Sn(OH)(H2O)n]+ and (CH3)2Sn(OH)2 (i.e., the hydrolysis products of aqueous (CH3)2SnCl2 at pH approximately 5 and pH approximately 7.4 respectively), was investigated by 119Sn Mössbauer spectroscopy. The addition of [(CH3)2SnCl2(C2H5OH)n] to DNA yielded a solid product, possibly (CH3)2Sn(DNA phosphodiester)2, where the environment of the tin atom is trans-octahedral with linear CSnC skeleton, and the equatorial atoms may consist of oxygen or nitrogen from water as well as from the nucleic acid constituents. No interaction with DNA apparently takes place due to hydrolyzed dimethyltin(IV) species, which occur in aqueous phases at approximate physiological pH values. The reaction pathway is then assumed to require weakly solvated, easily dissociable species such as [(CH3)2SnCl2(C2H5OH)n], which would imply in vivo reactivity of cellular DNA with organotins from hydrophobic sites.

A 119Sn Mössbauer spectroscopic study on the interaction of dimethyltin (IV) derivatives with rat hemoglobin, and of related model systems in aqueous solution

In the context of a study of the molecular basis of the antileukemia (murine) activity of diorganotin (IV) compounds, the interaction with rat hemoglobin (selected as a model protein) of the representative terms dimethyltin dichloride, dimethyltin glycylglycinate (Me2SnGlyGly), and dimethyltin L-cysteinate (Me2Sn-Cys) has been investigated by 119Sn Mössbauer spectroscopy. In order to possibly determine the reaction pathway, aqueous model systems in Hepes buffer at pH 7.4 were also considered. The structural characteristics of reactants and products were advanced on the basis of semiempirical calculations of Mössbauer nuclear quadrupole splitting parameters, delta E, by the point-charge model approach. In aqueous Hepes at pH 7.4, evidence was obtained for the formation of the five-coordinated species, trigonal bipyramidal type (tbp), Me2Sn(OH)2.Hepes(II), Me2Sn(OH)(GlyGly).Hepes(III), and Me2Sn(OH)Cys(IV) (see Fig. 1). Equatorial groups or atoms would be the Me radicals, as well as OH, N(peptide), and S(thiol), respectively. Hepes would coordinate to tin in axial position through the tertiary amino nitrogen, while cysteine would behave as a bidentate chelating agent, with an axially located amino group. Species (II), (III), and (IV) react with cysteine in aqueous Hepes at pH 7.4, yielding Me2Sn(OH)Cys(IV), as well as Me2SnCys2(V), where tin would be embedded into a tbp structure due to one cysteine probably chelating (equatorial S thiol and axial amino nitrogen), and one monodentate through S thiol. Species (II), (III), and (IV) react analogously with rat hemoglobin, primarily through the S thiol of a cysteine side chain, yielding pellets where the environment of tin could be tetrahedral, such as in Me2Sn(OH)(S thiol), (VI), and tetrahedral (IX) or tbp (V) in Me2Sn(Cys)(S thiol), where Cys would act either as chelating or monodentate. Further reaction of (VI) and (IX) could involve imidazole nitrogen atoms, N het, of histidine side chains, forming tetrahedral Me2Sn(S thiol)(N het), (VIII), or tbp Me2Sn(OH)(S thiol)(N het), (VII), and Me2Sn(Cys)(S thiol)(N het), (V) (see Figs. 1 and 5).

Yeast iso-1-cytochrome c. A 2.8 A resolution three-dimensional structure determination

A molecular replacement approach, augmented with the results of predictive modeling procedures, solvent accessibility studies, packing analyses and translational coefficient searches, has been used to elucidate the 2.8 A (1 A = 0.1 nm) resolution structure of yeast iso-1-cytochrome c. An examination of the polypeptide chain folding of this protein shows it to have unique conformations in three regions, upon comparison with the structures of other eukaryotic cytochromes c. These include: residues -5 to +1 at the N-terminal end of the polypeptide chain, which are in an extended conformation and project in large part off the surface of the protein; residues 19 to 26, which form a surface beta-loop on the His18 ligand side of the central heme group; and, the C-terminal end of the helical segment composed of residues 49 to 56, which serves to form a part of the heme pocket. Structural studies also show that the highly reactive sulfhydryl group of Cys102 is buried within a hydrophobic region in the monomer form of yeast iso-1-cytochrome c. Dimerization of yeast iso-1-cytochrome c through disulfide bond formation between two such residues would require a substantial conformational change in the C-terminal helix of this protein. Another unique structural feature, the trimethylated side-chain of Lys72, is located on the surface of yeast iso-1-cytochrome c near the solvent-exposed edge of the bound heme prosthetic group. On the basis of the results of these and other structural studies, an analysis of the spatial conservation of structural features in the heme pocket of eukaryotic cytochromes c has been conducted. It was found that the residues involved could be divided into three general classes. The current structural analyses and additional modeling studies have also been used to explain the altered functional properties observed for mutant yeast iso-1-cytochrome c proteins.