Two-dimensional NMR investigation of iron-sulfur cluster electronic and molecular structure of oxidized Clostridium pasteurianum ferredoxin. Interpretability of contact shifts in terms of cysteine orientation.

A two-dimensional NMR study has been carried out on the four-iron clusters of a bacterial oxidized ferredoxin for the purpose of investigating the relationship between contact shift patterns and the orientation of the individual coordinated cysteines. The ferredoxin from Clostridium pasteurianum, CpFdox, was selected because of its extensive sequence homology, and likely close structural similarity, to the crystallographically characterized ferredoxin from Peptococcus aerogenes, Pa Fdox (Adman, E.T., Sieker, L.C., and Jensen, L. H. (1973) J. Biol. Chem. 248, 3987-3996). Rapid data collection rates with minimal but adequate acquisition time allowed the detection of numerous CpFdox cross-peaks from the contact-shifted and strongly relaxed coordinated cysteinyl C beta H protons in the resolved 10-20 ppm window. Relatively strong magnitude COSY cross peaks from the resolved eight cysteinyl C beta H resonance unambiguously locate the geminal C beta H partner for each residue; weaker cross-peaks locate the C alpha Hs from three of the residues. The geminal nature of the magnitude-COSY detected partners to the resolved C beta H peaks is confirmed by strong NOESY cross-peaks. The NOESY spectra, moreover, assign an additional two cysteinyl C alpha H resonances. The present results confirm some previous one-dimensional NOE assignments, revise others, and locate resonances previously undetected (Bertini, I., Briganti, F., Luchinat, C., and Scozzafara, A. (1990) Inorg. Chem. 29, 1874-1880). A striking pairwise pseudo-symmetry in cysteinyl contact shift patterns is observed which is attributed to the previously recognized pseudo-symmetry in the crystal of PaFdox. A detailed analysis of the structural/electronic determinants of the coordinated cysteine C beta H contact shift pattern is made, and the NMR data necessary for unique interpretation are identified. It is shown that analysis of the relaxation properties of cysteine beta-methylene protons provides the stereospecific assignments necessary for comparison of shift ratios with crystallographic structural data. The available structural data on PaFdox (Backes, G., Mino, Y., Loehr, T., Meyer, T., Cusanovich, M., Sweeney, W., Adman, E., and Sanders-Loehr, J. (1991) J. Am. Chem. Soc. 13, 2055-2064) are qualitatively but not quantitatively consistent with the observed cysteinyl contact shift pattern, with the NMR data reflecting more asymmetry than previous studies. A tentative assignment of a single pair of symmetry-related cysteines is proposed.(ABSTRACT TRUNCATED AT 400 WORDS)     

Carrier-mediated transfer of d-glucose in brush border vesicles derived rom rabbit renal tubules. Na+-dependent versus Na+-independent transfer

A brush border preparation from rabbit renal tubules containing a high yield of vesicles has been used to study the transfer of d-glucose through the brush border membrane. In the presence of an Na⁺ gradient across the vesicular membrane, the vesicles could concentrate d-glucose to a factor of 1.5, whereas in the absence of an Na⁺ gradient, only equilibrium with the medium was achieved. Two types of transfer could be distinguished by their requirement of Na⁺, their sensitivity to phlorizin and their pH optimum. The Na⁺-independent transfer was about 100 times less sensitive to phlorizin than the Na⁺-dependent path and exhibited a pH optimum between 7 and 8, whereas the Na⁺-dependent transfer was highest at a pH between 8 and 9.The brush border preparation could be freed of most of the contaminating material derived from the basal and lateral tubular cell membrane by a discontinuous density gradient centrifugation. It still showed both forms of transfer to a similar extent, indicating that both are located in the brush border membrane.A study of the sensitivity of d-glucose transfer to phlorizin, in the presence and absence of Na⁺ at different temperature, suggests a single carrier species functioning in two interchangeable conformational states with different affinities for phlorizin rather than two transfer systems working independently.     

Novel imidazole-based histamine H3 antagonists

A novel series of imidazole containing histamine H₃ receptor ligands were investigated and found to be potent functional antagonists. After improving the stability of these molecules towards liver microsomes, these compounds were found to have no appreciable affinity for CYP P450s. Subsequent in vivo experiments showed significant brain uptake of (4-chloro-phenyl)-[2-(1-isopropyl-piperidin-4-ylmethoxy)-3-methyl-3H-imidazol-4-yl]-methanone 22.     

Osmotically reactive plasma membrane vesicles prepared from rabbit kidney tubules by mild hypotonic lysis

Plasma membrane vesicles were obtained by hypotonic lysis in an ice-cold medium containing EDTA and MgCl₂. The vesicles were isolated by differential centrifugation. Compared to a total kidney homogenate, a 10–12-fold enrichment of trehalase and alkaline phosphatase (marker enzymes for renal brush border), and a 6-fold enrichment of (Na⁺---:K⁺)-stimulated ATPase, (a marker enzyme for the basal and lateral plasma membrane of the tubule cell), was achieved. Contamination by other cell organelles was very low. The plasma membrane vesicles enclosed small amounts of the cytoplasmic enzymes lactate dehydrogenase and malate dehydrogenase, which exhibited full activity only after their release into the medium by sonication.Electron micrographs of this preparation showed microvilli with drumstick-like expansions, but also spherical vesicles. By measuring the distribution of radio-actively labelled compounds of different molecular weight in a packed sediment of the plasma membranes under isotonic conditions, an intravesicular volume of 82% or 9 μl/mg of membrane protein was estimated. The intravesicular volume decreased when the osmolality of the medium was augmented by raffinose. The scattering of light by the vesicular suspension could be used to monitor rapid volume changes. By this method, the following sequence of flux rates was established: glycerol>erythritol> adonitol>mannitol. The fluxes of LiCl, NaCl, and KCl were almost identical, but faster than those of adonitol and mannitol. The data indicate, that a large fraction of the plasma membrane isolated in this preparation have formed vesicles, and also that they have retained, as far as investigated, the permeability characteristics of the plasma membrane.     

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DNA stretching in the nucleosome facilitates alkylation by an intercalating antitumour agent

DNA stretching in the nucleosome core can cause dramatic structural distortions, which may influence compaction and factor recognition in chromatin. We find that the base pair unstacking arising from stretching-induced extreme minor groove kinking near the nucleosome centre creates a hot spot for intercalation and alkylation by a novel anticancer compound. This may have far reaching implications for how chromatin structure can influence binding of intercalator species and indicates potential for the development of site selective DNA-binding agents that target unique conformational features of the nucleosome.