Rapid group separations of nucleotides and related compounds on silica columns

Nucleosides, bases, and nucleotides can be separated from one another rapidly (10–15 min) on 1-ml silica cartridges. Samples adjusted to 4 mm ammonium borate, 90% acetonitrile are loaded onto 1-ml columns equilibrated with the same solvent. Bases do not absorb to the silica under these conditions. Nucleosides are eluted with 16 ml of 0.5 m acetic acid in 90% acetonitrile. Nucleotides are then eluted with water. The 1-ml silica columns have performed well with samples up to 10 ml in volume. We have found the procedure to be quantitative and the gels to have high capacity (61 μmol Cyd/ml silica). Acid extracts from a large number of cells (10⁸) have been processed on a single cartridge.     

Resistance of some common fungi to gamma irradiation

Ten species of fungi representing the genera Alternaria, Aspergillus, Caldosporium, Curvularia, Fusarium, and Penicillium were examined for their relative resistance to gamma irradiation from a 137Cs source. Inactivation doses for dematiaceous fungi in agar medium ranged from 0.6 to greater than 1.7 megarads, whereas those for moniliaceous fungi were less than 0.3 megarad. D10 values (the dose required to reduce the inoculum by 1 log) for Curvularia geniculata (greater than 0.29 megarad) exceeded those for control spores of Bacillus pumilus (0.15 megarad).     

Molecular cloning and expression of a pituitary-specific receptor for growth hormone-releasing hormone.

A novel cDNA was isolated from rat pituitary mRNA using the polymerase chain reaction to amplify sequences encoding G protein-coupled receptors. The human homolog of this cDNA was isolated and expressed in human kidney 293 cells, and membrane fractions from these cells were found to bind human GH-releasing hormone (GHRH) with high affinity and specificity. GHRH also stimulates intracellular cAMP production in these transfected cells. The encoded receptor protein contains seven potential membrane-spanning domains, a hallmark of G protein-coupled receptors, and is homologous to previously identified receptors for secretin and vasoactive intestinal peptide, ligands that are related to GHRH. The rat GHRH receptor mRNA is expressed predominantly, if not exclusively, in the anterior pituitary gland, the major target for GHRH action. These results define a mechanism for cellular signaling by GHRH and provide the opportunity to examine the role of the GHRH receptor in growth abnormalities that involve the GH axis.     

Epidermal growth factor from the mouse. Physical evidence for a tiered beta-sheet domain: two-dimensional NMR correlated spectroscopy and nuclear Overhauser experiments on backbone amide protons

When H2O-exchanged, lyophilized mouse epidermal growth factor (mEGF) is dissolved in deuterium oxide at low pH (i.e., below approximately 6.0), 13 well-resolved, amide proton resonances are observed in the downfield region of an NMR spectrum (500 MHz). Under the conditions of these experiments, the lifetimes of these amide protons in exchange for deuterons of the deuterium oxide solvent suggest that these amide protons are hydrogen-bonded, backbone amide protons. Several of these amide proton resonances show splittings (i.e., JNH alpha-CH) of approximately 8-10 Hz, indicating that their associated amide protons are in some type of beta-structure. Selective nuclear Overhauser effect (NOE) experiments performed on all amide proton resonances strongly suggest that all 13 of these backbone amide protons are part of a single-tiered beta-sheet structural domain in mEGF. Correlation of 2D NMR correlated spectroscopy data, identifying scaler coupled protons, with NOE data, identifying protons close to the irradiated amide protons, allows tentative assignment of some resonances in the NOE difference spectra to specific amino acid residues. These data allow a partial structural model of the tiered beta-sheet domain in mEGF to be postulated.     

Acyl carrier protein from Escherichia coli I. Aspects of the solution structure as evidenced by proton nuclear Overhauser experiments at 500 MHz

The downfield aromatic (6-8 ppm) and upfield ring current shifted methyl regions (1-0 ppm) in the proton nuclear magnetic resonance spectrum of acyl carrier protein (ACP) from Escherichia coli have been examined at 500 MHz by using nuclear Overhauser methods. The data are analyzed in terms of the secondary structural model of Rock & Cronan (1979) [Rock, C. O., & Cronan, J. E., Jr. (1979) J. Biol. Chem. 254, 9778-9785], which suggests the existence of four alpha-helical segments joined by three beta-turns, and a short coil at the C terminus of the protein. Nuclear Overhauser effects among Tyr-71, Ile-69, Ile-72, and His-75 allow refinement of the secondary structure of the C terminus. Nuclear Overhauser effects among Tyr-71, Phe-28, and three Ile's also place stringent limitations on the folding of the four alpha-helices. These data allow the proposal of a tertiary structural model for ACP.     

Epidermal growth factor receptor binding is not a simple one-step process

The binding kinetics of 125I-labeled mouse epidermal growth factor (EGF) to receptors on human fibroblast cells in monolayer culture were measured at 4 degrees C. Initial binding rates as a function of hormone concentration allowed estimation of simple two-state on-off rate constants of 1.2 x 10(6) M-1 s-1 and 4.9 x 10(-3) s-1, respectively. These two-state parameters gave inadequate computer fits to long term kinetic and equilibrium-binding data, suggesting that an additional process(es) was occurring. Nonlinear equilibrium Scatchard plots and transient "pseudo-Scatchard" plots taken at pre-equilibrium times support the idea that at least one other process is occurring during receptor binding. 125I-EGF-receptor dissociation kinetic plots were biphasic, yielding rate constants of 1.5 x 10(-2) s-1 and 5.6 x 10(-5) s-1 with the ratio of the two components changing with the time of initial incubation with 125I-EGF. Application of a ternary complex model which assumed complexation of the bound receptor with a cell surface interaction molecule gave satisfactory fits to all data.     

Use of an affinity label to probe the function of the NADPH binding component of the respiratory burst oxidase of human neutrophils

The respiratory burst oxidase of neutrophils can be activated in a cell-free system in which solubilized membranes, cytosol, and Mg2+ are required and in which sodium dodecyl sulfate is used to convert the dormant oxidase to an active form. The 2',3'-dialdehyde analog of NADPH was used as an affinity label for the cytosolic NADPH binding component of the respiratory burst oxidase from human neutrophils. When treated with this affinity label in the presence of sodium cyanoborohydride to reduce Schiff bases, neutrophil cytosol was shown to lose at least 90% of its activity in the cell-free system. In contrast to normal cytosol, treated cytosol had lost its ability to abolish the lag time required for activation of the oxidase, suggesting that the treated cytosol was no longer able to participate in the rate-limiting activation step. Furthermore, the treated cytosol had lost its ability to convert the oxidase from a form with a high Km to a form with a low Km for NADPH. The ability of dialdehyde-treated cytosol to activate the oxidase could be restored by untreated cytosol with a concentration dependence suggesting that only one kinetically active component of the oxidase was inhibited by treatment with the NADPH analog. Like the dialdehyde-treated cytosol, cytosols from patients with chronic granulomatous disease caused by a deficiency in a cytosolic Mr = 47,000 protein (pp47) fail to participate in the rate-limiting activation step (Curnutte, J. T., Scott, P. J., and Babior, B. M. (1989) J. Clin. Invest. 83, 1236-1240). These chronic granulomatous disease cytosols were nevertheless able to restore limited activity to the dialdehyde-inactivated cytosol in a cell-free activation system. These results are consistent with a model in which (a) the NADPH binding subunit of the oxidase exists in a very slowly dissociating complex with one or more additional cytosolic components, including pp47, and (b) the NADPH binding component of the oxidase controls the affinity of the enzyme for NADPH, either directly or through the binding of additional cytosolic factors.