Inactivation of bovine pancreatic DNase by 2-nitro-5-thiocyanobenzoic acid. I. A novel inhibitor for DNase I.

Though DNase does not contain any cysteine residues, incubation of the enzyme with 2-nitro-5-thiocyanobenzoic acid in the presence of Ca2+ at pH values above 7.5 results in an irreversible inactivation of the enzyme. The inactivation also occurs when Ca2+ is replaced by Mg2+, but not in their absence. Amino acid analyses after acid hydrolyses of the completely inactivated ant the native enzymes show no significant differences in composition, including tryptophan and half-cystine residues. However, sodium dodecyl sulfate gel electrophoresis indicates enzyme cleavage by the treatment with 2-nitro-5-thiocyanobenzoic acid. This reagent does not inactivate chymotrypsin and lysozyme, and under conditions where bovine DNase is inactivated, does not inactivate other nucleases such as ribonuclease, snake venom phosphodiesterase, and spleen acid DNase. However, it inactivates malt DNase and can, therefore, be considered a specific inhibitor of DNase I. The inactivation kinetics is pseudo-first order, resembling Michaelis-Menten, with an affinity constant of 16.7 mM. It is the cyano group, not the thionitrobenzoic acid of 2-nitro-5-thiocyanobenzoic acid that reacts to form cyano-DNase.     

5-Hydroxylation of benzimidazole by micromycetes I. Strains selection

Summary The ability of 610 strains of micromycetes to introduce a hydroxy-group into benzimidazole substrate has been investigated. Among them, 232 micromycetes have been selected and cultivated in a liquid synthetic medium. Convenient hydroxylation of benzimidazole was obtained with 78 fungi. Finally, 8 strains of micromycetes have been chosen for further investigation and optimization of this bioconversion.     

Thermodynamics of the disproportionation of adenosine 5'-diphosphate to adenosine 5'-triphosphate and adenosine 5'-monophosphate. I. Equilibrium model

The thermodynamic treatment of the disproportionation reaction of adenosine 5′-diphosphate to adenosine 5′-triphosphate and adenosine 5′-monophosphate is discussed in terms of an equilibrium model which includes the effects of the multiplicity of ionic and metal bound species and the presence of long range electrostatic and short range repulsive interactions. Calculated quantities include equilibrium constants, enthalpies, heat capacities, entropies, and the stoichiometry of the overall reaction. The matter of how these calculations can be made self-consistent with respect to both calculated values of the ionic strength and the molality of the free magnesium ion is discussed. The thermodynamic data involving proton and magnesium-ion binding data for the nucleotides involved in this reaction have been evaluated.     

Substrate specificity and biochemical properties of M3.BstF5I DNA methyltransferase from the BstF5I restriction-modification system

Optimal conditions for DNA methylation by the M3.BstF5I enzyme from Bacillus stearothermophilus and kinetic parameters of λ phage DNA modification and that of a number of oligonucleotide substrates are established. Comparison of M1.BstF5I and M3.BstF5I kinetic parameters revealed that with similar temperature optima and affinity for DNA, M3.BstF5I has nearly fourfold lower turnover number (0.24 min⁻¹) and modifies the hemimethylated recognition site with lower efficiency under optimal conditions than the unmethylated one. In contrast to another three methylases of the BstF5I restriction-modification system, the M3.BstF5I enzyme is able to optionally modify the noncanonical 5′-GGATC-3′ DNA sequence with a rate more than one order of magnitude lower than the methylation rate of the canonical 5′-GGATG-3′ recognition site.     

Phyllotaxis. I. A Mechanistic Model

A quantitative two-dimensional model for phyllotaxis is described.The model is based on the production, diffusion and degradationof a morphogen, and it is assumed that primordial initiationcan only take place in competent tissue when the morphogen concentrationdrops to a certain critical level. The model predicts the anglesbetween successive primordia; under appropriate conditions asteady state is achieved where the divergence angle is constant;and for limiting values of the parameters, the predicted steady-statedivergence angle approaches the Fibonacci angle. To accountfor non-spiral phyllotaxis, an additional hypothesis, calledspatial competence, is introduced. As well as a morphogen levelbelow the threshold level, and for competence in the usual sense,this assumes that particular spatial demands must be satisfiedbefore primordial initiation can proceed.     

Noncontact dipole effects on channel permeation. I. Experiments with (5F-indole)Trp13 gramicidin A channels.

Gramicidin A (gA), with four Trp residues per monomer, has an increased conductance compared to its Phe replacement analogs. When the dipole moment of the Trp13 side chain is increased by fluorination at indole position 5 (FgA), the conductance is expected to increase further. gA and FgA conductances to Na+, K+, and H+ were measured in planar diphytanoylphosphatidylcholine (DPhPC) or glycerylmonoolein (GMO) bilayers. In DPhPC bilayers, Na+ and K+ conductances increased upon fluorination, whereas in GMO they decreased. The low ratio in the monoglyceride bilayer was not reversed in GMO-ether bilayers, solvent-inflated or -deflated bilayers, or variable fatty acid chain monoglyceride bilayers. In both GMO and DPhPC bilayers, fluorination decreased conductance to H+ but increased conductance in the mixed solution, 1 M KCl at pH 2.0, where K+ dominates conduction. Eadie-Hofstee plot slopes suggest similar destabilization of K+ binding in both lipids. Channel lifetimes were not affected by fluorination in either lipid. These observations indicate that fluorination does not change the rotameric conformation of the side chain. The expected difference in the rate-limiting step for transport through channels in the two bilayers qualitatively explains all of the above trends.