Steroid dimer formation: metal reduction of methyl androst-4-ene-3, 17-dion-19-oate

Two isomeric dimeric steroids, 3,3'-bis(methyl 3-hydroxyandrost-4-en-17-on-19-oate-3-yl), with symmetrical (alpha, alpha') and unsymmetrical structures (alpha,beta'), have been obtained by reduction of methyl androst-4-ene-3,17-dion-19-oate with zinc in aqueous acetic acid together with the major products, the isomeric methyl 5alpha- and 5beta-androst-3-en-17-on-19-oates. The structures of the dimers and unsaturated products are supported by spectroscopic methods. The symmetrical dimer was also obtained from treatment of the 4-en-3-on-19-oate ester with lithium in ammonia.     

Regioselective derivatization of ouabain with trialkylsilyl reagents and selective oxidation of the unprotected alcohols

Many mammalian tissues contain cardiac glycoside-like steroids that inhibit the sodium pump. A ouabain-like compound has been described in the human circulation and suggested to be ouabain or a closely related isomer. Ouabain is a highly hydroxylated compound and one of the most potent inhibitors of the sodium pump. Trialkylsilyl derivatization of ouabain has been carried out to determine reagent selectivity among the eight hydroxy groups as a prelude to the synthesis of regiospecific isomers. Mono-, di-, tri-, and hexa-trialkylsilyl derivatives have been prepared with substitution at the 19-, the 3',19-, the 1,3',19-, and the 1,2',3',4',11, 19-positions, respectively. Mass spectrometry and NMR confirmed the substitutions. Selective protection of the hydroxy groups allows selective oxidation of the unprotected steroid ring alcohols without oxidation of the 2'- and 4'-rhamnoside alcohols. Pyridinium dichromate oxidation of the di-trialkylsilyl and tri-trialkylsilyl derivatives gave the 1,11-diketone and the 11-ketone analogues, respectively. These regioselective reactions open a route to the synthesis of a series of closely related isomers of ouabain and other derivatives that may have useful structure-activity relationships and utility in the elucidation of the biosynthesis of ouabain-like compounds.     

Changes in Electrogenesis in the Motor Nerve Terminal with Long-Lasting High-Frequency Activation as a Factor of the Control of Transmitter Release

Using the technique of extracellular recording from the region of the neuromuscular junction in the cutaneous-sternal muscle in the frog under conditions of a reduced concentration of Ca²⁺ in the surrounding milieu, we demonstrated that long-lasting (10 min) rhythmic stimulation of the motor nerve with a frequency of 10 sec^{− 1} leads to a gradual increase in the evoked transmitter release. These changes are accompanied by a decrease in the amplitude of electrical responses of the nerve terminal (NT) and by a retardation of its second phase, as well as by a diminution of the third phase. Under conditions of long-lasting (5 min) stimulation with a frequency of 50 sec⁻¹, we observed a two-phase change in the intensity of transmitter release: on the 2nd min, the initial rise was replaced by inhibition. Modifications of the response of the NT with different stimulation frequencies were qualitatively similar, but with a frequency of 10 sec⁻¹ they were clearly expressed. Mathematical simulation of ion currents in the NT demonstrated that voltage-dependent potassium and sodium channels are inactivated in the course of long-lasting high-frequency excitation; the shape of the action potential is modified with changes in the rate of such inactivation. This leads to either an increase or a decrease of the inward calcium current. We conclude that the change in electrogenesis in the NT with long-lasting high-frequency activation of neuromuscular junctions exerts a significant influence on the dynamics of transmitter release. Neirofiziologiya/Neurophysiology, Vol. 37, No. 2, pp. 108–115, March–April, 2005.     

Prevalence of non-strongyle gastrointestinal parasites of horses in Riyadh region of Saudi Arabia

This study aimed to provide recent data on the occurrence of non-strongyle intestinal parasite infestation in horses in the Riyadh region of Saudi Arabia as a basis for developing parasite control strategies. We conducted necropsy for 45 horses from September 2006 to November 2007 in the Riyadh region, Saudi Arabia. 39 out of 45 horses were infected with intestinal parasites with an infestation rate of 86.6%. Infestations with seven nematode species and two species of Gasterophilus larva were found. The most prevalent parasites were Strongyloides westeri (64.4%) and Parascaris equorum (28.8%) followed by Habronema muscae (22.2%). Trichostrongylus axei and Oxyuris equi were less common at (11.1%) and (8.8%), respectively. Habronema megastoma and Setaria equine were found in two horses only (4.4%). Gasterophilus intestinalis larvae were recovered from 39 horses (86.6%) and Gasterophilus nasalis larvae were found in 17 horses (37.7%). Season had a significant effect on the prevalence of P. equorum and G. nasalis, while age of horses had a significant effect only on the prevalence of P. equorum. The husbandry in Saudi Arabia appears to be conductive to parasites transmitted in stables or by insects rather than in pasture.     

Evolution of Pan-Genomes of Escherichia coli,Shigella spp., and Salmonella enterica

Multiple sequencing of genomes belonging to a bacterial species allows one to analyze and compare statistics and dynamics of the gene complements of species, their pan-genomes. Here, we analyzed multiple genomes of Escherichia coli, Shigella spp., and Salmonella enterica. We demonstrate that the distribution of the number of genomes harboring a gene is well approximated by a sum of two power functions, describing frequent genes (present in many strains) and rare genes (present in few strains). The virtual absence of Shigella-specific genes not present in E. coli genomes confirms previous observations that Shigella is not an independent genus. While the pan-genome size is increasing with each new strain, the number of genes present in a fixed fraction of strains stabilizes quickly. For instance, slightly fewer than 4,000 genes are present in at least half of any group of E. coli genomes. Comparison of S. enterica and E. coli pan-genomes revealed the existence of a common periphery, that is, genes present in some but not all strains of both species. Analysis of phylogenetic trees demonstrates that rare genes from the periphery likely evolve under horizontal transfer, whereas frequent periphery genes may have been inherited from the periphery genome of the common ancestor.     

Reintroducing electrostatics into macromolecular crystallographic refinement: application to neutron crystallography and DNA hydration

Most current crystallographic structure refinements augment the diffraction data with a priori information consisting of bond, angle, dihedral, planarity restraints, and atomic repulsion based on the Pauli exclusion principle. Yet, electrostatics and van der Waals attraction are physical forces that provide additional a priori information. Here, we assess the inclusion of electrostatics for the force field used for all-atom (including hydrogen) joint neutron/X-ray refinement. Two DNA and a protein crystal structure were refined against joint neutron/X-ray diffraction data sets using force fields without electrostatics or?with electrostatics. Hydrogen-bond orientation/geometry favors the inclusion of electrostatics. Refinement of Z-DNA with electrostatics leads to?a hypothesis for the entropic stabilization of Z-DNA that may partly explain the thermodynamics of converting the B form of DNA to its Z form. Thus, inclusion of electrostatics assists joint neutron/X-ray refinements, especially for placing and orienting hydrogen atoms.