Electron microscopic study of the products from dissolving unstabilized fibrin with complex heparin compounds

Effects of some heparin complex compounds (heparin-urea, adrenaline-heparin, fibrinogen-heparin complexes and secondary complex adrenalin-heparin-fibrinogen) on factor XIIIa unstabilized fibrin were studied using electron microscopy. Fibrillar network of unstabilized fibrin destroys with the formation of globular molecular particles similar to fibrinogen molecule or fibrin monomer ultrastructure. A mechanism of fibrinolytic action of all the complexes mentioned is probably the same, since under dissolving of unstabilized fibrin, structures are found, which are similar to those forming under dissolving of unstabilized fibrin with urea.     

Characteristics of functioning of electromechanical coupling in striated muscles of higher and lower vertebrates

A comparative pharmacological analysis of relative contributions of different signal transduction pathways in the activation of contraction (excitation-contraction coupling, ECC) in intact fast striated muscles of frog and lamprey was performed. It was found that the major mechanism responsible for the ECC in muscles of both animals is Ca2+ release from the sarcoplasmic reticulum through the ryanodine-sensitive channels. However, the ECC in lamprey muscle displays some important differences in the units of electromechanical coupling, which precede the calcium release from sarcoplasmic reticulum. The maximum contraction force in frog muscle develops during caffeine-induced contracture, which indicates that all Ca2+ stored in sarcoplasmic reticulum is released through ryanodine-sensitive channels. In contrast, in lamprey muscle, the maximum force develops not in response to high caffeine concentration, but in response to repetitive electrical stimulation. Hence, in addition to stores liberated by ryanodine-sensitive channels, some other sources of calcium ions should exist, which contribute to the contraction activation. A source of this additional Ca2+ ions can be external medium, because acetylcholine contracture is abolished in a calcium-free medium. In frog muscle, the acetylcholine contracture was abolished in a Na(+)-free solution. It was concluded that in frog muscle ECC can be triggered by changes in the transmembrane potential (depolarization-induced calcium release), while in lamprey muscle the entry of calcium ions into myoplasm as the trigger in ECC (calcium-induced calcium release). The lamprey muscle was found to be more resistant to tetrodotoxin and tetracaine, which is indicative of a role in the activation of contraction of tetrodotoxin-resistant Na+ and/or Ca2+ channels. It was concluded, that ECC mechanism in striated muscles of low vertebrates is not limited by the generally accepted scheme of depolarization-induced calcium release but can include some other schemes, which require the Ca2+ influx into the cell.     

Sequence-specific transitions of the torsion angle gamma change the polar-hydrophobic profile of the DNA grooves: implication for indirect protein–DNA recognition

Variations of the shape and polarity of the DNA grooves caused by changes of the DNA conformation play an important role in the DNA readout. Despite the fact that non-canonical trans and gauche- conformations of the DNA backbone angle γ (O5′–C5′–C4′–C3′) are frequently found in the DNA crystal structures, their possible role in the DNA recognition has not been studied systematically. In order to fill in this gap, we analyze the available high-resolution crystal structures of the naked and complexed DNA. The analysis shows that the non-canonical γ angle conformations are present both in the naked and bound DNA, more often in the bound vs. naked DNA, and in the nucleotides with the A-like vs. the B-like sugar pucker. The alternative angle γ torsions are more frequently observed in the purines with the A-like sugar pucker and in the pyrimidines with the B-like sugar conformation. The minor groove of the nucleotides with non-canonical γ angle conformation is more polar, while the major groove is more hydrophobic than in the nucleotides with the classical γ torsions due to variations in exposure of the polar and hydrophobic groups of the DNA backbone. The propensity of the nucleotides with different γ angle conformations to participate in the protein–nucleic acid contacts in the minor and major grooves is connected with their sugar pucker and sequence-specific. Our findings imply that the angle γ transitions contribute to the process of the protein–DNA recognition due to modification of the polar/hydrophobic profile of the DNA grooves.     

Changes in the electromechanical activity in the course of tetanic contraction

The functioning of excitation-contraction coupling during tetanic contraction was investigated on frog skeletal muscle. The effect of the calcium release blocker dantrolene was tested on electrically evoked twitches and tetanic contractions. It was shown that the first: developmental stage of tetanus is inhibited by dantrolene as well as a twitch contraction, and does not influenced by calcium-free medium. This substantiates it as based on "voltage dependent Ca-release" (VDCR) mechanism of activation, when depolarization directly opens the rhyanodin receptor calcium channels. The next stage: the long lasting plateau of tetanic contraction, is directly dependent on external Ca2+ entry and also inhibited by dantrolene, and therefore may be described as "calcium-induced Ca-release" (CICR) activation mechanism. It is proposed that such change in ECC mechanism taking place during tetanic contraction, can occur also in conditions of natural muscle activity, because of its rhythmical nature.     

DNA minor groove electrostatic potential: influence of sequence-specific transitions of the torsion angle gamma and deoxyribose conformations

The structural adjustments of the sugar-phosphate DNA backbone (switching of the γ angle (O5′–C5′–C4′–C3′) from canonical to alternative conformations and/or C2′-endo → C3′-endo transition of deoxyribose) lead to the sequence-specific changes in accessible surface area of both polar and non-polar atoms of the grooves and the polar/hydrophobic profile of the latter ones. The distribution of the minor groove electrostatic potential is likely to be changing as a result of such conformational rearrangements in sugar-phosphate DNA backbone. Our analysis of the crystal structures of the short free DNA fragments and calculation of their electrostatic potentials allowed us to determine: (1) the number of classical and alternative γ angle conformations in the free B-DNA; (2) changes in the minor groove electrostatic potential, depending on the conformation of the sugar-phosphate DNA backbone; (3) the effect of the DNA sequence on the minor groove electrostatic potential. We have demonstrated that the structural adjustments of the DNA double helix (the conformations of the sugar-phosphate backbone and the minor groove dimensions) induce changes in the distribution of the minor groove electrostatic potential and are sequence-specific. Therefore, these features of the minor groove sizes and distribution of minor groove electrostatic potential can be used as a signal for recognition of the target DNA sequence by protein in the implementation of the indirect readout mechanism.     

Electron microscopic study of the depolymerization process on unstabilized fibrin as affected by urea or monochloracetic acid

With the help of the electronmicroscopic method the non-stabilized fibrin depolymerization has been studied. It has been established that under the action of urea or monochloridacetic acid the gradual transition of fibril clot's structure in globular fibrinolike material takes place. These globules have morphological likeness with the fibrin-monomer molecules and have analogy of the morphological properties to the non-stabilized fibrin dissolution products by complex compounds of heparin. The elimination of urea or monochloridacetic acid from media gives possibility to reconstruct the fibrillar fibrin structure.