Effects of cytochalasin B on Na+ content and cell volume of Entamoeba histolytica

Cells of Entamoeba histolytica accumulated K+ and extruded Na+ compared to the concentrations of those ions present in the growth medium. Pinocytic activity, measured by the uptake of horseradish peroxidase of 125I-polyvinylpyrrolidone, was high (up to 0.3 ml/ml cells per h). Upon addition of cytochalasin B, at a concentration (20 microM) that completely blocked pinocytosis, cells lost up to 40% of their Na+ content within 90 min; K+ content was not affected or increased slightly compared to control cells without the inhibitor. Cation loss was associated with cell shrinkage. The dose-response curves for the effects of cytochalasin B on pinocytosis and Na+ content were identical. These data provide direct evidence that pinocytosis is an important component of the homeostatic system for Na+.     

On the toroidal condensed state of closed circular DNA

The influence of double helix torsional elasticity on the compaction and structure of circular DNA compact form is studied theoretically in the case when the compact (globular) form has torus shape. For closed circular DNA the topological invariant, the linking number, yields a strict connection between conformation of the double helix considered as unifilar homopolymer and elastic energy of torsional twisting. The contribution of torsional elasticity to the free energy of the toruslike globule is calculated. This contribution is shown to be proportional to the square of superhelical density. Allowance of the torsional elasticity decreases the equilibrium radius of the toruslike globule formed by circular DNA. Closure of linear DNA into a ring widens the stability range of the relatively short DNA compact form and tightens it for long DNA.     

Import and processing of the precursor of cytochrome P-450(SCC) by bovine adrenal cortex mitochondria

Isolated bovine adrenal cortex mitochondria imported in vitro synthesized pre-P-450(SCC) and processed it to the mature form. Partial radio-sequencing of the processed P-450(SCC) gave a result identical with that for authentic P-450(SCC). Rat liver mitochondria also imported pre-P-450(SCC) and processed it to the mature form, whereas bovine heart mitochondria were unable to import and process pre-P-450(SCC) although both mitochondrial preparations imported and processed pre-adrenodoxin. The pre-P-450(SCC) processing activity of bovine adrenal cortex mitochondria was associated with the matrix side surface of the inner membrane. The processing protease could be solubilized by sodium cholate and partially purified by ammonium sulfate fractionation. The partially purified processing protease cleaved pre-P-450(SCC) at the correct position. It was also active in processing pre-P-450(11 beta) but inactive toward pre-adrenodoxin. Bovine heart mitochondria lacked the processing activity to pre-P-450(SCC). The localization of pre-P-450(SCC) and mature P-450(SCC) in bovine adrenal cortex mitochondria was examined. Mature P-450(SCC) processed by the mitochondria was found associated with the matrix-side surface of the inner membrane, which is the correct location of P-450(SCC) in the cell. In the presence of o-phenanthroline, pre-P-450(SCC) was imported into the organelles without being processed and remained soluble in the matrix. The incorporation of newly processed mature P-450(SCC) into the inner membrane was also observed when pre-P-450(SCC) was incubated with inner membrane vesicles. Mature P-450(SCC) generated in vitro from pre-P-450(SCC) by the partially purified processing protease was incorporated not only into the inner membrane vesicles but also into bovine adrenal cortex microsomes. These findings suggested that the processing of pre-P-450(SCC) occurred prior to the incorporation of mature-P-450(SCC) into the inner membrane.     

The role of histones H3 and H1 in the formation of thymine-lysine cross-links during UV-irradiation of deoxyribonucleoprotein

The effect of UV light (lambda = 254 nm) on calf thymus DNP at low ionic strengths was studied. It was found that at the irradiation doses used the protein in the DNA-protein complex increases as the irradiation dose rises. Thermal treatment and acid hydrolysis resulted in a predominant release of histones H3 and H1 from the complex. Data from liquid high performance chromatography, amino acid analysis, thin-layer chromatography point to the induction by UV-light of a thymine-lysine bond, whose formation involves DNA thymines and histone lysine residues, predominantly H3 and H1 fractions.     

Effect of serotonin on the yield of UV- and x ray-induced DNA damage

Using two-dimensional thin-layer chromatography, the effect of serotonin on the yield of thymine dimers and on cleavage of the N-glycosidic bond in the DNA irradiated with ultraviolet (UV) light and X-ray was studied. Bound serotonin was shown to reduce the synthesis of UV-induced thymine dimers but had no effect on the number of X-ray-induced breaks in the N-glycoside bonds in thymidine residues. The data obtained are discussed in terms of the mechanisms of serotonin involvement in the photoprotection of yeast cells from the lethal action of UV and X-ray irradiations.     

Soluble, prolonged-acting insulin derivatives. II. Degree of protraction and crystallizability of insulins substituted in positions A17, B8, B13, B27 and B30

It has previously been found that insulins, to which positive charge has been added by substitutions in position B30, thus raising the isoelectric point towards pH 7, had a prolonged action when injected as slightly acidic solutions because such derivatives crystallize very readily upon neutralization. Positive charge has now been added by substituting the B13 and A17 glutamic acid residues with glutamines and B27 threonine with lysine or arginine. These substitutions were introduced by site-specific mutagenesis in a gene coding for a single-chain insulin precursor. By tryptic transpeptidation the single-chain precursors were transformed to the double-chain insulin structure, concomitantly with incorporation of residue B30. Thus insulins combining B13 glutamine, A17 glutamine and B27 lysine or arginine with B30 threonine, threonine amide or lysine amide were synthesized. The time course of blood glucose lowering effect and the absorption were studied after subcutaneous injection in rabbits and pigs. The prolonged action of B30-substituted insulins was markedly enhanced by B27 lysine or arginine substitutions and by B13 glutamine. The B27 residue is located on the surface of the hexamer, so a basic residue in this position presumably promotes the packing of hexamers at neutral pH. The B13 residues cluster in the centre of the hexamer. When the electrostatic repulsive forces from six glutamic acid residues are abolished by substitution with glutamine, a stabilization of the hexamer can be envisaged.(ABSTRACT TRUNCATED AT 250 WORDS)