The accessibility of DNA to dimethylsulfate in complexes with recA protein.

recA protein coats DNA co-operatively to form filaments approximately 100 A thick, which in the presence of ATP, and more stably so in the presence of the non-hydrolyzable analog ATP gamma S, have a helical appearance with a deep cleft in the protein coat. This protein helix follows the DNA helix, to which it imparts a new helicity of 18.5 bp per turn of 97 A pitch. Here we test the accessibility of the DNA in the complex to modification by dimethylsulfate, and find that the complexed DNA is approximately 2-fold more reactive on the major groove side than it was in B-DNA (methylation of guanine N7), while it is protected approximately 2-fold on the minor groove side (methylation of adenine N3), suggesting that the protein coats the DNA along the minor groove. Furthermore, N3 of cytosine, a residue involved in base pairing, is found exposed in complexes with single strands as it is in naked single-stranded DNA, while it remains inaccessible in complexes with double strands, suggesting that the latter is not melted at this stage of the strand exchange reaction.     

Purification and characterization of glutathione transferase of human thyroid

An anionic (pI 4.6) isoenzyme of glutathione transferase was purified to homogeneity from human thyroid by affinity chromatography followed by isoelectric focusing. The content of enzyme was calculated to constitute about 0.2% of soluble proteins. The enzyme is formed by two identical subunits of 23,000 daltons approximately. The thyroid transferase did not catalyze the reduction of peroxides. Physical, catalytic and immunological analyses demonstrated extensive similarities between the thyroid transferase and the transferase from placenta, erythrocytes and breast. On the other hand, the thyroid transferase appears catalytically different from transferase 7-7, even if both cross-react with the antibodies raised against human placenta transferase.     

Kinetics of binding of macrolides, lincosamides, and synergimycins to ribosomes

The synergistic effect of type A (virginiamycin M (VM)) and type B (virginiamycin S (VS)) synergimycins and their antagonistic effect against erythromycin (a 14-membered macrolide) for binding to the large ribosomal subunit (50 S) have been related. This investigation has now been extended to 16-membered macrolides (leucomycin A3 and spiramycin) and to lincosamides (lincomycin). A dissociation of VS-ribosome complexes was induced as well by 16-membered macrolides as by lincosamides. The observed dissociation rate constant of VS-ribosome complexes was identified with the kappa-vs in the case of 16-membered macrolides, but linearly related to lincomycin concentration, suggesting a direct binding of the latter antibiotic to VS-ribosome complexes and the triggering of a conformational change of particles entailing VS release. Two different mechanisms were also involved in the VM-promoted reassociation to ribosomes of VS previously displaced by either macrolides or lincosamides. By binding to lincosamide-ribosome complexes, VM induced a conformational change of ribosomes resulting in higher affinity for VS and lower affinity for lincosamides. On the contrary, an incompatibility for a simultaneous binding of VM and 16-membered macrolides to ribosomes was observed. These results have been interpreted by postulating specific (nonoverlapping) and aspecific (overlapping) antibiotic binding sites at the peptidyltransferase domain. All the kinetic constants of five antibiotic families (type A and B synergimycins, 14- and 16-membered macrolides, and lincosamides) and a topological model of peptidyltransferase are presently available.     

Voltage-dependent activation and inactivation of calcium channels in PC12 cells. Correlation with neurotransmitter release

The existence and mechanisms of inactivation of voltage-gated Ca2+ channels are important, but still debatable, physiological problems. By using the Ca2+ indicators quin2 and fura-2, we demonstrate that in PC12 cells voltage-gated Ca2+ channels undergo inactivation dependent on both voltage and [Ca2+]i. Inactivation, however, is never complete and a small number of channels remains open during prolonged depolarization, explaining the steady state elevation of [Ca2+]i observed in cells depolarized with high KCl. A close parallel exists between Ca2+ channel inactivation and the transient nature of neurotransmitter release: secretion is rapidly stimulated during the first 30 s of depolarization, when a transient overshoot in [Ca2+]i can be demonstrated, while it is negligible during the following period, despite the persistence of an elevated [Ca2+]i; predepolarization in Ca2+-free medium and subsequent addition of Ca2+ (a condition which allows the development of the voltage inactivation) abolishes the fast phase of secretion, while not modifying the steady state [Ca2+]i eventually attained; and increases in the intracellular Ca2+ buffering decreases the amplitude of the fast secretion phase induced by KCl without altering the steady state [Ca2+]i. We suggest that localized [Ca2+]i gradients form close to the plasma membrane shortly after depolarization and that the [Ca2+]i reached in these regions is the relevant parameter in the regulation of secretion.     

The nature of the neutral Na+−Cl−-coupled entry at the apical membrane of rabbit gallbladder epithelium: II. Na+−Cl− symport is independent of K+

Summary In the epithelium of rabbit gallbladder, in the nominal absence of bicarbonate, intracellular Cl^– activity is about 25mm, about 4 times higher than intracellular Cl^– activity at the electrochemical equilibrium. It is essentially not affected by 10^–4 m acetazolamide and 10^–4 m 4-acetamido-4-isothiocyanostilbene-2,2-disulfonate (SITS) even during prolonged exposures; it falls to the equilibrium value by removal of Na⁺ from the lumen without significant changes of the apical membrane potential difference. Both intracellular Cl^– and Na⁺ activities are decreased by luminal treatment with 25mm SCN^–; the initial rates of change are not significantly different. In addition, the initial rates of change of intracellular Cl^– activity are not significantly different upon Na⁺ or Cl^– entry block by the appropriate reduction of the concentration of either ion in the luminal solution. Luminal K⁺ removal or 10^–5 m bumetanide do not affect intracellular Cl^– and Na⁺ activities or Cl^– influx through the apical membrane. It is concluded that in the absence of bicarbonate NaCl entry is entirely due to a Na⁺–Cl^– symport on a single carrier which, at least under the conditions tested, does not cotransport K⁺.     

The nature of the neutral Na+−Cl−-coupled entry at the apical membrane of rabbit gallbladder epithelium: I. Na+/H+, Cl−/HCO 3 − double exchange and Na+−Cl− symport

Summary Cl^– influx at the luminal border of the epithelium of rabbit gallbladder was measured by 45-sec exposures to³⁶Cl^– and³H-sucrose (as extracellular marker). Its paracellular component was evaluated by the use of 25mm SCN^– which immediately and completely inhibits Cl^– entry into the cell. Cellular influx was equal to 16.7eq cm^–2 hr^–1 and decreased to 8.5eq cm^–2 hr^–1 upon removal of HCO ₃ ^– from the bathing media and by bubbling 100% O₂ for 45 min. When HCO ₃ ^– was present, cellular influx was again about halved by the action of 10^–4 m acetazolamide, 10^–5 to 10^–4 m furosemide, 10^–5 to 10^–4 m 4-acetamido-4-isothiocyanostilbene-2,2-disulfonate (SITS), 10^–3 m amiloride. The effects of furosemide and SITS were tested at different concentrations of the inhibitor and with different exposure times: they were maximal at the concentrations reported above and nonadditive. In turn, the effects of amiloride and SITS were not additive. Acetazolamide reached its maximal action after an exposure of about 2 min. When exogenous HCO ₃ ^– was absent, the residual cellular influx was insensitive to acetazolamide, furosemide and SITS. When exogenous HCO ₃ ^– was present in the salines, Na⁺ removal from the mucosal side caused a slow decline of cellular Cl^– influx; conversely, it immediately abolished cellular Cl^– influx in the absence of HCO ₃ ^– . In conclusion, about 50% of cellular influx is sensitive to HCO ₃ ^– , inhibitable by SCN^–, acetazolamide, furosemide, SITS and amiloride and furthermore slowly dependent on Na⁺. The residual cellular influx is insensitive to bicarbonate, inhibitable by SCN^–, resistant to acetazolamide, furosemide, SITS and amiloride, and immediately dependent on Na⁺. Thus, about 50% of apical membrane NaCl influx appears to result from a Na⁺/H⁺ and Cl^–/HCO ₃ ^– exchange, whereas the residual influx seems to be due to Na⁺–Cl^– contranport on a single carrier. Whether both components are simultaneously present or the latter represents a cellular homeostatic counterreaction to the inhibition of the former is not clear.     

Subcellular Location and Neuronal Release of Diazepam Binding Inhibitor

Diazepam binding inhibitor (DBI), a peptide located in CNS neurons, blocks the binding of benzodiazepines and beta-carbolines to the allosteric modulatory sites of gamma-aminobutyric acid (GABAA) receptors. Subcellular fractionation studies of rat brain indicate that DBI is compartmentalized. DBI-like immunoreactivity is highly enriched in synaptosomes obtained by differential centrifugation in isotonic sucrose followed by a Percoll gradient. In synaptosomal lysate, DBI-like immunoreactivity is primarily associated with synaptic vesicles partially purified by differential centrifugation and continuous sucrose gradient. Depolarization induced by high K+ levels (50 mM) or veratridine (50 microM) released DBI stored in neurons of superfused slices of hypothalamus, hippocampus, striatum, and cerebral cortex. The high K+ level-induced release is Ca2+ dependent, and the release induced by veratridine is blocked by 1.7 microM tetrodotoxin. Depolarization released GABA and Met5-enkephalin-Arg6-Phe7 together with DBI. DBI is also released by veratridine depolarization, in a tetrodotoxin-sensitive fashion, from primary cultures of cerebral cortical neurons, but not from cortical astrocytes. Depolarization fails to release DBI from slices of liver and other peripheral organs. These data support the view that DBI may be released as a putative neuromodulatory substance from rat brain neurons.     

Serum beta 2-microglobulin in patients with monoclonal gammopathies

Beta-2-microglobulin concentrations were determined in serum samples from 45 patients with benign and malignant monoclonal gammopathies. In the group of patients suffering from multiple myeloma or Waldenstr?m macroglobulinemia the mean beta 2-microglobulin level was significantly higher than in the group with monoclonal gammopathy of undetermined significance. Values above 3 mg/L were highly indicative of a neoplastic process and were observed in all the Waldenstr?m patients and in greater than 90% of myeloma patients. No significant correlation was noticed between beta 2-microglobulin and monoclonal protein levels in any of the groups examined.     

Intracellular transport of thialysine and selenalysine in CHO cells

The intracellular transport of thialysine and selenalysine in CHO cells has been studied. Data have been obtained indicating that the two lysine analogs can be transported by both the cationic aminoacid transport system and by the L transport system. The affinity of the cationic aminoacid transport system is similar for the two lysine analogs but lower than that for lysine and the affinity of the L transport system for the two lysine analogs is lower than that for leucine.