Mechanism of the radioprotective effect of cysteamine

The radioprotective effect of cysteamine combined with the modification of the chromatin state by sodium butyrate has been studied using V-79 and CHEL lines of Chinese hamster cells and HeLa cells. Sodium butyrate enhances the chromatin sensitivity to nucleases and removes the radioprotective effect of cysteamine as measured by the yield of cells with chromosome aberrations. As is indicated by changes in the intensity of fluorescence of the DNA-ethidium bromide complex, measured by laser flow cytometry, the protective agent decreases the binding of the dye with both irradiated and nonirradiated DNA whereas ionizing radiation and sodium butyrate increase thereof. It is concluded that the radioprotective effect of cysteamine depends in its ability to reduce the susceptibility of DNA to nucleases.     

Possible explanation for the metabolic radioprotective effect of cysteine on Escherichia coli B

The growth of logarithmic-phase cultures of E. coli B cells on glucose-mineral medium was partially inhibited by 0.2 mM L-cysteine-HCl. Twenty other amino acids failed to show a similar inhibiting effect even at a concentration of 10 mM. After incubation of the log-phase culture in the presence of 2.0 mM cysteine for 30 minutes (inhibition about 70 %) the cells were centrifuged, resuspended, and diluted 200-fold in cysteine-free phosphate buffer. These cells exhibited increased resistance to the effect of X-rays as measured by the number of colony-forming units (DRF = 2.1). Both the growth-inhibiting and the radioprotective effects of cysteine could be diminished by the presence of 0.5 mg of casein hydrolyzate per milliliter during the 30-minute preirradiation culture period. This "anti-cysteine" effect was caused mainly by L-leucine, L-isoleucine, L-valine, and L-threonine from among the amino acids of casein hydrolyzate. It is suggested that cysteine inhibits the biosynthesis of some amino acids, thereby blocking protein synthesis, which may result in increased radioresistance.     

Mechanisms involved in the depleting effect of cysteamine on pancreatic somatostatin

We investigated the effects of cysteamine on the pancreatic islet hormones and found that pancreatic somatostatin contents depleted 60 min after the oral administration of cysteamine (300 mg/kg) to rats, yet the insulin and glucagon contents remained unchanged. When pancreatic islets isolated by collagenase digestion were incubated for 60 min in Krebs-Ringer bicarbonate buffer containing 0.1, 1, or 10 mM cysteamine, cysteamine dose-dependently decreased the somatostatin content, however, only a high concentration (10 mM) decreased the insulin level, and cysteamine exerted no effect on the glucagon content. The islet hormones (synthetic somatostatin-14, synthetic somatostatin-28, extracted pork insulin and extracted pork glucagon) were incubated for 60 min with cysteamine (0.1, 1, or 10 mM) and somatostatin-14 was found to be markedly decreased by 1 mM cysteamine. Pork insulin but not pork glucagon was dose-dependently decreased by 0.1-10 mM cysteamine. Cysteamine, 0.1-1 mM, did not interfere with the radio-immunoassay system for somatostatin or insulin, although 10 mM cysteamine did so. This compound exerted no effect on the radioimmunoassay system for glucagon. Our studies support earlier findings that cysteamine administered to experimental animals plays a role of relatively specific depletor of somatostatin. The possibility that the depletion of somatostatin is in part due to the remarkable sensitivity of the intracellular compartments of the D cells to the drug and in part due to the remarkable sensitivity of the molecular structure of somatostatin has to be considered.     

Effect of hyperthermia on the radioprotective action of cysteamine and isoproterenol

Heating of mouse bone marrow cells up to 42 degrees C was shown to increase their radiosensitivity (DMF = 0.80 +/- 0.12). At this temperature, the radioprotective efficiency of cysteamine was lost completely (DMF = 0.78 +/- 0.09), and radioprotective activity of d,l-isoproterenol significantly decreased (DMF declined from 2.41 +/- 0.23 to 1.67 +/- 0.16). It is assumed that the radioprotective effect of cysteamine on mammalian cells is associated with the processes of the postirradiation DNA repair for just these processes are inhibited by heating. The mechanism of action of a beta-agonist of isoproterenol is perhaps only partially associated with DNA repair.     

Interrelationship of Repair Mechanisms in Ultraviolet-Irradiated Escherichia coli

Investigation of the effect of photoreactivation, excision, and recombination repair, individually and in combination, on the survival of ultraviolet-irradiated Escherichia coli K-12 mutants has led to a possible explanation of the loss of photoreactivability and of the complex changes in viability observed during liquid-holding. It is suggested that at higher ultraviolet light doses the excision repair mechanism becomes saturated due to overlapping of excised regions on opposite strands of the deoxyribonucleic acid helix. The results also provide support for the existing hypothesis that states that the shape of shouldered survival curves of ultraviolet-irradiated bacteria can be described in terms of the probability of occurrance of overlapping excised regions. Using the data obtained with repair-deficient mutants with closely related genetic makeup, we present a mathematical model that accurately predicts the shape of the observed survival curves and provides an estimate of the number of nucleotides in each fragment of deoxyribonucleic acid removed by the excision repair mechanism.     

Mechanisms of replication fork restart in Escherichia coli

Replication of the genome is crucial for the accurate transmission of genetic information. It has become clear over the last decade that the orderly progression of replication forks in both prokaryotes and eukaryotes is disrupted with high frequency by encounters with various obstacles either on or in the template strands. Survival of the organism then becomes dependent on both removal of the obstruction and resumption of replication. This latter point is particularly important in bacteria, where the number of replication forks per genome is nominally only two. Replication restart in Escherichia coli is accomplished by the action of the restart primosomal proteins, which use both recombination intermediates and stalled replication forks as substrates for loading new replication forks. These reactions have been reconstituted with purified recombination and replication proteins.     

鲎源抗菌肽对大肠杆菌的抑杀机理

【目的】研究鲎源抗菌肽鲎素对大肠杆菌抑杀的作用机理,为防治大肠杆菌引起的肠道疾病提供新的潜在抗菌药物。【方法】利用牛津杯法和微量MH肉汤稀释法测定其抗菌活性,激光共聚焦扫描显微镜观察鲎素对大肠杆菌的结合、分布及杀伤过程,透射电镜观察鲎素对大肠杆菌超微结构的影响,并采用琼脂糖凝胶阻滞电泳研究其对大肠杆菌基因组DNA和RNA的影响。【结果】鲎素对大肠杆菌的最小抑菌浓度与最低杀菌浓度分别为5 mg/L和20 mg/L;激光共聚焦扫描显微镜和透射电镜观察发现鲎素能快速作用于细胞表面,并发生聚集现象,随着作用时间的延长能导致细胞膜结构的破坏和细胞内含物的释放;通过琼脂糖电泳结果显示鲎素也能够作用于大肠杆菌基因组DNA,并呈浓度依赖关系,10 mg/L鲎素对基因组DNA无明显影响,80 mg/L鲎素能导致DNA断裂;凝胶阻滞电泳显示鲎素也能与基因组DNA和RNA发生结合。【结论】研究结果为深入探讨鲎素抑杀大肠杆菌的分子机制提供重要的理论依据。     

Mechanisms of acid resistance in enterohemorrhagic Escherichia coli.

Enterohemorrhagic strains of Escherichia coli must pass through the acidic gastric barrier to cause gastrointestinal disease. Taking into account the apparent low infectious dose of enterohemorrhagic E. coli, 11 O157:H7 strains and 4 commensal strains of E. coli were tested for their abilities to survive extreme acid exposures (pH 3). Three previously characterized acid resistance systems were tested. These included an acid-induced oxidative system, an acid-induced arginine-dependent system, and a glutamate-dependent system. When challenged at pH 2.0, the arginine-dependent system provided more protection in the EHEC strains than in commensal strains. However, the glutamate-dependent system provided better protection than the arginine system and appeared equally effective in all strains. Because E. coli must also endure acid stress imposed by the presence of weak acids in intestinal contents at a pH less acidic than that of the stomach, the ability of specific acid resistance systems to protect against weak acids was examined. The arginine- and glutamate-dependent systems were both effective in protecting E. coli against the bactericidal effects of a variety of weak acids. The acids tested include benzoic acid (20 mM; pH 4.0) and a volatile fatty acid cocktail composed of acetic, propionic, and butyric acids at levels approximating those present in the intestine. The oxidative system was much less effective. Several genetic aspects of E. coli acid resistance were also characterized. The alternate sigma factor RpoS was shown to be required for oxidative acid resistance but was only partially involved with the arginine- and glutamate-dependent acid resistance systems. The arginine decarboxylase system (including adi and its regulators cysB and adiY) was responsible for arginine-dependent acid resistance. The results suggest that several acid resistance systems potentially contribute to the survival of pathogenic E. coli in the different acid stress environments of the stomach (pH 1 to 3) and the intestine (pH 4.5 to 7 with high concentrations of volatile fatty acids). Of particular importance to the food industry was the finding that once induced, the acid resistance systems will remain active for prolonged periods of cold storage at 4 degrees C.     

Mechanisms of the mediated neutrophil reactivity to Escherichia coli peptidoglycan

The opsonic properties of normal serum with respect to E. coli peptidoglycan was studied under the actual conditions of the oxygen-dependent metabolism of neutrophils. In the course of the differentiated study of the influence of antibodies, the classical and the alternative cascades of complement the serum was heated, treated with ethylenediaminetetraacetate and ethylene glycol tetraacetate, exhausted in the cold. In serial experiments the opsonic activity of purified fibronectin was studied. The indirect reactions were shown to be the leading mechanisms of the neutrophil-stimulated activity of E. coli peptidoglycan. IgG was found to be in the center of the opsonic cooperation and thus to determine the quantitative manifestation of the total phenomenon. Complement proved to be of lesser importance; depending on the conditions of the experiment, the activation of complement occurred by the alternative way (after the removal of antibodies) or the classical way (whole serum). The actual contribution of IgG-independent and complement-independent opsonins was insignificant. Fibronectin in physiological concentrations showed no opsonic activity.     

Evidence for Two Mechanisms of Photoreactivation in Escherichia coli B

Escherichia coli B phr^-, which is not photoreactivable under certain conditions, has been shown to exhibit photoreactivation of killing in the logarithmic growth phase at 3341 A. Dependence of the reaction upon (a) wavelength, (b) dose, and (c) dose rate of the reactivating radiation, as well as upon (d) temperature during reactivation treatment, is very similar to that of photoprotection. We conclude that this photoreactivation is similar in mechanism to photoprotection, believed to be an indirect repair process, the initial step of which is non-enzymatic and leads to a growth-division delay. We therefore call the present phenomenon “indirect photoreactivation.” Similar studies suggest that indirect photoreactivation of killing occurs also in the parent strain, E. coli B (Harm). It has often been supposed that all photoreactivation results from a photoenzymatic reaction similar to that found to operate in vitro on transforming DNA. Our data provide the first evidence for two distinct types of photoreactivation of cell killing, one of which appears not to involve photoenzymes. These experiments also show that photoprotection results from intracellular events that can be induced by treatment after, as well as before, far ultraviolet irradiation.     

Mechanisms of action of ostrich beta-defensins against Escherichia coli

To understand their mechanism of antimicrobial activity against Gram-negative bacteria, ostrich beta-defensins, ostricacins-1 and 2 (Osp-1 and Osp-2), were compared with those of sheep myeloid antimicrobial peptide (SMAP)-29 and human neutrophil peptide (HNP)-1, well-characterized sheep alpha-helical and human alpha-defensin peptides, respectively. Fluorescence-based biochemical assays demonstrated that the ostricacins bound lipopolysaccharides and disrupted both outer and cytoplasmic membrane integrity. The ostricacins' permeabilizing ability was weaker than that of SMAP-29, but stronger than HNP-1. As ostricacins have previously shown the ability to inhibit bacterial growth, these peptides were suggested to be bacteriostatic to Gram-negative bacteria, which are caused by the interaction between the peptides and cytoplasmic targets causing the inhibition of DNA, RNA, and protein synthesis as well as enzymatic activities. These findings indicated promising possibilities for the peptides to be used in the development of therapeutic and topical products.