Formation of a complex between chromomycin and DNA has been shown to depend on the guanine content and on base pairing of the polydeoxynucleotides. A limit of association is reached when one chromomycin molecule is bound per four nucleotide base pairs. The rate of complex formation is independent of the size of the sugar side chains of the antibiotic and of the nature and base composition of DNA. I n contrast the rate of complex dissociation increases with a decrease in size of the sugar side chains. The inhibition by chromomycin and its derivatives of DNA and RNA polymerase reactions exhibits similar features. Probably the dissociation rate of the complex determines the inhibitory activity. Chromomycin by complex formation protects guanine rich segments of DNA very efficiently against the attack of nucleases. After enzymatic hydrolysis chromomycin containing base paired oligonucleotide fragments remain in the reaction mixture and can be isolated by electrophoresis. The antibiotic is not covalently bound to these fragments and can be removed by thorough extraction with ether.Low concentrations of the antibiotic chromomycin Id and of closely related compounds such as olivomycin and mithramycin have been shown to inhibit selectively the DNA directed cellular and enzymatic RNA synthesis [l-91. The DNA polymerase reaction is also retarded [6]. The following RO
The purpose of this study was to measure the spatially varying 31P MR signals in global and regional ischemic injury in the isolated, perfused rat heart. Chronic myocardial infarcts were induced by occluding the left anterior descending coronary artery eight weeks before the MR examination. The effects of acute global low-flow ischemia were observed by reducing the perfusate flow. Chemical shift imaging (CSI) with three spatial dimensions was used to obtain 31P spectra in 54-microl voxels. Multislice 1H imaging with magnetization transfer contrast enhancement provided anatomical information. In normal hearts (n = 8), a homogeneous distribution of high-energy phosphate metabolites (HEP) was found. In chronic myocardial infarction (n = 6), scar tissue contained negligible amounts of HEP, but their distribution in residual myocardium was uniform. The size of the infarcted area could be measured from the metabolic images; the correlation of infarct sizes determined by histology and 31P MR CSI was excellent (P < 0.006). In global low-flow ischemia (n = 8), changes of HEP showed substantial regional heterogeneity. Three-dimensional 31P MR CSI should yield new insights into the regionally distinct metabolic consequences of various forms of myocardial injury.
schrecken metastabil crhalten werden. Die Struktur fur 15 Ca0.2 P205.3 B203 i s t nicht bekannt; aus Pulveraufnahmen ist jedoch auf eine wcite Verbreitung des Typs zu schlie-13en: isostrukturelle Phaseii rnit groBerem Stabilitatsbereich und einer gewissen Phasenbreite finden sich in den Systemen SrO/P205/B203 und BaO/PzOs/Bz03, und Verbindungen des Typs 6 MO.PzOs.Bz03 ( M = Mg, Zn, Cd, Mn, Co, Ni) zeigen ahnliche Pulverdiagramme. 2CaO.PzOyB203 kristallisiert hexagonal und ist ein Glied einer isotypen Reihe 2MO-Xz05. BzO3, von der bisher dic Verbindungen rnit M = Ca, Sr, Ba und X = P, As dargestellt werden konnten. Neben den ternaren Verbindungen wurden im System CaO/P205/B203 zwei Bereiche fester Losungen gefunden, die strukturell dem Apatittyp [l] und dern Typ des p-Tricalciumphosphats [3] zuzuordnen sind. Den festen Losungen mit Apatitstruktur liegt eine kombinierte Substitution (von BO&-fur Pod3--Gruppen) und Addition (Eintritt von B3+ auf Zwischengitterplatze und von gegebenenfalls zusatzlich eingebrachtem 0 2auf die zweizahligen Punktlagen des Apatitgitters) zugrunde, wahrend die festen Losungen mit p-Tricalciumphosphat-Struktur nach einem reinen Additionsmechanismus gebildet werden. Beide Typen fester Losungen finden sich wiederum auch in anderen Systemen MO/X205/B203, Apatite rnit M == Ca, Sr, Ba und X = P, As sowie rnit M = Ca und X = V; der zweite Typ findet sich noch im System CaO/As205/B203.
The antibiotics of the rifamycin, actinoniycin, chromomycin, and anthracycline groups have been found to be specific inhibitors for the DNA-controlled synthesis of RNA [21 in vitro. Streptomycin, chloramphenicol, and puromycin can specifically suppress certain steps in the biosynthesis of proteins. The investigation of the mode of action of such substances may heIp us to gain a better insight into the transmission of hereditary information.Seven years ago Reich, Franklin, Shatkin, and Tatum [31 found that small concentrations of the antibiotic actinomycin in tissue cultures selectively suppress the synthesis of RNA whereas DNA and protein syntheses, on the other hand, remained temporarily almost unchanged. This discovery attracted great interest in connection with the intensive efforts being made to explain the nucleic acid and protein syntheses, since specific inhibitors had already been of great value in the elucidation of the individual reaction steps of other important metabolic chains.
A high signal-to-noise ratio (SNR) in 31P-nuclear magnetic resonance (31P-NMR) spectroscopy can be obtained only with good B0-field homogeneity and optimal coil sensitivity. This demands double-tuned coils with a highly sensitive 31P channel and an additional 1H channel for 1H-magnetic resonance imaging, shimming, 1H decoupling, and nuclear Overhauser enhancement (NOE). For studies on an 11.75 T magnet, we built coils derived from the four-ring birdcage design originally described by Murphy-Boesch. A comparison with conventional, single-tuned coils shows that, in spite of double tuning, there is no significant loss in 31P sensitivity while the 1H channel provides the requested performance. The coil design offers the advantage of circular polarization on both channels.
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