Melting of DNA-actinomycin clusters

[Energy of interaction in actinomycin-nucleotide complexes]

Variety of different compounds has been used for delivery of antibiotics to the tumor cells. In this work, using the highly sensitive spectrophotometry, the natural complexes of heterocyclic antibiotic actinomycin D (AMD) with such possible curriers like purine and pyrimidine nucleotides as well as fragmented DNA and phospholipid liposomes were studied. The antibiotic is not only adsorbed on the surface of purine clusters, but also is embedded in them. The antibiotic is especially well integrated into the unwound DNA regions. Embedding is accompanied by a long-wavelength shift in the absorption spectrum. The magnitude of the shift was used for calculation of the interaction energy. In the case of AMD with caffeine and adenosine, the value of energy is 2.4 and 2.7 kcal/mol and in the case of guanosine and fragmented DNA--considerably higher: 3.3 and 3.7 kcal/mol. It can be assumed that guanosine, adenosine, caffeine, and the fragmented DNA could serve as carriers of antibiotic.

Nucleotide carriers for anti-tumour actinomycin antibiotics

We have investigated a number of complexes of 7-aminoactinomycin D (7AAMD), with its potential carriers: caffeine, folic acid (FA), purine bases-guanine and adenine, pyrimidine base-thymine and with fragmented DNA to determine more stable and suitable complex. The process of binding of the fluorescent antibiotic with clusters of caffeine, guanine, adenine, thymine and with fragmented DNA was accompanied by a considerable long-wavelength shift in excitation spectrum. The energy of interaction between phenoxazine hetero-cycle of 7AAMD and chromophores of the carriers studied has been found. In the case of 7AAMD with guanine, adenine, thymine and caffeine, the energy is about of 7 kcal/mol, which is a little lower than in the case with DNA (7.7 kcal/mol). On the basis of emission spectra, in all examined compounds, with the exception DNA, the 7AAMD molecule emits photons from water phase, not from a cluster, since photo-excitation leads to desorption of the antibiotic from a cluster surface. We observed also the mutual fluorescence quenching of 7AAMD and FA in their complex. It may well be that this complex forms due to interaction of peptide-lactone rings of 7AAMD with system of FA. In the case of DNA, the complex with 7AAMD has very high stability that is determined not only by interaction between phenoxazine of 7AAMD and the DNA bases, but it is largely owing to the interaction between two peptide-lactone rings of 7AAMD and the DNA deoxyribose-phosphate chains.

[Fluorescence study of energetics in nucleotide-actinomycin complexes]

Variety of different compounds use for delivery of antibiotics to the tumor cells. In this work, using a highly sensitive fluorescence analysis, we have studied complexes of fluorescent analog of the natural heterocyclic antibiotic actinomycin D (7-aminoactinomycin D) with potential carriers: purine bases and fragmented DNA. The antibiotic is not only adsorbed on the surface ofpurine clusters, but also is embedded in them. The antibiotic is especially well integrated into the unwound DNA regions. Embedding is accompanied by a long-wavelength shift in the excitation spectrum. The magnitude of the shift was used for calculation of the interaction energy. In the case of AMD with guanine, caffeine and adenine, the value of energy was about of 7 kcal/mol and in the case of fragmented DNA it was only a bit higher: 7.7 kcal/mol. It can be assumed that guanine, adenine, caffeine, and the fragmented DNA could apply as carriers of antibiotic.

Force spectroscopy reveals the DNA structural dynamics that govern the slow binding of Actinomycin D

Actinomycin D (ActD) is a small molecule with strong antibiotic and anticancer activity. However, its biologically relevant DNA-binding mechanism has never been resolved, with some studies suggesting that the primary binding mode is intercalation, and others suggesting that single-stranded DNA binding is most important. To resolve this controversy, we develop a method to quantify ActD’s equilibrium and kinetic DNA-binding properties as a function of stretching force applied to a single DNA molecule. We find that destabilization of double stranded DNA (dsDNA) by force exponentially facilitates the extremely slow ActD-dsDNA on and off rates, with a much stronger effect on association, resulting in overall enhancement of equilibrium ActD binding. While we find the preferred ActD–DNA-binding mode to be to two DNA strands, major duplex deformations appear to be a pre-requisite for ActD binding. These results provide quantitative support for a model in which the biologically active mode of ActD binding is to pre-melted dsDNA, as found in transcription bubbles. DNA in transcriptionally hyperactive cancer cells will therefore likely efficiently and rapidly bind low ActD concentrations (∼10 nM), essentially locking ActD within dsDNA due to its slow dissociation, blocking RNA synthesis and leading to cell death.