Ligand binding by recombinant domains from insect ecdysone receptors.

The ligand binding domains (LBDs) from the EcR and USP proteins of four insect pests (Lucilia cuprina, Myzus persicae, Bemisia tabaci, Helicoverpa armigera) were purified as recombinant heterodimers. The K(d) values for [(3)H]-ponasterone A binding by LBD heterodimers that included the hinge regions (i.e., DE/F heterodimers) ranged 0.7-2.5 nM, with K(i) values for ecdysteroid and dibenzoylhydrazine ligands ranging from 0.1 nM to >448 microM. The K(d) and K(i) values for a recombinant H. armigera LBD heterodimer that lacked D-regions (i.e., an E/F heterodimer) were approximately 4 times higher than those for its DE/F counterpart. Rate constants were estimated for the L. cuprina LBD heterodimer. A fluorescein-inokosterone conjugate (K(i)~40 nM) was used to develop a novel binding assay based on fluorescence polarization. This assay, which ranked the affinity of competitor ecdysteroids in the same order as the [(3)H]-ponasterone A binding assay, is well suited to high-throughput screening. Ponasterone A had a higher affinity than muristerone A for the recombinant hemipteran LBD heterodimers, whereas the reverse was true for the recombinant dipteran one. The same preference was observed when these ligands were tested as inducers of ecdysone receptor-controlled gene expression in transfected mammalian cells. The binding data obtained in vitro using recombinant LBD heterodimers reflects the ability of agonists to induce transgene expression in recombinant mammalian cells, and can also reflect their efficacy as larvicides.

Amplifications of phleomycin and bleomycin-induced antibiotic activity in Escherichia coli by aromatic cationic compounds.

A wide range of aromatic compounds has been shown to amplify phleomycin-induced cell killing in Escherichia coli. They include acridines, acridinium chlorides, dihydroanthracenes, anthracenes, dianthracenes, phenanthridinium salts, phenazinium chlorides, phenoxazones, triphenyl methane dyes, benzoquinolizinium chloride, diphenylmethane derivatives, stilbene and diphenyl derivatives. Low concentrations of these amplifiers also amplified the DNA breakage and degradation effects of phleomycin. The minimum structural specification for activity as an amplifying agent is suggested. A representative sample of compounds effective as amplifiers of phleomycin also amplified the antibiotic effects of bleomycins B4 and B6. The amplifiers described are known to vary in their ability to penetrate and accumulate in different organisms or tissues. This suggests the possibility of developing a series of antibiotic regimes using these amplifiers (or the large number of derivative compounds also likely to be active) where the therapeutic index is determined by the properties of the amplifier chosen rather than of the phleomycin or the bleomycin.