An antitumor polypeptide antibiotic neocarzinostatin: the mode of apo-protein--chromophore interaction

Comparison of Antibiotic Resistance Mechanisms in Antibiotic-Producing and Pathogenic Bacteria

Antibiotic resistance poses a tremendous threat to human health. To overcome this problem, it is essential to know the mechanism of antibiotic resistance in antibiotic-producing and pathogenic bacteria. This paper deals with this problem from four points of view. First, the antibiotic resistance genes in producers are discussed related to their biosynthesis. Most resistance genes are present within the biosynthetic gene clusters, but some genes such as paromomycin acetyltransferases are located far outside the gene cluster. Second, when the antibiotic resistance genes in pathogens are compared with those in the producers, resistance mechanisms have dependency on antibiotic classes, and, in addition, new types of resistance mechanisms such as Eis aminoglycoside acetyltransferase and self-sacrifice proteins in enediyne antibiotics emerge in pathogens. Third, the relationships of the resistance genes between producers and pathogens are reevaluated at their amino acid sequence as well as nucleotide sequence levels. Pathogenic bacteria possess other resistance mechanisms than those in antibiotic producers. In addition, resistance mechanisms are little different between early stage of antibiotic use and the present time, e.g., β-lactam resistance in Staphylococcus aureus. Lastly, guanine + cytosine (GC) barrier in gene transfer to pathogenic bacteria is considered. Now, the resistance genes constitute resistome composed of complicated mixture from divergent environments.

Aponeocarzinostatin—A superior drug carrier exhibiting unusually high endurance against denaturants

The enediyne antitumor antibiotic chromoproteins are very potent in causing DNA damages. During the drug delivery time course, the stability of the carrier protein becomes an important concern. To simulate conceivably offensive environment in biological contexts, such as cell membrane, we studied structural endurance of aponeocarzinostatin against several denaturants by circular dichroism and nuclear magnetic resonance spectroscopy. For comparison, we also examined proteins known to be stable and similar in size to aponeocarzinostatin. The results highlight the unusual structural stability of aponeocarzinostatin against chemical denaturants, suggesting the potential of aponeocarzinostatin as an inherently superior carrier in drug delivery systems.

Cold instability of aponeocarzinostatin and its stabilization by labile chromophore

The conformational stability of aponeocarzinostatin, an all-beta-sheet protein with 113 amino-acid residues, is investigated by thermal-induced equilibrium unfolding between pH 2.0 and 10.0 with and without urea. At room temperature, the protein is stable in a pH range of 4.0-10.0, whereas the stability of the protein drastically decreases below pH 4.0. The thermal unfolding of aponeocarzinostatin is reversible and follows a two-state mechanism. By two-dimensional unfolding studies, the enthalpy change, heat capacity change, and free energy change for unfolding of the protein are estimated. Circular dichroism profiles suggest that this protein undergoes both heat- and cold-induced unfolding. The ellipticity changes at far- and near-UV circular dichroism suggest that the tertiary structure is disrupted but the secondary structure remains folded at low temperatures. Interestingly, the labile enediyne chromophore, which is highly stabilized by the protein, is able to protect the protein against cold-induced unfolding, but not the heat-induced unfolding.

Solution structures of C-1027 apoprotein and its complex with the aromatized chromophore

C-1027 is one of the most potent antitumor antibiotic chromoproteins, and is a 1:1 complex of an enediyne chromophore having DNA-cleaving ability and a carrier apoprotein. The three-dimensional solution structures of the 110 residue (10.5 kDa) C-1027 apoprotein and its complex with the aromatized chromophore have been determined separately by homonuclear two-dimensional nuclear magnetic resonance methods. The apoprotein is mainly composed of three antiparallel beta-sheets: four-stranded beta-sheet (43-45, 52-54; 30-38; 92-94; 104-106), three-stranded beta-sheet (4-6; 17-22; 61-66), and two-stranded beta-sheet (70-72; 83-85). The overall structure of the apoprotein is very similar to those of other chromoprotein apoproteins, such as neocarzinostatin and kedarcidin. A hydrophobic pocket with approximate dimensions of 14 A x 12 A x 8 A is formed by the four-stranded beta-sheet and the three loops (39-42; 75-79; 97-100). The holoprotein (complex form with the aromatized chromophore) structure reveals that the aromatized chromophore is bound to the hydrophobic pocket found in the apoprotein. The benzodihydropentalene core of the chromophore is located in the center of the pocket and other substituents (beta-tyrosine, benzoxazine, and aminosugar moieties) are arranged around the core. Major binding interactions between the apoprotein and the chromophore are likely the hydrophobic contacts between the core of the chromophore and the hydrophobic side-chains of the pocket-forming residues, which is supplemented by salt bridges and/or hydrogen bonds. Based on the holoprotein structure, we propose possible mechanisms for the stabilization and the release of chromophore by the apoprotein.

Kinetic analysis of the in vitro cell-killing action of neocarzinostatin

The relationship between the drug concentration and exposure time of neocarzinostatin (NCS) for a definite cell-killing effect was kinetically analyzed, taking into consideration its loss in biological activity during incubation. Its cell-killing activity was determined by a colony-forming inhibition assay, which was conducted at room temperature (25 degrees C) for 0.5-30 min exposure and at 37 degrees C for 5 min - 96 h exposure. Drug degradation at both temperatures was also investigated by bioassay. NCS lost its biological activity much faster at 37 degrees C than at 25 degrees C and the rate of loss in activity was higher at the lower initial concentration. Thus, the initial NCS concentrations necessary for 90% cell kill corresponding to each exposure time and a drug degradation constant were applied to a mathematical equation for the cell-killing effect of cell-cycle-phase-nonspecific agents. As a result, the curves for IC90-exposure time relationships predicted from drug degradation constants for 37 degrees C and 25 degrees C were fairly well fitted to the respective experimental data. These results indicate that the cell-killing action of NCS can be expressed by this mathematical equation with scrutiny of drug degradation and is dependent on the concentration-time product (C x T).