Module 3 : Designed Enediyne Model Systems

Lecture 15: Biological Actions of Some Synthetic Models

3.10. Biological Actions of Some Synthetic Models

In general two types of DNA cleavages occur- (a) the single- and (b) double-strand cuts (Figure 4a). In a typical experiment to check the DNA-cleaving activity a supercoiled plasmid DNA is used. Single-strand cleavage means the breakage or nicking of only one of the two strands, which enables the DNA to relax. On the other hand, a linear form will result if cuts are produced on complimentary sites (or close to complimentary sites) of both of the strands. The mobilities of all three forms in a gel under electric field are different and hence can be easily identified via gel electrophoresis technique (Figure 4d). Calicheamicin show its potential DNA cleaving activity at nanomolar concentrations (1 pg μL-1). However, all the reported synthetic enediynes show lower DNA cleavage activity compared to natural calichiamicin. These synthetic enediynes show their potency at micromolar concentrations. Thus, in comparison to the natural enediynes, the cleavage efficiency of designed enediynes is a moderate.

Figure 4. (a) Single- and double-strand DNA cuts. (b) Supercoiled plasmid DNA. (c) Relaxed plasmid DNA. (d) Gel electrophoresis pattern of various forms of plasmid DNA [form I : supercoiled ; form II: nicked, single-stranded (ss) cleavage ; form III: linear, double-stranded (ds) cleavage].

3.10. 1. Biological Activity of Basak's β -Lactam Fused Enediyne

Under basic conditions (pH = 8), the β -lactam fused enediyne as is shown in Figure 5 showed cleavage of supercoiled plasmid DNA (pBR 322) at concentrations of 50 μmol. No such cleavage was observed under neutral conditions. In alkaline condition, the β -lactam hydrolyses to produce the amine, which is predominantly present in the protonated form and is the actual DNA cleaving agent. The result clearly showed the potential of the β -lactam fused enediyne as a cytotoxic agent.

Figure 5. Interaction of supercoiled DNA (in Tris-acetate buffer, pH 8.0) and enediyne in acetonitrile. Lanes 1: DNA; Lanes 2: DNA + enediyne 1.4.46 (50 mmol); Lanes 3: DNA.