the blank (179 cpm) were at the origin (Fig. 3a). This result confirmed that it was the MurG activity of the membrane preparation that was deficient. MurG was assayed under similar conditions. Lipid II was synthesized when 10 ng of pure E. coli MurG was added to these membranes along with Triton X-100 (Table 2). The identity of the product was confirmed by paper chromatography analysis (Fig. 3b) where radioactivity was detected
at the solvent front (Rf ~ 0.9) ABT-737 where lipid II migrates. Thus, the MurG activity in the MurG-deficient membranes could be reconstituted, and this assay for convenience is further referred to as the ‘reconstituted MurG assay’. In the reconstituted MurG assay, the product formed was dependent on the quantity of MurG added and the time of the reaction (Fig. 4). Using 10 ng of MurG, the reaction was linear up to ~ 30 min. Synthesis of lipid II was linear to ~ 20 ng and saturated above 100 ng. In membrane-based assays of MurG, both the quantity of the substrate, lipid I, and the quantity of enzyme are undefined (Mengin-Lecreulx et al., 1991; Ravishankar et al., 2005). However,
in the reconstituted MurG assay, the quantity of enzyme is defined, allowing the specific activity of MurG with the natural substrate to be defined for the first time. In the SPA, the efficiency of counting and capture is difficult to estimate, and hence, results are reported in cpm and not nmols. However, using the paper chromatography analysis, presuming the efficiency of counting of lipid II on the paper is similar to Fluorouracil solubility dmso that of UDP-[3H]GlcNAc (~ 10%), and the specific activity of E. coli MurG was 1.4 nmol min−1 mg−1; some batches had activity five times higher than this. Interestingly, the specific activity appears similar to that reported (Ha et al., 2000), Anidulafungin (LY303366) despite
the fact that the published assay used a synthetic lipid analogue and MurG was in solution. MurG activity in the reconstituted MurG assay was 60- to 100-fold higher in the presence of Triton X-100 than in its absence. In contrast, peptidoglycan synthesis activity of the MurG-reconstituted membranes was inhibited by Triton X-100. This is not unexpected, because peptidoglycan synthesis in wild-type membranes was inhibited 50% by 0.05% TritonX-100, most likely due to the inhibition of the transglycosylase (Branstrom et al., 2000; Chandrakala et al., 2001). Triton X-100 did not stimulate MurG in wild-type membranes, so it is likely that the detergent improved accessibility of the purified soluble MurG to the lipid substrate and other components present in the membranes. Nisin and vancomycin inhibited the reconstituted MurG assay with IC50s of 3.5 μg mL−1 and 32 μM, respectively; these were similar to the IC50s for MurG in wild-type membranes (nisin:10 μg mL−1 and vancomycin: 30 μM). Thus, the reconstituted MurG assay closely resembles the assay of MurG in wild-type membranes.