The ChargeSwitch Plasmid ER Mini Kit from Invitrogen addresses the need for endotoxin-free/reduced mini prep kits within the market. DNA from bacterial cell cultures is purified using magnetic beads and pH-switching; the protocol also includes an endotoxin-reducing step. Briefly, the magnetic beads bind the DNA within the sample in the presence of a low pH buffer, allowing for removal of contaminants with wash buffers. Then, by switching to a higher pH buffer, the DNA is released into the elution buffer. Our lab uses this kit to purify luciferase expression vectors from 5ml cultures of transformed E.coli in LB. We typically transfect primary cell cultures with endotoxin-reduced DNA using a lipid-based transfection agent. Most of our experiments consist of transfecting several wells of a tissue culture dish. Each well is transfected with 2 – 4 different reporters, requiring a total of ~4-8ug of each purified DNA sample. Therefore, using a maxi/mega prep kit is impractical and expensive. The ChargeSwitch Plasmid ER Mini Kit fits our needs perfectly.
One of the features of the kit is the speed of purification. Invitrogen advertises that sample purification takes ~10 minutes after sample preparation (i.e. lysing the cells). When purifying a few samples, this holds true, bringing the total time to ~20 minutes. Obviously, it takes a bit longer when purifying DNA from a larger number of samples, but even when working with up to 24 samples, the ChargeSwitch Plasmid ER Mini Kit is faster than spin-format mini prep kits.
The MagnaRack required to use the kit is an additional up-front expense, but it is a one-time purchase and can be used with several other applications. The layout of the magnets on the rack and the effectiveness of the magnets themselves are quite impressive. When the tubes are placed in the rack the magnets are strong enough to attract all beads in tubes. The magnets hold the beads on the side of the tube (not the bottom) which allows for easier pipetting during wash and elution steps.
Overall, the product was easy to use (more so than we initially expected) and delivered results as indicated in the product literature (i.e. we generally get just over 20ug of clean, high-quality, plasmid DNA per prep). The ChargeSwitch Plasmid ER Mini Kit is especially nice for isolating small quantities of purified DNA to use for optimization and generating preliminary data, but the yields are also high enough to conduct most small-scale experiments.
Steven J. Prior, Ph.D.
Research Fellow
Division of Gerontology
University of Maryland School of Medicine