| Abstract: |
The induction of electrophile-processing Phase II enzymes (i.e., glutathione S-transferases, UDP-glucuronosyltransferases, and quinone reductase) is a major mechanism whereby a large group of heterogeneous compounds prevent the toxic, mutagenic, and neoplastic effects of carcinogens. This paper reviews the direct assay of quinone reductase in cultured cells as a method to rapidly detect potentially anticarcinogenic substances. Cells (usually Hepa 1c1c7 murine hepatoma cells) growing in 96-well microtiter plates are exposed to test compounds for 24 to 72 hrs and are then lysed and assayed for quinone reductase activity by measuring the formation of formazan dye from the menadione-dependent reduction of MTT (3-(4,5-dimethylthiazo-2-yl)-2,5-diphenyltetrazolium bromide). Reaction velocities are linearly dependent on quinone reductase over a wide range of enzyme concentrations, and the specificity of MTT reduction can be ascertained with dicoumarol, a potent inhibitor of quinone reductase. This in vitro technique reliably detects compounds that effectively induce Phase II enzymes in vivo, and the screening assay has identified hitherto unrecognized and novel inducers from synthetic and natural sources. As inducers of Phase II enzymes are receiving serious consideration as potential human anticarcinogens, the assay described offers the opportunity to rapidly identify, isolate, and characterize inducers of medicinal interest. © 1994. |
