A simple quantitative assay for chloramphenicol acetyltransferase by direct extraction of the labeled product into scintillation cocktail

Abstract

A simple, rapid, sensitive, quantitative, and inexpensive assay for chloramphenicol acetyltransferase (CAT) is described. The assay is based on the direct extraction of the products of the reaction into toluenebased liquid scintillation cocktail. The assay is carried out in 7-ml scintillation vials using 1 m m chloramphenicol and either 100 μ m acetyl-CoA and 0.1 μCi of [ 3H]acetyl-CoA or 1 m m acetyl-CoA and 0.5 μCi of [ 3H]-acetyl-CoA. After incubation, the reaction is terminated with 0.5 ml of 0.1 m sodium borate-5 m NaCl, pH 9. The acetylchloramphenicols are extracted with 5 ml of 0.4% 2,5-diphenyloxazole-0.005% 1,4-bis(5-phenyloxazol-2-yl)benzene in toluene by a 30-s shaking. After a short centrifugation to clarify the layers, the vials are counted in a liquid scintillation counter. Extracted products are stable in the organic layer. Under these conditions, nearly 100% extraction of acetylchloramphenicols is shown using nonlabeled compounds and spectrophotometric methods. Using pure enzyme in the assay, linearity of activity with enzyme concentration, time, and temperature of incubation is demonstrated. Assays may even be carried out at 60°C, where the enzyme activity is 3.4-fold higher than that at 23°C. The increase in enzyme activity with increasing temperature is due to the increased formation of predominantly 3-acetyl- and 1-acetylchloramphenicols and not to 1,3-diacetylchloramphenicol. The present assay compared very well with the standard assay using [ 14C]chloramphenicol and TLC. Using this assay, we measured quantitatively the CAT activity in extracts of pSV2-CAT-transfected CV-1 cells in 10 min and NIH 3T3 cell extracts in 60 min at 60°C. CAT-like activity and acetyl-CoA hydrolase activities are not detectable in control cell extracts under the present assay conditions.