The microplate format has become a standard for many biological assays such as high throughput screening for drug discovery and radioimmunoassays. Productivity with these high volume procedures has been limited by the amount of sample handling required and low sample counting throughput. Now, however, microplate assays can be efficiently counted in microplates with the Packard TopCount Microplate Scintillation and Luminescence Counter. This paper summarizes TopCounts performance for counting samples in organic solvents or bound to the wells of microplates (in-plate assays) using MicroScintTM-O scintillation cocktail, a special formulation for samples in organic solvents. Results from an enzyme inhibition assay and a microplate immunoassay obtained with TopCount are comparable to those obtained by conventional liquid scintillation and gamma counting.
Enzyme inhibition assays are among the many tests used by pharmaceutical companies to screen new compounds for biological activity. These assays measure the inhibitory effect of test compounds on target enzymes such as HIV proteases, various kinases, RNA and DNA transcriptases and polymerases. Enzyme activity is often determined by measuring the conversion of a radiolabeled substrate to a product that can be separated from the substrate by chromatography or organic extraction. After the separation step, the 3H- or 14C-labeled products, often in organic solvents, are counted by liquid scintillation counting (LSC). The assays are often done in microplates, and it is highly desirable to quantitate the radiolabeled product in this format, especially when the product can be eluted directly into microplates. By quantitating in microplates, additional sample handling and liquid scintillation vials can be eliminated. In addition, sample and cocktail volumes, and the resulting waste d