Radioimmunoassay of digitonin

We describe a radioimmunoassay for digitonin which utilizes the ability of digotonin to compete with 125I-labeled digoxigenin for binding to anti-digoxin antiserum. As performed using the Gammaflo automated radioimmunoassay system and commercially available reagents, the assay can detect as little as 20 micrograms/ml (15 microM) digitonin. The assay is insensitive to interference by cholesterol or other cell membrane constituents and is useable above and below the critical micelle concentration of digitonin. It should be useful for the monitoring of digitonin concentrations in solubilized biochemical preparations.