A simple, highly sensitive assay for measurement of digitonin during receptor solubilization

A simple and highly sensitive assay for measuring total digitonin in biological samples is described. The assay is based on the ability of digitonin to hemolyze red blood cells. The precision and reproducibility of the assay was excellent with intra- and inter-assay variabilities of less than 1% and 6%, respectively. The assay was used to evaluate several potential methods for removing digitonin from biological samples (digitonin extracts from guinea pig brain membranes). Dialysis and G-25 Sephadex chromatography were ineffective. However, protein and digitonin can be effectively separated by ammonium sulfate precipitation followed by dialysis. The kappa 1 opioid receptor survived these procedures with no change in affinity for [3H]U-69,593. In conclusion, the hemolytic assay for digitonin appears to provide a practical means for determining detergent concentrations during receptor purification and characterization and for evaluating potential methods for detergent removal. Although an in depth analysis of the assay was carried out only for digitonin, CHAPS and deoxycholate also caused 50% hemolysis at concentrations well below those commonly used for receptor solubilization and, therefore, the general assay procedures might have applicability for measurement of these and perhaps other detergents used in receptor solubilization as well.