A modified digitonin-precipitation radioassay for 3 β-hydroxy-Δ 5-steroid dehydrogenase

An improved digitonin-precipitation radioassay for 3 β-hydroxy-Δ 5-steroid dehydrogenase is presented. It is linear for enzyme protein concentrations up to 0.6 mg and for 37°C incubation times up to 10 min. Sensitivity and reproducibility are improved nine- and sevenfold, respectively, over the method of Philpott and Peron. There is a good correlation with the Philpott and Peron assay ( r = 0.958) and with a thin-layer chromatography method ( r = 0.997). Beyond being faster, simpler, and more sensitive than other analyses, this radioassay uses one-tenth the quantity of [ 3H]pregnenolone substrate and is, therefore, a safer and less expensive procedure.