Double isotope derivative assay of aldosterone in biological extracts.

The selective measurement of aldosterone in biological fluids has been handicapped by the lack of specific and sensitive assays. Bioassay methods (l-4) have been limited by the variation inherent in animal response. Chemical methods (5-9) require extensive purification procedures to remove contaminants, and are therefore subject to variable recoveries of the steroid. Furthermore, the quantity of steroid isolated after purification may be too small to permit a reliable analysis, and in most instances the resulting purified aldosterone has been estimated by visual examination of soda fluorescent spots. A partial correction for loss has been provided by using a single tracer, but the method depends on a fmal chemical reaction that is relatively insensitive and may lack precision (10). The use of radioactive indicators in the analytical assay often makes possible a quantitative assay without a quantitative isolation of the compound of interest. Udenfriend et al. (11-15) developed isotope derivative assays for the determination of amino acids by means of their pipsyl derivatives, labeled with 1131 and S36. One of the radioactive isotopes served as an indicator to provide for internal yield correction and final quantitation of the unknown. Double isotope derivative methods have recently been developed for the measurement of adrenal steroids (16, 17) and these have been applied to the measurement of aldosterone (18). An internal correction for yield in each sample, and quantitation by radioactivity measurements allows for the assay of as little as 0.01 pg of aldosterone after an extensive purification process. This report includes a detailed description of the method, assay results for human urine and dog adrenal vein plasma, and a critical evaluation of the specificity, precision, and accuracy of this technique.