An enzyme immunoassay for serum 18-hydroxycorticosterone was established using alkaline phosphatase as a label. The antiserum for 18-hydroxycorticosterone was produced by immunization of rabbits with 18-hydroxycorticosterone 3- (o-carboxymethyl) oxime conjugated to bovine serum albumin. Sephadex LH-20 column chromatography was used to separate 18-hydroxycorticosterone from other steroids in serum samples. The minimal detectable amount of 18-hydroxycorticosterone was 50 pg/tube, and the measurable range was from 5 to 1000 ng/dl when a 1.0 ml serum sample was used. Intra- and inter-assay coefficients of variance were 5.0% (n=6) and 5.8% (n=6), respectively. In normal controls, the serum 18-hydroxycorticosterone level was 4.8 - 34.0 ng/dl (mean ± S.D. = 17.1 ± 9.0 ng/dl) on an unrestricted diet. Seven out of 8 patients with aldosterone-producing adenoma had above-normal serum 18-hydroxycorticosterone levels.
Serum 18-hydroxycorticosterone increased and decreased significantly following ACTH and dexamethasone administration, respectively. In essential hypertensive patients, serum 18-hydroxycorticosterone was high during a low-sodium diet and was suppressed remarkably by captopril. These observations support the previous reports that adrenal 18- hydroxycorticosterone synthesis is dependent on both ACTH and the renin-angiotensin system. The present method is sufficiently sensitive and producible, avoids the use of radioisotopes and is quite satisfactory for clinical use.
