Radioimmunoassay of human sex hormone binding globulin: improved radioiodination procedure.

Sex hormone binding globulin (SHBG), purified by affinity chromatography from retroplacental blood plasma, was reacted with 3-(p-hydroxyphenyl) propionic acid N-hydroxysuccinimidyl ester (PHPPS, Bolton-Hunter reagent). The derivative of SHBG obtained (parahydroxyphenylpropionyl-SHBG; PHPP-SHBG) was stable and could, in contrast to underived SHBG, be efficiently 125I-iodinated with a lactoperoxidase technique. The PHPP-SHBG labelled with 125I had good antiserum binding and stability properties and was used for radioimmunoassay (RIA) of SHBG in serum. The RIA requires a total incubation time of 3 h. It has been standardized with purified SHBG and has a sensitivity of 5 micrograms/l, giving a lowest detectable concentration in the routine procedure (samples diluted 1:40) of about 0.2 mg/l. Variation within and between assay was 4.1% and 7.2%, respectively, for samples with values within the normal range. Values obtained by this RIA procedure correlate well with those obtained by a dihydrotestosterone binding method and by an electroimmunoassay technique. The mean serum concentration of SHBG in healthy, regularly menstruating women (n = 42) was 3.7 +/- 1.0 (SD, standard deviation) mg/l and in healthy men (n = 100) 2.0 +/- 0.9 mg/l.