The preparation and use of [ 125I]steroid-albumin conjugates as tracers in steroid radioimmunoassays

Protein conjugates of a variety of steroids have been synthesised and labelled with 125I for use as tracers in radioimmunoassays. Using a coated tube technique these tracers have been compared with 3H tracers for their effect on the sensitivity and specificity of the assays. The [ 125I]steroid-protein conjugates were easily prepared with specific activities g3nerally in the range of 100–250 μC/μg. They were stable for severalweeks at 4°. When estrogen-protein conjugates were iodinated 125I was incorporated into the A ring of the estrogen as well as into the protein. The sensitivity and specificity of the assays was found to depend upon the nature of the tracer but not upon its specific activity. The effects seen depended on the nature of the antiserum and the conditions of the assay.