Preparation of125I-Labeled Human Growth Hormone of High Quality Binding Properties Endowed with Long-Term Stability

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Abstract
125I-labeled human growth hormone (125I-labeled. hGH) was prepared by using two variants of the chloramine T labelling procedure and purified by polyacrylamide gel electrophoresis (PAGE) of the reaction mixture. Variant A produced a tracer with high specific activity (100 ± 10 μCi/μg), high maximal binding capacity to antibodies (93%) and long-term stability (at least 150 days at -20°C). No diiodinated tyrosil residues could be detected in this tracer. Variant B was devised to obtain higher yields of labeled hormone. The electrophoresis of the iodination mixture revealed two radioactive components with Rm values of 0.49 and 0.55 which result from the iodination of hGH variants preexisting in the starting material. Both tracers had similar specific activities (70 ± 10 μCi/μg), high maximal binding capacity to antibodies or receptors (80–100%, after 80 days of their obtention) and high stability (at least 100 days at −20°C). It is concluded that the iododerivatives of hGH obtained by either method are adequate to perform radioimmunoassay and receptor studies and have longterm stability.