METHOD

Preparation of 13II-labelled growth hormone. Reaction is carried out in the rubber-capped vial (10 ml.) in which the ['31I] iodide sample is packaged. Care is taken that the vial is kept upright in transport by requesting delivery in a 1 in. lead pot. This ensures that the small volume (0 05 ml.) of [1311] iodide solution is not spread over the surface of the vial and its cap.

The radioactivity is counted by using a Panax scintillation counter (USC-B) with a well-type sodium iodide crystal with the geometry modified such that 2mc in 0-05 ml. gives approx. 400 counts/see. The lid of the lead castle is removed and the sample, ina 1 in. lead pot, placed 30 cm. above the aperture. The rubber cap is removed after the preparation is completed, and counted separately, with the same geometry. To the sample is added 0-025 ml. of 0-5M-phosphate buffer, pH 7-5. Immediately thereafter growth hormone (5, ug.) followed by fresh chloramine-T (100, ug.), each in 0-025 ml. of 0 05M-phosphate buffer, pH 7.5, are added. After each addition, made by injection through the rubber cap with an Agla micrometer syringe. the contents of the vial are briefly mixed. Immediately after mixing the chloramine-T, sodium metabisulphite is added [0-1 ml. of a solution (2-4 mg./ml.) in 0-05M-phosphate buffer, pH 7-5]. This addition prevents any subsequent iodination of the Sephadex-gel bed by converting [131I] iodine into [181I]-iodide. This residual iodide is diluted with carrier KI [0-2 ml. of a solution (10mg./ml.) in the 0 05M-phosphate buffer]. The reaction mixture is transferred to a Sephadex column followed by a single wash with further KI [0-4 ml. of a solution (10 mg./ml.) in the 005M-phosphate buffer]. The reaction vial and pipette are then counted. Separation of 131I-labelled growth hormone from the reaction mixture is carried out by gel-filtration with a