Iodine-131 labelled human growth hormone

A simple and rapid method is presented for the preparation, of ¹³¹I-labeled human growth hormone of high specific radioactivity (240-300 μc./γ).Low .amounts of carrier-free ¹³¹I^- (2 mc.) are allowed to react, without prior treatment, with small quantities of protein (5 γ) in a high-yield reaction (approx. 70% transfer of ¹³¹I to protein).The degree of chem. substitution is minimized (0.5-1.0 atom of I/mol. of protein) by the use of carrier-free ¹³¹I^-.The ¹³¹I-labeled hormone (up to 300 μc./γ) contains no detectable degradation products and is immunologically identical with the unlabeled hormone.The loss of immunological reactivity at high specific radioactivities or at high levels of chem. substitution with ¹²⁷I^- is demonstrated.

RECENT procedures for the immunological assay of protein hormones in human plasma1–3 require the routine preparation of hormones labelled with iodine-131 of high specific activity.