Conversion of Radiolabeled Human Growth Hormone into Higher Molecular Weight Moieties in Human Plasmain Vivoandin Vitro

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Abstract
Purified human growth hormone (hGH) was iodinated with 131I ([131I]iodo-hGH) and purified on Sephadex G-100. The monomeric [131I]iodo-hGH (MW about 20,000 daltons) was injected as a bolus i.v. in healthy volunteers and plasma obtained at 10,20 and 40 min. Then continuous infusion with [131I]iodo-hGH was started and further plasma samples obtained after 120 min when a plateau of immunoreactive hGH (IR-hGH) had been attained. In 1 individual the bolus injection was followed further to 60 and 120 min. Fresh plasma was immediately chromatographed on Sephadex G-100 and the IR-hGH profiles evaluated. In all instances following i.v. injection, larger and occasionally smaller immunoreactive moieties than the injected [131]iodo-hGH appeared. When [131I]iodo-hGH was incubated at 37.degree. C with fresh human plasma from 4 different individuals from 10 to 120 min, the larger MW forms, noted in the in vivo studies, were again demonstrable. The monomeric [131I]iodo-hGH was found when [131I]iodo-hGH was incubated at 37.degree. C for 120 min with human serum albumin or .gamma. globulin.

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