Endocrinology, Vol 101, 350-359, Copyright © 1977 by Endocrine Society

ARTICLES

Conversion of radiolabeled human growth hormone into higher molecular weight moieties in human plasma in vivo and in vitro

IZ Beitins, MC Rattazzi and MH MacGillivray

Purified human growth hormone (hGH) was iodinated with 131I ([131I]iodo- hGH) and purified on Sephadex G-100. The monomeric [131I]iodo-hGH (mol wt about 20,000 daltons) was injected as a bolus iv 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 one 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 iv injection, larger and occasionally smaller immunoreactive moieties than the injected [131I]iodo-hGH appeared. When [131I]iodo-hGH was incubated at 37 C with fresh human plasma from 4 different individuals from 10 to 120 min, the larger molecular weight forms, noted in the in vivo studies, were again demonstrable. Only the monomeric [131I]iodo-hGH was found when [131I]iodo-hGH was incubated at 37 C for 120 min with human serum albumin or gamma globulin.