Preparation and biological reactivity of polyiodinated human growth hormone

Studies with iodinated human GH (hGH) were undertaken to characterize the iodinated product and to evaluate the possibility that iodination might distort the hGH molecule sufficiently to interfere preferentially with binding to one or more classes of receptors on adipocytes. hGH containing an average of 1.2 atoms of 125I/molecule was subjected to cleavage with cyanogen bromide, and the cleavage products were examined by autoradiography after electrophoresis of the lyophilized reaction mixture on polyacrylamide gel. 125I was found to be distributed in the fragments according to their content of tyrosine, suggesting that there is no single tyrosine residue that is iodinated preferentially. Digestion of the iodinated hormone with pronase followed by paper chromatography revealed that 7.6 times as much iodine appeared as MIT than as DIT, suggesting that the incorporation of 125I into a tyrosine residue had little influence on the incorporation of a second iodine atom. By increasing the molar ratios of iodide and chloramine-T to hGH in the reaction mixture, increasing amounts of iodine were incorporated into hGH in a highly predictable and reproducible manner until a product containing 11-12 atoms of iodine/molecule hGH was obtained. The ratio of MIT to DIT decreased progressively with increasing incorporation of iodine. The data suggest that only six of the eight tyrosine residues of hGH are accessible for iodination. Iodinated hGH retained full biological potency with respect to stimulation of glucose oxidation (insulin-like effect) and lipolysis even after incorporation of 4.6 atoms of I/molecule hGH. Two preparations containing 9.4 and 9.6 atoms of I/molecule hGH retained full biological potency, whereas another preparation containing 9.4 atoms/molecule had approximately one third the potency of uniodinated hGH with respect to both activities. Some biological potency, at least with respect to stimulation of lipolysis, was retained even in maximally iodinated preparations. We conclude that iodination occurs randomly among the six accessible tyrosine residues, and even after polyiodination, the interaction of hGH with its receptors in adipocytes is not completely compromised.