Structural characterization of iodinated bovine growth hormone

Bovine growth hormone (bGH) was submitted to iodination using limited amounts of oxidizing reagent, yielding a derivative with no more than 1-g-atom of iodine per mole of hormone. Analysis of the hydrolysis products indicated that monoiodotyrosine was almost the only product of substitution. Isolation and identification of the tryptic fragments showed that half of the 125I-labeled bGH molecules were iodinated in Tyr 174, followed by Tyr 158 (16%) and Tyr 42 (14%). Frontal gel chromatography indicated that the preparation did not contain significant amounts of unreacted bGH. Circular dichroism evidenced structural similarity between the native and the iodinated bGH. The iodinated hormone, like the native protein, undergoes self-association. The dissociation constant of the iodo-labeled bGH self-association equilibrium showed a two-fold increase when compared to that corresponding to the unlabeled hormone. At pH 8.5, where the equilibrium constant was estimated, one tenth of the molecules bear a charged iodotyrosyl residue (average pKapp = 9.3), which could account for part, if not all, of the observed difference regarding self-association. By this criterion, the presence of the iodine atom does not disturb the area engaged in dimer formation.