Ethoxyformylation of bovine growth hormone

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Abstract
Reactivity of histidines in bovine growth hormone towards ethoxyformic anhydride was investigated and localization in the molecule of 2 kinetically distinguishable classes was achieved, a slow class including only histidine residue 169 (k = 0.180 min-1) and a fast one composed of histidines 19 and 21 (k = 0.900 min-1). Total ethoxyformylation of bovine growth hormone brought about a complete loss of its capacity to compete with 125I-labeled hormone for rat liver binding sites, but modification of approximately 1/2 of the fast histidine group was enough to produce an important decrease in this capacity. Circular dichroism studies indicated no significant changes in protein conformation with all 3 histidine residues modified. Practically full binding capacity was restored when these residues were regenerated by treatment with hydroxylamine. Apparently, one or both of the fast reacting histidine residues are involved in bovine growth hormone binding to its specific receptors.