Acetylation of bovine growth hormone by N-acetylimidazole

Bovine growth hormone was chemically modified with N-acetylimidazole and the acetylated tyrosine residues were identified. Tyrosine 42 was the most reactive followed by tyrosines 158, 35 and 109 and to a lesser degree by tyrosines 141 and 174. Several lysine residues were also acetylated. The modified hormone retained the native alpha-helix content but showed a weak growth promoting activity and was unable to complete in vivo with 125I-labeled bovine growth hormone for the liver binding sites. The loss in growth promoting activity induced by acetylation was prevented by the amidination of lysine residues. These findings combined with results already published suggest that tyrosine residues are not important in the manifestation of the hormone growth promoting activity.