The Effect of Acetic Acid on the Association and the State of the Tyrosine Residue of an Active Fragment of Bovine Growth Hormone

Self-association and the state of a tyrosine residue of the active fragment of bovine growth hormone were investigated by gel filtration and difference spectroscopy with various concentrations of acetic acid. In acetic acid solutions more concentrated than 1.5 m, the active fragment was found to exist as a monomeric form. Decrease in the concentration of acetic acid promoted self-association of the active fragment. Dimeric and trimeric forms were observed in 0.1 m acetic acid. By difference spectroscopy, the tyrosine residue in the active fragment showed a typical blue shift in 2.5 M acetic acid compared to 0.5 M acetic acid. The transition concentration, 1.25 M of acetic acid for the association-dissociation of the fragment was consistent with that for the appearance of the blue shift difference spectrum. It was concluded that a tyrosine residue in the active fragment was exposed on the surface of the fragment molecule in the monomer form and was held between the fragment molecules under conditions in which the active fragment forms dimer and trimer.