Receptor binding and conformational properties of bovine and ovine prolactins after chemical modification of the two tryptophan residues.

The two tryptophan residues in ovine and bovine prolactins were modified by reaction with 2-nitrophenylsulfenyl chloride in 75% formic acid. These derivatives exhibited an important loss of receptor affinity (less than 1% of the native hormones) to a rabbit mammary gland preparation. To a lesser degree, they also lost their binding affinity to specific guinea pig antibodies as detected by radioimmunoassay. The chemical modifications induced a change in the folding of the polypeptide chain, which in itself could be partly or totally responsible for the loss of biological or binding activities. This conformational change has been analyzed by circular dichroism and by prediction of secondary structures from the amino acid sequence using the method of Garnier et al. (Garnier, J., Osguthorpe, D.J. and Robson, B. (1978) J. Mol. Biol. 120, 97--120). The comparison of predicted prolactin and somatotropin structures revealed almost identical alpha-helix, turns and coil regions with an overall content of 67% alpha-helix, 5% beta-sheet, 17% turn and 11% of aperiodic structures. These values were close to those obtained from circular dichroism. The conformational change of the chemically modified hormones as compared to native folding, can be described as a partial loss of alpha-helical structure and an increase in beta-sheet content.