Modification of arginine residues in ovine prolactin by 1,2‐cyclohexanedione: Effect on binding capacity to lactogenic receptors

The reactivity of arginine residues in ovine prolactin was studied by reaction with 1,2‐cyclohexanedione. Kinetic analysis of the data showed a good fit with two simultaneous pseudo‐first‐order equations with apparent velocity constants of 0.28 and 1.2 × 10⁻² min⁻¹, corresponding to 1.8 ‘fast’ and 8.7 ‘slow’ residues, respectively. Modification led to a decrease in binding capacity to lactogenic rat liver receptors, and apparently the modification of the two ‘fast’ reacting arginine residues is responsible for the rapid loss of this capacity. The presence of a non‐reacting arginine has been described in human and bovine growth hormones, and it is located near the carboxy‐terminus. This lack of reactivity is probably due to the formation of a salt bridge, since the arginine residue becomes susceptible to modification once the peptide is separated from the rest of the molecule. This salt bridge is absent in ovine prolactin, since the homologous arginine residue is reactive with cyclohexanedione. This result suggests that there could be a difference between the three‐dimensional structure of ovine prolactin and of the growth hormones, at least near the carboxy‐terminal region of the molecule.