Studies on prolactin. 40. Chemical modification of tyrosine residues in the ovine hormone.

ABSTRACT

The seven tyrosine residues in ovine prolactin have been modified, in separate experiments, with tetranitromethane and iodine at 0 degrees C. Both the nitration and iodination studies indicate Tyr-44 to be the most reactive; this residue was converted almost completely to the dinitro and diiodo derivative under the reaction conditions employed. All other reactive tyrosine residues formed the monosubstituted derivative. The relative chemical reactivity of the seven tyrosine residues can be described as follows Tyr-44 and Tyr-195 are highly reactive, Tyr-147, Tyr-169 and Tyr-185 are partially reactive, with Try-96 and Tyr-28 being relatively unreactive under the conditions employed. The various derivatives have been characterized by exclusion chromatography, circular dichroism, rate of tryptic digestion, and biological activity. The modification of Tyr-44 possibly produces a conformational change which is observed on Sephadex G-100 as an equilibrium between two monomeric forms eluting at slightly different positions. These two forms also appear to display different side-chain CD spectra. Nitration or limited iodination does not appear to decrease the biological activity of the hormone, as measured in the pigeon crop-sac assay, although more extensive iodination does produce a significant loss in potency.