Luteinizing Hormone. 2. Relative Reactivities of Tyrosyl Residues of the Porcine Hormone Towards Iodination

The iodination of porcine luteinizing hormone and its subunits, in the presence of lactoperoxidase and hydrogen peroxide has been studied as a function of iodide concentration. The maximum incorporations were 11 iodine atoms per molecule of hormone while it was possible to incorporate up to 10 and 5 iodine atoms per molecule in the isolated α and β subunits respectively. Both tyrosyl residues of the β subunit (β-37 and β-59), readily iodinated when the subunit is isolated, are found to be non-reactive in the native hormone. They could thus be located on poly peptide portions involved in the binding of the subunits. The reactivities of the tyrosines of porcine luteinizing hormone were found to be in the order: α-21 > α-92 ≃α-93 > α-41 > α-30 ≃β-37 ≃β-59. The susceptibility of peptidic bonds α-20—α-21 and α-24–α-25 to trypsin indicates that this part of the molecule, containing Tyr-α-21, is at the surface of the hormone in aqueous solution. Although the Lys-α-95—Ser-α-96 bond is susceptible to trypsin it is not cleaved by carboxypeptidases. Thus, the carboxy-terminal part of the α chain in the native hormone is at the surface of the molecule but its carboxyl end is probably shielded.