Studies on the Disulfide Bonds of Glycoprotein Hormones

To gain information concerning the location of the five disulfide bonds of the common, α, subunit of pituitary thyrotropin and luteinizing hormone, bovine thyrotropin-α and luteinizing hormone-α were partially reduced with low concentrations of dithioerythritol, and the resulting cysteine residues were labeled with [1-14C]iodoacetic acid. The labeled subunits were then completely reduced and carboxymethylated with nonradioactive iodoacetic acid. Peptides containing S-carboxymethylcysteine were isolated from samples labeled after different conditions of partial reduction. Determination of the specific radioactivities of the S-carboxymethylcysteines indicated the initial simultaneous opening of two bridges involving half-cystines at positions 11, 14, 35, and 36 of the α sequence (Liao, T.-H., and Pierce, J. G. (1971) J. Biol. Chem. 246, 850–865) with disulfide bonds most likely between half-cystines 11, 14 with either 35 and 36, or 36 and 35. The distribution of radioactivity in the other half-cystine positions indicates bonds between residues 32–64, 63–91, and 86–88.