Tyrosine residues of bovine thyrotropin. Accessibility to iodination in the intact hormone and isolated subunits.
The susceptibility of the tyrosines of bovine thyrotropin (bTSH) and of its isolated alpha and beta subunits to lactoperoxidase catalyzed iodination was examined as a probe of the reactivity of these residues. In isolated TSH alpha, tyrosines at positions 21, 41, 92 and 93 were labeled with radioactive iodine to a nearly equivalent extent while residue 30 did not incorporate iodine. In intact TSH tyrosine 41, as well as 30, was not modified showing that, upon association with beta subunit, residue 41 becomes substantially less reactive. The other three tyrosines of the alpha subunit retained reactivity; residue 21 incorporated relatively more label than residues 92 and 93. The pattern of reactivity of the TSH alpha tyrosines in intact TSH parallels previous studies on the alpha subunits of lutropin (LH) and human choriogonadotropin (hCG). Thus the different primary sequences of the beta subunits do not influence environments around tyrosines at positions which are spaced throughout the alpha subunit structure. Most of the 11 tyrosine residues of TSH beta incorporate significant amounts of iodine. In intact TSH, beta tyrosines 25, 37, and 119 are most readily iodinated and tyrosines 14, 25 and 37 are the most reactive in the isolated subunit. Of particular interest in intact TSH is that tyrosine 37 and to a lesser extent 61 are modified. These tyrosines are found in analogous and highly conserved domains in the beta subunits of all glycoprotein hormones whose sequences have been determined, but in LH and hCG they can only be iodinated in isolated beta subunits.