TYROSINE RESIDUES OF BOVINE THYROTROPIN - ACCESSIBILITY TO IODINATION IN THE INTACT HORMONE AND ISOLATED SUBUNITS

  • 1 January 1984
    • journal article
    • research article
    • Vol. 24  (4) , 328-336
Abstract
The susceptibility of the Tyr [tyrosines] of bovine TSH (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., Tyr at positions 21, 41, 92 and 93 were labeled with radioactive iodine to a nearly equivalent extent while residue 30 did not incorporate I. In intact TSH Tyr 41, as well as 30, was not modified showing that, upon association with .beta. subunit, residue 41 becomes substantially less reactive. The other 3 Tyr 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. Tyr 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 Tyr at positions which are spaced throughout the .alpha. subunit structure. Most of the 11 Tyr residues of TSH.beta. incorporate significant amounts of I. In intact TSH, .beta. Tyr 25, 37 and 119 were most readily iodinated and Tyr 14, 25 and 37 were the most reactive in the isolated subunit. Of particular interest in intact TSH is that Tyr 37 and to a lesser extent 61 were modified. These Tyr 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.