J. Biol. Chem., Vol. 262, Issue 4, 1623-1632, 02, 1987
The iodination sites of bovine thyrotropin
PG Stanton and MT Hearn
Iodination of bovine thyrotropin (TSH) using a lactoperoxidase- catalyzed
labeling method at pH 5.6 results in modification of both the alpha- and
beta-subunits. In particular, 3 of the 5 tyrosine residues of the
alpha-subunit and 9 of the 11 tyrosine residues in the beta- subunit are
accessible to surface iodination. However, the reactivity of these tyrosine
residues in bovine TSH toward iodination under these enzyme-catalyzed
conditions follows the order alpha-Tyr-21 much greater than alpha-Tyr-92,
-93, approximately equal to beta-Tyr-45, -54 greater than beta-Tyr-74
greater than beta-Tyr-18 approximately equal to beta- Tyr-112 greater than
beta-Tyr-104 approximately equal to beta-Tyr-92 greater than beta-Tyr-7
greater than beta-Tyr-77. From reversed-phase high-performance liquid
chromatography tryptic mapping, leucyl aminopeptidase M digestion, and
microsequence analysis, it is clear that diiodination of the tyrosine
residues is not favored for the beta- subunit with the exception of
beta-Tyr-7, whereas diiodination was observed with alpha-Tyr-21 and
alpha-Tyr-92/93. These data on iodination sites are evaluated in terms of
the known receptor binding features of iodinated bovine TSH preparations as
well as in terms of the surface accessibility of these specific residues as
predicted from topographical algorithms based on an analysis of hydrophilic
and hydrophobic regions of the subunits. The results provide an explanation
for the anomalously low bound/total tracer ratio frequently observed in
radioreceptor assay procedures for TSH and suggest a basis for further
evaluation of the determinant loops associated with the hormone specificity
of the beta-subunit.
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati What's this?
