Characterization of T₄-Binding Globulin Cleaved by Human Leukocyte Elastase

Abstract

T(4)-binding globulin (TBG) serves to maintain an important serum pool of thyroid hormones and to prevent their excessive loss in urine. TBG has also been implicated in the tissue distribution and targeted delivery of the hormones, the mechanisms of which remain unclear. By virtue of sequence homology, TBG belongs to the serine proteinase inhibitors superfamily of proteins that are characterized by a reactive site loop serving as a recognition site for serine proteinases. However, both TBG and another serpin with hormone transport function, corticosteroid-binding globulin, are noninhibitory. Cleavage of corticosteroid-binding globulin by human leukocyte elastase results in the reduction of its hormone-binding affinity and capacity. In this communication we confirm previous observations that TBG is also cleaved by elastase and undergoes the characteristic conformational changes. In addition, contrary to a previous report, the present work demonstrates that the cleaved product has reduced T(4)-binding affinity and, as expected, increased heat stability. Additional fragmentation of the molecule results in the loss of the hormone-binding site that is in agreement with a recent in vivo observation of apparent consumption at sites of inflammation. These data suggest that TBG may play a role in the targeted delivery of thyroid hormones to tissues rich in proteinases.

Chemistry and Biology in the Biosynthesis and Action of Thyroid Hormones

“…[53] Cleavage of the RCL in TBG by proteases during sepsis results in an almost threefold decrease in the thyroxinebinding affinity. [54] In the blood, T4 is mainly carried by TBG, with adissociation constant (K d )for the TBG-T4 complex of 0.1 nm. [50] Ar ecent crystal structure of the TBG-T4 complex shows that protein carries T4 in as urface cavity between helices Ha nd Aa nd strands 3-5 of the B-sheet (Figure 4A).…”

Effect of mutations of the human serpin protein corticosteroid-binding globulin on cortisol-binding, thermal and protease sensitivity

“…The importance of the RCL-region for heat and protease sensitivity and the impact of this region on the serpin conformation and the ligand binding properties were shown by Janssen et al and Grasberger et al [24,25]. They designed TBG-alpha1-proteinase inhibitor chimeras modified in the loop region.…”

The S-to-R Transition of Corticosteroid-Binding Globulin and the Mechanism of Hormone Release

“…A similar but less marked (3-fold) decrease in affinity occurs with the S-to-R change in TBG, similarly resulting in the release of thyroxine at inflammatory sites. 13 However, little is known about how exactly the S-to-R conformational changes of the serpins affect the hormone binding site. Recent crystal structures of hormone-complexed native TBG and CBG show that there is unlikely to be any *Corresponding author.…”