Characterization of hormonogenic sites in an N-terminal, cyanogen bromide fragment of human thyroglobulin.

We have confirmed the observation of Marriq et al. (FEBS Lett. 207, 302-306, 1986) that substantial thyroxine formation occurs on enzymatic iodination of a 171-residue, N-terminal cyanogen bromide (CNBr) fragment of human goiter thyroglobulin. Marriq et al. concluded from their studies that Tyr130 is the donor for the preferential hormonogenic acceptor site Tyr5. However, in the present study we observed that thyroxine formation in the CNBr fragment occurred not at Tyr5, but rather at Tyr130. Moreover, we did not confirm the report of Marriq et al. that formation of thyroxine in the iodinated CNBr fragment involves cleavage of the Val129-Tyr130 bond. Our finding of thyroid hormone at Tyr130 was unexpected, as Tyr5 is known to be the major site of hormone formation in thyroglobulin isolated from the thyroids of humans and other mammals. In the CNBr fragment, however, Tyr5 appeared to act as a donor rather than an acceptor. Thus, while we have confirmed the finding that the N-terminal CNBr fragment of human goiter thyroglobulin is by itself capable of efficient thyroxine formation when enzymatically iodinated in vitro, we observed that the role of specific tyrosines in the hormone-forming process was different from that in intact thyroglobulin. We question, therefore, whether the CNBr fragment is a valid model for defining the role and location of hormonogenic sites in intact thyroglobulin.