Tetra-iodothyronine

The formation of triiodothyronine (T3) and tetra-iodothyronine (thyroxine T4) (see Figure 42—2) illustrates many of the principles of diversity discussed in this chapter. These hormones require a rare element (iodine) for bioactivity they are synthesized as part of a very large precursor molecule (thyroglobuhn) they are stored in an intracellular reservoir (colloid) and there is peripheral conversion of T4 to T3, which is a much more active hormone. 

Two amino acids, tyrosine and arginine are of particular importance as precursors of signalling molecules. As outlined in Figure 4.3, tyrosine is the amino acid precursor of thyroid hormones tri-iodothyronine (T3) and tetra-iodothyronine (T4) and also of catecholamines adrenaline (epinephrine) and noradrenaline (norepinephrine). 

Representatives of non-steroidal ligands, binding to nuclear receptors are T4 (tetra-iodothyronine thyroxine) and T3 (tri-iodothyronine) (Fig. 11.9). 

A lecturer in pathological chemistry at the University College Hospital Medical School in London, who was later to become Sir Charles Harrington, elucidated the structure of thyroxine. The degradation of the crystalline compound and the synthesis of an active hormone established that thyroxine was 3,5,3 5 -tetra-iodothyronine. 

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