Photochemical transformation of the thyroid hormone levothyroxine in aqueous solution.

PURPOSE

The direct aqueous photolysis of the thyroid hormone levothyroxine (T(4)) has been studied.

METHODS AND RESULT

One of the major photoproducts, i.e., 4-[4-(2-amino-2-carboxy-ethyl)-2,6-diiodo-phenoxy]-penta-2,4-dienoic acid (P1), was isolated by liquid chromatography and structurally assigned by mass spectrometric (MS) and nuclear magnetic resonance spectroscopic methods. The identity of a second major product, i.e., 3,5-diiodo-L: -thyrosine (P3), was confirmed through access to a commercially available standard. Furthermore, the structures of three additional transformation products are proposed on the basis of data obtained by high-resolution MS analyses. UV absorption spectra were determined for T(4) and the two photoproducts P1 and P3. Disappearance quantum yields were calculated for T(4) (ϕ = 0.014 at pH 12) and P3 (ϕ = 0.024 at pH 12 and ϕ = 0.010 at pH 8.5), whereas the compound P1 was found to be stable under the studied conditions (T(1/2) = 600 min).

CONCLUSION

The results indicate that solar UV light may have a significant impact on the fate of T(4) in the aquatic environment.