Transformation of arachidonic acid into an Iodolactone by the rat thyroid

In the presence of iodide and hydrogen peroxide, lactoperoxidase, an enzyme model for thyroid peroxidase, catalyzed the conversion of arachidonic acid into several iodinated products. The major product was identified as 6-iodo-5-hydroxy-eicosatrienoic acid, δ-lactone (iodolactone), on the basis of ¹²⁵I incorporation, mass spectrometry, proton magnetic resonance spectroscopy, and chemical modifications. Using this compound as a standard, two methods were developed to establish and quantitate the production of iodolactone by the rat thyroid in vitro: ¹²⁵I labeling following by reversed phase high pressure liquid chromatography and combined gas chromatography-mass spectrometry. Addition of iodide and arachidonic acid to rat thyroid lobes resulted in the formation and release of the iodolactone, which was inhibited by methimazole. These data suggest that peroxidases capable of oxidizing halides could provide a new pathway of arachidonic acid metabolism, besides cyclooxygenase and lipoxygenases.