Identification of a major iodolipid from the horse thyroid gland as 2-iodohexadecanal

The incorporation of iodide into proteins (PBI) and lipids (LBI) of horse thyroid slices was measured in various conditions. Their dependency on the concentration of extracellular iodide was strikingly different. For PBI the relationship was biphasic with a decrease above 10 microM, likely to correspond to the Wolff-Chaikoff effect. On the contrary, LBI increased as a function of iodide concentration up to 100 microM. Methimazole (MMI) inhibited the incorporation of iodide into both LBI and PBI, but higher concentrations of MMI were required to depress LBI as compared to PBI. The inhibition of active iodide transport by NaCIO4 reduced both PBI and LBI. Chromatography on silica gel resolved almost equal amounts of low and high polarity iodolipids. The main unpolar iodolipid was identified as 2-iodohexadecanal (2-IHDA), on the basis of proton nuclear magnetic resonance spectroscopy, mass spectrometry, and co-elution with authentic 2-IHDA obtained by chemical synthesis in reversed-phase high performance liquid chromatography and gas chromatography. The presence of 2-IHDA was also detected in dog thyroid slices, following incubation with KI (50 microM) and in the rat thyroid, 4 hours after intraperitoneal injection of KI (650 micrograms). An incubation of bovine brain plasmalogens with lactoperoxidase, iodide, and H2O2 generated 2-IHDA. In conclusion, we have identified a major thyroid iodolipid as 2-iodohexadecanal. The biosynthesis of this compound is likely to involve the addition of iodine to the vinyl ether group of plasmalogens.