Biosynthesis and regulation of iodolipids in calf thyroid

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Abstract
The incorporation of 125I into iodolipids was investigated in calf thyroid slices. Time course studies showed that the iodination of lipid reaches a plateau after 30 min of incubation. A highly significant correlation was found between iodination of lipid and of protein (r = 0.906), suggesting that both reactions may be related. Addition of propylthiouracil or methylmercaptoimidazol caused a significant inhibition of lipid iodination, indicating that a peroxidase could be involved in this reaction. The iodinated lipids are not attached to protein, since they migrated in BEA [n-butanol:ethanol:ammonium] chromatography and pancreation digestion of the samples did not modify the percentage radioactivity. The iodolipid fraction observed in BEA chromatography was eluted and analyzed by thin layer chromatography. Iodinated free fatty acids and neutral lipids comprised most of the radioactivity. Although iodolipids are present in every subcellular fraction, the 20,000 x g pellet had the greatest proportion of iodinated fatty acids and neutral lipids. The iodination of lipids is under the control of both TSH and triiodothyronine. These hormones caused a significant increase at 30-45 min of pulse labeling with 125I. Inhibition of prostaglandin synthesis with indomethacin caused a significant increase in lipid iodination. When phospholipase A2 was blocked by mepacrine, labeling of iodolipid was strongly decreased.