Iodinated thyrolipids: their possible role in homonogenesis

A total of 42 human thyroid glands (nodular goitre 9, adenoma with cystic degeneration 6; toxic goitre 10; carcinoma 14; and normal thyroid gland 3) were examined in vitro for iodination of an unidentified polar non-phosphatide lipid fraction (fraction II). The radioiodine incorporation in fraction II was 49.3 %, 43.6 %, 32.8 %, 20.7 %, and 22.0 % respectively in normal, nodular goitre, adenoma with cystic degeneration, toxic goitre and thyroid carcinoma.

In vitro studies with surviving sheep thyroid slices did not show any relation between the iodination of fraction II and thyroxine formation over a period of 120 min.

However, a highly significant correlation (r-value= 0.96375) was observed between the iodination of fraction II and thyroxine formation in vivo in the rat thyroid gland over a period of 24 h.

We have previously postulated that iodination of fraction II may be interrelated to thyroxine formation. In the light of this hypothesis and the above results we suggest that the iodination of fraction II and thyroxine formation in the thyroid gland may be interrelated, the degree of iodination of fraction II being modulated by the amount of thyroxine formed within the thyroid gland.

Iodinated thyrolipids

Sheep, bovine, mouse, dog and human thyroids incorporate 35–55% (41.5%, 45.9%, 34.4%, 55.4% and 41.1%, respectively) of total lipid radiodine in an unidentified polar thyrolipid fraction moving between phosphatidyl ethanolamine and phosphatidic acid on silica gel G thin layer chromatography in a solvent system CHCl3:MeOH:H2O (80:25:3). This unidentified lipid fraction (Fr. II) is ninhydrin negative, phosphorus negative, alkali labile, acid stable, phospholiphase A stable and orcinol positive indicating the absence of a free amino group, phosphorus group, vinyl ether linkage, vicinal phosphorus ester linkage and the presence of a sugar moiety.

Iodination of fraction II seems to be specific for the thyroid gland only, as the other iodide concentrating tissues such as the submaxillary salivary gland and stomach localise most of the radioiodine with phosphatidyl choline and lyso-phosphatidyl ethanolamine fraction respectively. Biologically inactive thyroid tissue or non-enzymic iodination of isolated thyrolipids indicate phosphatidyl choline and/or phosphatidyl ethanolamine as major iodinated lipid fractions. The possible role of fraction II in iodination for homornogenesis in thyroid tissue is discussed.