Iodinated phospholipids and the in vitro iodination of proteins of dog thyroid gland

Abstract

Slices of dog thyroid gland were incubated with liposomes consisting of 125I-labelled phosphatidylcholine (the iodine was covalently linked to unsaturated fatty acyl chains). The 125I label of 125I-labelled liposomes was incorporated into thyroid protein and/or thyroglobulin at a higher rate than was the 131I label of either Na131I or 131I2. The iodine was shown to be protein-bound by the co-migration of the labelled iodine with protein under conditions where free iodine, iodide and lipid-bound iodine were removed from protein. The uptake of iodine from the iodinated phospholipid was probably due to phospholipid exchange between the iodinated liposomes and the thyroid cell membrane, since (a) 14C-labelled phospholipid was metabolized to 14CO2 and (b) many lipids in the tissue slice became 14C-labelled. A very strong inhibition of iodide ‘uptake’ from Na131I, caused by thiosulphate, produced only a minor inhibition of the incorporation of 125I from 125I-labelled liposomes into thyroid protein and/or thyroglobulin. This implies that free iodide may not necessarily be formed from the iodinated phospholipids before their entrance or utilization in the cell. Synthetic polytyrosine polypeptide suspensions showed some iodination by 131I-labelled liposomes. In tissues with low tyrosine contents, such as liver and kidney, only a trace uptake was observed. Salivary gland showed some uptake. Endoplasmic reticulum of thyroid gland showed a higher iodine uptake than that of the corresponding plasma membranes. These experiments, together with the demonstration of the diet-dependent presence of iodinated phospholipids in dog thyroid, leads us to suggest that iodination of the membrane phospholipids of thyroid cells may be directly or indirectly involved at some stage in the synthesis of thyroglobulin, or exists as a scavenger mechanism, to re-utilize and/or recover released iodine from unstable compounds inside the thyroid cell.