Differential transmembrane diffusion of triiodothyronine and thyroxine in liposomes:: Regulation by lipid composition

How thyroid hormones move across biological or model membranes is a subject of controversy. The passage of the 3,5,3' triiodo L-thyronine and 3,5,3',5' tetraiodo L-thyronine across model membranes was evaluated by the addition of the hormones to liposomes containing 2,4,6-trinitrobenzene sulfonic acid. Results indicate that hormones can react with an amino-reactive compound pre-encapsulated into phosphatidylcholine liposomes. The transversal motions of thyroid hormones were characterized by using physiological concentration levels of (I-125) 3,5,3' triiodo L-thyronine and (I-125) 3,5,3',5' tetraiodo L-thyronine. The hormone distribution between the two monolayers was time-dependent and kinetic data were fitted to a single exponential. Results obtained show that 3,5,3' triiodo L-thyronine can permeate phospholipid membranes and the diffusion time increases in the gel and liquid-ordered phase. On the contrary, 3,5,3', 5' tetraiodo L-thyronine could not diffuse the liposomal membrane from dimyristoyl and dipalmitoyl phosphatidylcholine in gel phase and egg yolk phosphatidylcholine:cholesterol in the liquid-ordered phase. Our results in the liquid-ordered phase suggest that diffusion movement of thyroid hormones across cell membranes depends on the amount of cholesterol in the bilayer.