Rn. Chehin et al., Differential transmembrane diffusion of triiodothyronine and thyroxine in liposomes: Regulation by lipid composition, J MEMBR BIO, 167(3), 1999, pp. 251-256
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 additio
n of the hormones to liposomes containing 2,4,6-trinitrobenzene sulfonic ac
id. Results indicate that hormones can react with an amino-reactive compoun
d pre-encapsulated into phosphatidylcholine liposomes. The transversal moti
ons of thyroid hormones were characterized by using physiological concentra
tion levels of (I-125) 3,5,3' triiodo L-thyronine and (I-125) 3,5,3',5' tet
raiodo L-thyronine. The hormone distribution between the two monolayers was
time-dependent and kinetic data were fitted to a single exponential. Resul
ts obtained show that 3,5,3' triiodo L-thyronine can permeate phospholipid
membranes and the diffusion time increases in the gel and liquid-ordered ph
ase. On the contrary, 3,5,3', 5' tetraiodo L-thyronine could not diffuse th
e liposomal membrane from dimyristoyl and dipalmitoyl phosphatidylcholine i
n gel phase and egg yolk phosphatidylcholine:cholesterol in the liquid-orde
red phase. Our results in the liquid-ordered phase suggest that diffusion m
ovement of thyroid hormones across cell membranes depends on the amount of
cholesterol in the bilayer.