The binding of thyroid hormones to phospholipid membranes.

1. Thyroxine and tri-iodothyronine are bound by liposomes (bimolecular phospholipid membranes produced by dispersing egg-yolk lecithin in water). At pH 7.4 the ;partition coefficient' between water and the phospholipid is 1.2 x 10(4) for thyroxine and 2.2 x 10(4) for tri-iodothyronine. The partition coefficient falls off rapidly in more alkaline solutions as the phenolic group on the hormones becomes ionized.2. The binding of thyroxine by membranes is very rapid, occurring at rates equivalent to those found in the binding of thyroxine by serum.3. Lecithin membranes are readily permeable to thyroxine (the oil:water partition coefficient is 1.7).4. The binding of thyroid hormones by perfused rat hearts and the binding of thyroid hormones by liposomes are similar in their sensitivity to pH. Simple alcoholic extracts of tissues have strong thyroxine-binding activity.5. These results are discussed in relation to the nature and function of the tissue thyroxine-binding reaction.