Homeostatic regulation of free retinol-binding protein and free thyroxine pools of plasma by their plasma carrier proteins in chicken.

The mechanism underlying homeostatic regulation of the plasma levels of free retinol-binding protein and free thyroxine, the systemic distribution of which is of great importance, has been investigated. A simple method has been developed to determine the rate of dissociation of a ligand from the binding protein. Analysis of the dissociation process of retinol-binding protein from prealbumin-2 reveals that the free retinol-binding protein pool undergoes massive flux, and that prealbumin-2 participates in homeostatic regulation of the free retinol-binding protein pool. Studies on the dissociation process of thyroxine from its plasma carrier proteins show that the various plasma carrier proteins share two roles. Of the two types of protein, the thyroxine-binding globulin (the high affinity binding protein) contributes only 27% of the free thyroxine in a rapid transition process, despite its being the major binding protein. But prealbumin-2, which has lower affinity towards thyroxine, participates mainly in a rapid flux of the free thyroxine pool. Thus thyroxine-binding globulin acts predominantly as a plasma reservoir of thyroxine, and also probably in the 'buffering' action on plasma free thyroxine level, in the long term, while prealbumin-2 participates mainly in the maintenance of constancy of free thyroxine levels even in the short term. The existence of these two types of binding protein facilitates compensation for the metabolic flux of the free ligand and maintenance of the thyroxine pool within a very narrow range.