The effects of homogeneous human prealbumin on in vitro and in vivo immune responses in the mouse

Transthyretin as a thyroid hormone carrier: function revisited

Thyroid hormones are essential for normal mammalian development and for normal metabolism. Thyroxine (T4) is the principal product synthesized by the thyroid follicles, and triiodothyronine (T3), the biologically active hormone, derives mainly from tissue T4 deiodination. More than 99% of the circulating hormone is bound to plasma proteins, mainly to thyroxine-binding globulin, transthyretin and albumin in man, and to transthyretin and albumin in rodents. The role of plasma proteins in the transport of hormones to target tissues has, for a long time, been controversial. The liver and the choroid plexus are the major sites of transthyretin synthesis, tissues from which transthyretin is secreted into the blood and the cerebrospinal fluid, respectively. Transthyretin has been proposed to mediate thyroid hormone transfer into the tissues, particularly into the brain across the choroid-plexus-cerebrospinal fluid barrier. Studies in a transthyretin-null mice strain have shown conclusively that transthyretin is not indespensable for thyroid hormones' entry into the brain and other tissues, nor for the maintenance of an euthyroid status. An euthyroid status is also observed in man totally deprived of thyroxine-binding globulin and in rats without albumin. Taken together, these results exclude dependence of thyroid hormone homeostasis on any major plasma carrier per se. This evidence agrees with the free hormone hypothesis which states that the biologically significant fraction, that is taken up by the tissues, is the free circulating hormone.

Transthyretin microheterogeneity and thyroxine binding are influenced by non-amino acid components and glutathione constituents

Two non-amino acid components as well as the glutathione constituents in labile associations with transthyretin (TTR) have been detected by preparative polyacrylamide gel electrophoresis from preparations isolated by affinity chromatography on Sepharose-bound retinol-binding protein (RBP). Incubation of native or reduced TTR with these novel components influenced the quaternary structure and caused reactions with reduced TTR in particular. Reduction of isolated TTR monomers released cysteine from the quantitatively major monomer, but non-amino-acid components from another dominating monomer. The reaction patterns also influence thyroxine (T4) binding. These relationships indicate that interactions in serum of TTR with constituents of glutathione and components different from T4 and retinol-RBP are important for the metabolism and function of TTR.

Immuno-enhancing activity of the amino-terminal domain of human prealbumin: isolation, characterization and synthesis

A decapeptide isolated from highly purified preparations of human prealbumin was able to restore azathioprine (Az) sensitivity, a property of a sub-class of T-lymphocytes, to the spleen rosette-forming cells (RFC) of adult thymectomized (ATx) mice in vitro. The peptide was sequenced by the Edman method and shown to correspond to the ten amino-terminal residues of prealbumin, Gly-Pro-Thr-Gly-Thr-Gly-Glu-Ser-Lys-Cys. Synthesis of this peptide by solid phase methodology confirmed its activity both in vitro and in vivo. Synthesis of a number of structural analogues indicated that the amino-terminal deca, undeca and dodecapeptides of prealbumin as well as some of their derivatives were also able to restore Az sensitivity to RFC in vitro and in vivo. The Cys10 residue and the Glu7 residues both contributed significantly to potency in vitro. Removal of up to three amino acids from the N-terminus of the decapeptide led to a progressive loss of activity. The data indicates that the ability of human prealbumin to restore the Az sensitivity to the RFC of adult Tx mice is intrinsic to the protein and resides in the amino-terminal domain of the molecule.