Characteristics of tryptophan binding in the serum of the newborn rat.

During the very first period of postnatal life, tryptophan is almost entirely free in the serum of rats. This situation is in sharp contrast with the well-known ability of serum albumin to bind the essential amino acid in the adult. Three main factors accounted for the relative lack of binding during the early postnatal life when compared to the adult: (1) the lower concentration of serum albumin, the binding protein; (2) the inhibition of binding by nonesterified fatty acids, which were at a high level in the serum of young rats until weaning, and (3) the decreased number of available binding sites for tryptophan on the defatted serum albimin, whereas the apparent association constant of tryptophan binding to serum albumin was similar in newborn and adult. Since immunological characterization of newborn and adult serum albumins did not reveal a specific fetal serum albumin, we suggest that discrete changes at the association site for tryptophan are sufficient to induce large alteration in the binding capacity of the protein. In contrast to the situation observed in adult rats, serotonin synthesis in the brain of newborn animals is therefore not dependent on the equilibrium between bound and free tryptophan in serum.