Free and bound tryptophan in human plasma during the perinatal period

The concentration of tryptophan and the degree of binding of the amino acid to protein were examined in human plasma during the perinatal period. Both total and unbound (free) tryptophan were higher in cord vein plasma than in the maternal circulation, the concentration gradient being approximately 1 : 2. The proportion of the total plasma tryptophan concentration that was not bound to protein was less in cord vein plasma than in the maternal circulation. After birth the proportion in infant plasma fell significantly. Both total and free tryptophan fell during the first 24 hours of postnatal life. Total tryptophan returned to the cord vein plasma level 6--8 days after birth whilst free tryptophan failed to increase during the period of the observations. In premature infants total and free tryptophan also declined in concentration 12--24 hours after birth, suggesting the phenomenon to be related to birth rather than to gestational age. Phenylalanine remained unchanged whilst tyrosine increased in concentration during the first 80 hours of postnatal life. Thus, the availability of tryptophan to the tissues appears to decline during the immediate postnatal period and the results suggest that the requirement for tryptophan during this time may exceed the supply from standard artifical milk preparations.