Tryptophan Binding to Serum Protein

For many years, it has been known that L-tryptophan in blood binds to serum albumin, which is then transported in this manner throughout the circulatory system. McMenamy153 described this binding, and it is also reviewed in Chapter 5. Normally, L-tryptophan in the blood is approximately 85% bound to albumin and 15% free. 

It became of interest to determine whether L-tryptophan that enters cells may bind to cytosolic proteins. This was tested by orally administering L-tryp-tophan or nothing to fasted rats, followed by killing the rats after 15 min or 2 h. Following the tube-feeding of tryptophan (30 mg per 100 g body weight) to fasted rats, total tryptophan concentrations after 15 min became markedly elevated in serum (+727%) and in liver (+797%), and after 2 h the levels were still elevated in serum (+458%) and in liver (+95%). Similar changes were 

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