Tryptophan in human plasma

Abstract

Investigations were carried out on the occurrence of tryptophan in plasma. Tryptophan occurs in plasma in two forms: free and bound to plasma albumins. The binding proves labile on deproteinisation with trichloracetic acid. Precipitation with ammonium sulphate does not affect the tryptophan-protein complex. There is no transformation of energy in the reaction of tryptophan binding and heavy metals are not involved. The carboxyl group of tryptophan plays an active part; of other indole derivatives, indole acids and especially indolepropionic acid are active in binding to plasma proteins. Serotonin shows little activity. Dibasic amino acids and organic acids compete with tryptophan. The optimum temperature for the binding reaction is 37° and the optimum pH varies between 8.5 and 9.5. The amount of bound tryptophan is influenced by the volume of plasma and by the concentration of tryptophan added. Preparations of l-tryptophan are more active than those of dl-tryptophan. In human physiological plasma, the ratio of endogenous bound tryptophan to endogenous free tryptophan is constant and varies between 0.8 and 1.0. Marked variations from this proportion were found in cases of neoplasms. Plasma is able to bind some additional amounts of tryptophan. 1 ml of plasma binds about 50% of 40 μg of dl-tryptophan added.