(Inst. of Med. Chem. and Biochem., Univ. of Innsbruck, Fritz Pregl Str. 3; 1Central Inst. of Blood Transfusion and Immunol., A-6020 Innsbruck, Austria;* author for correspondence: fax 43-512-507-2865, e-mail Dietmar. Fuchs@ uibk. ac. at) l-Tryptophan is an essential amino acid that is required for the biosynthesis of proteins and is the precursor for several important biological compounds, eg, 5-hydroxytryptamine (serotonin). In an alternative reaction series, the so-called kynurenine pathway, tryptophan is catabolized to form nicotine amides and the vitamin niacin [1] as end products. A cytokine-inducible indoleamine-(2, 3)-dioxygenase (IDO) catalyzes the first step of tryptophan degradation [2–5], forming the intermediate kynurenine. Interferon-has been shown to be a potent stimulating cytokine for IDO in vitro [6, 7]. Similarly in vivo, a decrease of serum tryptophan and an increase of kynurenine in parallel was described [8–12], indicating an enhanced cytokine-induced degradation of tryptophan, when the cellular immune system is activated. In particular, the ratio of kynurenine to tryptophan seems to be a sensitive indicator for interferon--induced tryptophan degradation and therefore for an activated immune system [10, 13]. Thus, simultaneous measurements of kynurenine and tryptophan, allowing calculation of the ratio, enables indirect examination of endogenous interferon-formation.

We describe a new HPLC method to determine the amounts of kynurenine and tryptophan in serum simultaneously with use of an external albumin-based calibrator and an internal calibrator. The human subjects involved within the scope of this method development correspond to the ethical standards of our institutions’ responsible committee.