Serum tryptophan, kynurenine, phenylalanine, tyrosine and neopterin concentrations in 100 healthy blood donors

Formation of neopterin, a biomarker of the activated human immune system, is linked with tryptophan (TRP) and phenylalanine (PHE) metabolism. To obtain normal values, in this study, serum concentrations of neopterin as well as of TRP, PHE and their respective metabolites kynurenine (KYN) and tyrosine (TYR) were investigated in 100 successive blood donor serum specimens from the University Clinics of Innsbruck, Austria. In addition, nitrite concentrations were determined. Donors had passed anamnestic examination at entry and were therefore considered as healthy. The mean age of participants was 49 +/- 11.4 (mean +/- SD) years; 18% were older than 60 years. Both genders were included in the analysis. Neopterin concentrations measured by enzyme-linked immunosorbent assay were 5.9 +/- 1.6 nmol/L (mean +/- SD). Levels of amino acids and metabolites were determined by HPLC. Mean KYN and TRP concentrations were 1.78 +/- 0.42 mu mol/L and 67.4 +/- 10.2 mu mol/L, respectively. KYN to TRP ratio (KYN/TRP), an estimate for the activity of tryptophan-degrading enzyme indoleamine 2,3-dioxygenase, was 26.7 +/- 6.2 mu mol/mmol. Mean PHE and TYR concentrations were 65.2 +/- 11.1 mu mol/L and 90.6 +/- 22.9 mu mol/L. PHE to TYR ratio (PHE/TYR), an estimate for the activity of PHE-converting enzyme phenylalanine hydroxylase, was 0.75 +/- 0.14 mu mol/mu mol. Nitrite concentrations, estimated by Griess-Ilosvay reagent, were 44.9 +/- 32.0 mu mol/L. Males were taller and heavier than females (both p < 0.01), but body mass index did not differ. Males presented with significantly higher TRP and TYR concentrations than females (both p < 0.05). There existed significant correlations between neopterin and KYN (rs = 0.368), KYN/TRP (rs = 0.453), TYR (rs = -0.267; all p < 0.01) and PHE/TYR (rs = 0.236; p < 0.05) concentrations. Data indicate that also in a population of healthy individuals an association exists between low-grade immune activation as is indicated by slightly higher neopterin concentrations and biochemical alterations in the amino acid metabolism. Although minor, such changes may interfere with psycho-neuroimmunological regulatory networks and thus be of clinical relevance.