Evaluation of the antioxidant properties of tryptophan and its metabolites in in vitro assay
Authors: Nayak Bichitra N. , Buttar Harpal S. 
Published: December 2015

Background: Human milk contains a number of nutrients and bioactive ingredients which play an important role in the growth and development of infants. One important nutrient and bioactive ingredient of human milk is L-tryptophan. L-Tryptophan is an essential aromatic α-amino acid and is required in the diet of children and adult humans. As an essential amino acid, it is needed for protein synthesis and as a precursor of key biomolecules such as serotonin, melatonin, tryptamine, niacin, quinolinic acid and kynurenic acid, nicotinamide adenine dinucleotide. The aim of the study was to evaluate the antioxidant, anti-inflammatory and antiproliferative properties of tryptophan isolated from enzymatic hydrolysates from human milk and its metabolites on human glioma U251 cells and to evaluate the effects of human recombinant (hrIFNγ) on molecular ions of tryptophan and its metabolites in human glial U251 cells.

Methods: The cytotoxicity was determined by MTT (3-(4, 5-dimethylthiazolyl-2)-2, 5-diphenyltetrazolium bromide) assay. The antioxidant property was assessed by the oxygen radical scavenging capacity (ORAC) method. The anti-inflammatory effect was determined by the enzyme-linked immunosorbent assay (ELISA) against cytokines IL-6 and TNF-α. The effects of recombinant human (rhIFNγ) on molecular ions of tryptophan and its catabolites were evaluated by mass spectrometry. The tryptophan was isolated from milk peptides following enzymatic digestion, followed by separation by chromatographic and mass spectrometric methods.

Results: Tryptophan from human milk exhibited profoundly higher oxygen radical absorption capacity (7,986±468 µm Trolox equivalent (TE)/g) than that of whole human milk (80.4±13.3 µm TE/g). Tryptophan showed a moderate degree of anti-inflammatory activity against TNF-α and IL-6. rhIFNγ inhibited tryptophan metabolism. A low concentration of L-tryptophan (10–25 μg/mL) inhibited nearly 25% of cell growth. When U251 cells were treated with 25 μg/mL L-tryptophan and subsequently challenged with 30 ng/mL of human recombinant IFNγ, a significant inhibitory effect on cell growth was observed. Low concentrations of Xanthurenic acid, L-kynurenine, and 3-OH DL kynurenine were found to inhibit cell growth except melatonin and 3-OH anthranilic acid. Melatonin was a strong inducer of TNF-α in RAW cells, whereas 3-OH kynurenine at 25, 50 and 100 µg/mL inhibited IL-6 in RAW cells. No significant change was observed in the IL-8 profile in tryptophan-treated U251 cells except that L-kynurenine at 10 µg/mL produced significantly high level of an inflammatory cytokine IL-8. Melatonin, 3-OH, DL kynurenine at high concentrations (100 µg/mL) induced proliferation of U251 cells. Melatonin seemed to show synergistic effects with recombinant human IFNγ (rhINFγ) in promoting growth of human glioma cells. While treatment of U251 cells with tryptophan alone and subsequent treatment with rhIFNγ inhibited the growth of human cancer glioma cells, and conversely melatonin combined with rhIFNγ promoted growth of the U251 cells.

Conclusions: The findings from this study suggest that human milk-derived tryptophan and its metabolites possess strong antioxidant properties. Such effects might play a significant role in regulating the cell proliferation and growth of human cancer cells in a concentration-dependent manner.