Tryptophan from human milk induces oxidative stress and upregulates the Nrf-2-mediated stress response in human intestinal cell lines

Chemical screening of digested human milk protein using the oxygen radical absorbance capacity (ORAC FL) antioxidant assay confirmed the presence of a peptide fraction (PF23) with high antioxidant activity [5.53 mmol Trolox equivalents (TE)/g] that contained tryptophan as a main component. We evaluated the effects of both PF23 and tryptophan alone on the modulation of oxidative stress in cultured intestinal cells using a dichlorofluorescein diacetate probe. Despite the high ORAC FL value, PF23 enhanced (P< 0.05) 2, 2'-azobis (2-amidinopropane) dihydrochloride (peroxyl radical generator)-induced intracellular oxidation in the Caco-2 human adenocarcinoma cell line, suggesting prooxidant activity. Compared to selected peptide fractions with relatively lower ORAC FL values, PF23 induced oxidative stress more than all other peptide fractions tested (P< 0.05) and contained more tryptophan than the others (P< 0.05). Similar prooxidant activity was observed for tryptophan when it was added to culture medium for both the Caco-2 cells and FHs 74 Int primary fetal enterocytes, while also exhibiting a high ORAC FL value (9.69 mmol TE/g). The effect of tryptophan that involves activation of the Nrf-2 pathway and transcription of antioxidant enzymes was therefore investigated in FHs 74 Int cells. Exposure of infant intestinal cells to tryptophan resulted in Nrf-2 activation and an increase in the gene transcript level of glutathione peroxidase 2. We conclude that tryptophan-induced oxidative stress associated with tryptophan-containing milk peptides induces an adaptive response that involves the activation of the antioxidant responsive signaling pathway in intestinal cells

Tryptophan released from mother's milk has antioxidant properties

Bioactive factors in human milk (HM) are crucial to the health of newborns, especially preterm infants. These compounds assist in reducing the oxidative stress that may occur as a result of combined exposure to supplemental oxygen and immature physiologic defenses. To identify the components in HM that contribute to its greater resistance to oxidative stress compared with infant formulae, enzymatic hydrolysates of HM were prepared, ultrafiltered, separated, and analyzed for antioxidant potential. The antioxidant activity [μM Trolox equivalent (TE/g)] of nondigested milk, whole digested milk, and derived ultrafiltrates were 80.4 ± 13.3, 159.0 ± 5.6, and 127.4 ± 3.1, respectively. An HPLC fraction denoted as fraction 23 (5274 ± 630 μM TE/g) was obtained and its constituents identified as tryptophan (Trp), peptides HNPI, and PLAPQA. Scavenging activity was not observed for PLAPQA, whereas moderate activity was associated with HNPI (144 ± 10.7 μM TE/g) and very high activity to Trp (7986 ± 468 μM TE/g). Trp addition to HM and two infant formulas significantly increased formulae antioxidant properties. Trp appeared to be a powerful free radical scavenger naturally present in HM. Its antioxidant effects and potential application in the diets of infants, particularly preterm, must be examined further