Oxidation rate of free and protein‐bound tryptophan by hydrogen peroxide and the bioavailability of the oxidation products

Abstract

AbstractThe oxidation of free and protein‐bound tryptophan by hydrogen peroxide (H2O2) has been used to evaluate the potential risk of oxidative damage of tryptophan during processing of foods. In the presence of excess H2O2 the first order rate constant for the oxidation of free tryptophan was 0.362 h−1 (at pH 8.4 and 50°C). This was 5 and 15 times faster than the rate of oxidation of tryptophan bound in casein or lactalbumin, respectively. To determine to what extent the oxidation products are utilisable in vivo, three levels of tryptophan oxidation were tested in a metabolic study. L[3‐14C]‐tryptophan was treated with H2O2 to give 0, 54 and 100% oxidation of the amino acid. The products were then given to rats fed ad libitum on a diet limiting in tryptophan. Radioactivity measurements of liver protein showed that oxidation products were not utilised for protein synthesis. For unreacted tryptophan only 4.6%±0.4 of the ingested dose was recovered in urine and 6.5%±0.8 in faeces (42 h period following ingestion). For completely oxidised tryptophan the recovered proportions increased to 32.6%±1.8 and 21.1%±1.5, respectively, which again indicates a reduced ability of the rat to use the oxidation products for anabolic purposes. Finally, H2O2‐treated casein was fed to rats in a growth assay which showed no significant decrease in tryptophan bioavailability. This supports the results obtained chemically. Even under the relatively strong oxidising conditions used in this study, the results indicate that protein‐bound tryptophan is not very sensitive to oxidation and, therefore, loss of tryptophan would not be an important limiting factor in food processing.